Unknown

TM 5-881-8/AFJMAN 32-1030

ECHNICAL

ANUAL

HEADQUARTERS

. 5-818-8

DEPARTMENTS OF THE ARMY

ORCE

ANUAL

AND THE AIR FORCE

32-1030

ASHINGTON

, DC,

20 July 1995

ENGINEERING USE OF GEOTEXTILES

Paragraph

Page

HAPTER

INTRODUCTION
Purpose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1-1

Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1-2

1-1

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1-3

1-1

Geotextile Types and Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1-4

1-1

Geotextile Durability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1-5

1-2

Seam Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1-6

1-2

Geotextile Functions and Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1-7

1-3

GEOTEXTILES IN PAVEMENT APPLICATIONS
Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2-1

Paved Surface Rehabilitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2-2

2-1

Reflective Crack Treatment for Pavements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2-3

2-1

Separation and Reinforcement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2-4

2-2

Design for Separation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2-5

2-4

Geotextile Survivability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2-6

2-4

Design for Reinforcement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2-7

2-4

FILTRATION AND DRAINAGE
Water Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3-1

Granular Drain Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3-2

3-1

Geotextile Characteristics Influencing Filter Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3-3

3-1

Piping Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3-4

3-1

Permeability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3-5

3-2

Other Filter Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3-6

3-3

Strength Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3-7

3-4

Design and Construction Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3-8

3-4

GEOTEXTILE REINFORCED EMBANKMENT ON SOFT FOUNDATION
General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4-1

Potential Embankment Failure Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4-2

4-1

Recommended Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4-3

Example Geotextile-Reinforced Embankment Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4-4

4-7

Bearing-Capacity Consideration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4-5

4-8

RAILROAD TRACK CONSTRUCTION AND REHABILITATION
General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5-1

Material Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5-2

5-1

Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5-3

5-1

Depth of Placement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5-4

5-1

Protective Sand Layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5-5

5-2

Drainage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5-6

5-3

Typical Sections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5-7

5-3

Special Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5-8

5-3

EROSION AND SEDIMENT CONTROL
Erosion Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

6-1

Bank Erosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

6-2

6-1

Precipitation Runoff Collection and Diversion Ditches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

6-3

Miscellaneous Erosion Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

6-4

6-3

Sediment Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

6-5

6-4

REINFORCED SOIL WALLS
Geotextile-Reinforced Soil Walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

7-1

Advantages of Geotextile-Reinforced Walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

7-2

7-1

Disadvantages of Geotextile- Reinforced Walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

7-3

7-1

Uses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

7-4

7-1

General Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

7-5

7-1

Properties of Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

7-6

7-2

Design Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

7-7

7-3

Design Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

7-8

7-3

APPROVED FOR PUBLIC RELEASE; DISTRIBUTION IS UNLIMITED

TM 5-881-8/AFJMAN 32-1030

Paragraph

Page

PPENDIX

REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

A-1

BIBLIOGRAPHY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . BIBLIOGRAPHY-1

LIST OF FIGURES

Figure

1-1.

Dimensions and Directions for Woven Geotextiles.

1-2

1-2.

Woven Monofilament Geotextiles Having Low Percent Open Area (Top), and High Percent Open

1-3

Area (Bottom).

1-3.

Woven Multifilament Geotextile.

1-4

1-4.

Woven Slit-Film Geotextile.

1-4

1-5.

Needle-Punched Nonwoven Geotextile

1-5

1-6.

Heat-Bonded Nonwoven Geotextile.

1-6

1-7.

Seam Types Used in Field Seaming of Geotextiles.

1-7

1-8.

Stitch Types Used in Field Seaming of Geotextiles.

1-8

2-1.

Geotextile in AC Overlay.

2-2

2-2.

Guidance for Geotextile Use in Minimizing Reflective Cracking.

2-3

2-3.

Relationship Between Shear Strength, CBR, and Cone Index.

2-6

2-4.

Thickness Design Curve for Single-Wheel Load on Gravel-Surfaced Roads.

2-7

2-5.

Thickness Design Curve for Dual-Wheel Load on Gravel- Surfaced Roads.

2-8

2-6.

Thickness Design Curve for Tandem-Wheel Load on Gravel-Surfaced Roads.

2-9

3-1.

Trench Drain Construction.

3-5

4-1.

Potential Geotextile-Reinforced Embankment Failure Modes.

4-2

4-2.

Concept Used for Determining Geotextile Tensile Strength Necessary to Prevent Slope Failure.

4-4

4-3.

Assumed Stresses and Strains Related to Lateral Earth Pressures.

4-7

4-4.

Embankment Section and Foundation Conditions of Embankment Design Example Problem.

5-1.

Typical Sections of Railroad Track with Geotextile.

5-4

6-1.

Relationship between Atterberg Limits and Expected Erosion Potential.

6-2

6-2.

Pin Spacing Requirements in Erosion Control Applications.

6-3

6-3.

Geotextile Placement for Currents Acting Parallel to Bank or for Wave Attack on the Bank.

6-4

6-4.

Ditch Liners.

6-5

6-5.

Use of Geotextiles near Small Hydraulic Structures.

6-6

6-6.

Use of Geotextiles around Piers and Abutments.

6-6

6-7.

Sedimentation behind Silt Fence.

6-7

7-1.

General Configuration of a Geotextile Retained Soil Wall and Typical Pressure Diagrams.

7-2

7-2.

Procedures for Computing Live Load Stresses on Geotextile Reinforced Retaining Walls.

7-4

LIST OF TABLES

Table

2-1.

Property Requirements of Nonwoven Geotextiles.

2-3

2-2.

Construction Survivability Ratings (FHWA 1989).

2-4

2-3.

Relationship of Construction Elements to Severity of Loading Imposed on Geotextile in Road-

2-5

way Construction (FHWA 1989).

2-4.

Minimum Geotextile Strength Properties for Survivability (FHWA 1989).

2-5

3-1.

Geotextile Filter Design Criteria.

3-1

3-2.

Geotextile Strength Requirements for Drains.

3-4

5-1.

Recommended Geotextile Property Requirements for Railroad Applications.

5-2

6-1.

Recommended Geotextile Minimum Strength Requirements.

6-2

6-2.

Pin Spacing Requirements in Erosion Control Applications.

6-3

TM 5-818-8/AFJMAN 32-1030

CHAPTER 1

INTRODUCTION

1-1. Purpose

This manual describes various geotextiles, test

methods for evaluating their properties, and rec-
ommended design and installation procedures.

1-2. Scope

This manual covers physical properties, functions,
design methods, design details and construction

procedures for geotextiles as used in pavements,
railroad beds, retaining wall earth embankment,
rip-rap, concrete revetment, and drain construc-

tion. Geotextile functions described include pave-
ments, filtration and drainage, reinforced embank-
ments, railroads, erosion and sediment control,
and earth retaining walls. This manual does not
cover the use of other geosynthetics such as geo-
grids, geonets, geomembranes, plastic strip drains,

composite products and products made from natu-
ral cellulose fibers.

1-3. References

Appendix A contains a list of references used in
this manual.

1-4. Geotextile Types and Construction

a. Materials.

Geotextiles are made from poly-

propylene, polyester, polyethylene, polyamide

(nylon), polyvinylidene chloride, and fiberglass.
Polypropylene and polyester are the most used.

Sewing thread for geotextiles is made from
Kevlar

or any of the above polymers. The physi-

cal properties of these materials can be varied by
the use of additives in the composition and by

changing the processing methods used to form the
molten material into filaments. Yarns are formed

from fibers which have been bundled and twisted
together, a process also referred to as spinning.
(This reference is different from the term spinning

as used to denote the process of extruding fila-
ments from a molten material.) Yarns may be
composed of very long fibers (filaments) or rela-
tively short pieces cut from filaments (staple
fibers).

b. Geotextile Manufacture.

(1) In woven construction, the warp yarns,

which run parallel with the length of the geotex-
tile panel (machine direction), are interlaced with
yarns called till or filling yarns, which run perpen-

dicular to the length of the panel (cross direction

Kevlar is a registered trademark of Du Pont for their aramid

fiber.

as shown in fig 1-1). Woven construction produces
geotextiles with high strengths and moduli in the
warp and fill directions and low elongations at
rupture. The modulus varies depending on the rate
and the direction in which the geotextile is loaded.

When woven geotextiles are pulled on a bias, the
modulus decreases, although the ultimate break-
ing strength may increase. The construction can

be varied so that the finished geotextile has equal
or different strengths in the warp and fill direc-
tions. Woven construction produces geotextiles

with a simple pore structure and narrow range of
pore sizes or openings between fibers. Woven

geotextiles are commonly plain woven, but are
sometimes made by twill weave or leno weave (a

very open type of weave). Woven geotextiles can be
composed of monofilaments (fig 1-2) or multifila-

ment yarns (fig 1- 3). Multifilament woven con-
struction produces the highest strength and modu-
lus of all the constructions but are also the highest
cost. A monofilament variant is the slit-film or
ribbon filament woven geotextile (fig 1-4). The
fibers are thin and flat and made by cutting sheets
of plastic into narrow strips. This type of woven
geotextile is relatively inexpensive and is used for
separation, i.e., the prevention of intermixing of

two materials such as aggregate and fine-grained
soil.

(2) Manufacturers literature and textbooks

should be consulted for greater description of
woven and knitted geotextile manufacturing pro-
cesses which continue to be expanded.

(3) Nonwoven geotextiles are formed by a

process other than weaving or knitting, and they
are generally thicker than woven products. These

geotextiles may be made either from continuous
filaments or from staple fibers. The fibers are
generally oriented randomly within the plane of

the geotextile but can be given preferential orien-
tation. In the spunbonding process, filaments are
extruded, and laid directly on a moving belt to
form the mat, which is then bonded by one of the
processes described below.

(a) Needle punching.

Bonding by needle

punching involves pushing many barbed needles
through one or several layers of a fiber mat
normal to the plane of the geotextile. The process

causes the fibers to be mechanically entangled (fig

1-5). The resulting geotextile has the appearance

of a felt mat.

(b) Heat bonding.

This is done by incorpo-

1-1

TM 5-818-8/AFJMAN 32-1030

Figure 1-1. Dimensions and Directions for Woven Geotextiles.

rating fibers of the same polymer type but having
different melting points in the mat, or by using

heterofilaments, that is, fibers composed of one
type of polymer on the inside and covered or
sheathed with a polymer having a lower melting
point. A heat-bonded geotextile is shown in figure

1-6.

(c) Resin bonding.

Resin is introduced into

the fiber mat, coating the fibers and bonding the
contacts between fibers.

(d) Combination bonding.

Sometimes a com-

bination of bonding techniques is used to facilitate
manufacturing or obtain desired properties.

(4) Composite geotextiles are materials which

combine two or more of the fabrication techniques.
The most common composite geotextile is a non-
woven mat that has been bonded by needle punch-
ing to one or both sides of a woven scrim.

1-5. Geotextile Durability

Exposure to sunlight degrades the physical proper-
ties of polymers. The rate of degradation is re-
duced by the addition of carbon black but not

eliminated. Hot asphalt can approach the melting
point of some polymers. Polymer materials become
brittle in very cold temperatures. Chemicals in the
groundwater can react with polymers. All poly-
mers gain water with time if water is present.
High pH water can be harsh on polyesters while
low pH water can be harsh on polyamides. Where

a chemically unusual environment exists, labora-
tory test data on effects of exposure of the geotex-

tile to this environment should be sought. Experi-
ence with geotextiles in place spans only about 30
years. All of these factors should be considered in
selecting or specifying acceptable geotextile mate-

rials. Where long duration integrity of the mate-
rial is critical to life safety and where the in-place

1-2

material cannot easily be periodically inspected or
easily replaced if it should become degraded (for
example filtration and/or drainage functions
within an earth dam), current practice is to use
only geologic materials (which are orders of magni-

tude more resistant to these weathering effects
than polyesters).

1-6. Seam Strength

a. Joining Panels.

Geotextile sections can be

joined by sewing, stapling, heat welding, tying,

and gluing. Simple overlapping and staking or

nailing to the underlying soil may be all that is
necessary where the primary purpose is to hold

the material in place during installation. However,
where two sections are joined and must withstand
tensile stress or where the security of the connec-

tion is of prime importance, sewing is the most
reliable joining method.

b. Sewn Seams.

More secure seams can be pro-

duced in a manufacturing plant than in the field.
The types of sewn seams which can be produced in
the field by portable sewing machines are pre-
sented in figure 1-7. The seam type designations
are from Federal Standard 751. The SSa seam is

referred to as a “prayer” seam, the SSn seam as a
“J” seam, and the SSd as a “butterfly” seam. The
double-sewn seam, SSa-2, is the preferred method

for salvageable geotextiles. However, where the
edges of the geotextile are subject to unraveling,

SSd or SSn seams are preferred.

c. Stitch Type.

The portable sewing machines

used for field sewing of geotextiles were designed
as bag closing machines. These machines can
produce either the single-thread or two-thread
chain stitches as shown in figure 1-8. Both of
these stitches are subject to unraveling, but the

single-thread stitch is much more susceptible and

TM 5-818-8/AFJMAN 32-1030

Figure 1-5. Needle-Punched Nonwoven Geotextile.

placed in contact with and down gradient of soil to

be drained. The plane of the geotextile is normal
to the expected direction of water flow. The capac-
ity for flow of water normal to the plane of the
geotextile is referred to as permittivity. Water and
any particles suspended in the water which are
smaller than a given size flow through the geotex-

tile. Those soil particles larger than that size are
stopped and prevented from being carried away.
The geotextile openings should be sized to prevent

soil particle movement. The geotextiles substitute

for and serve the same function as the traditional

granular filter. Both the granular filter and the

geotextile filter must allow water (or gas) to pass
without significant buildup of hydrostatic pres-
sure. A geotextile-lined drainage trench along the

edge of a road pavement is an example using a
geotextile as a filter. Most geotextiles are capable
of performing this function. Slit film geotextiles

are not preferred because opening sizes are unpre-
dictable. Long term clogging is a concern when
geotextiles are used for filtration.

c. Drainage.

When functioning as a drain, a

geotextile acts as a conduit for the movement of
liquids or gases in the plane of the geotextile.

Examples are geotextiles used as wick drains and

blanket drains. The relatively thick nonwoven

geotextiles are the products most commonly used.

Selection should be based on transmissivity, which
is the capacity for in-plane flow. Questions exist as

to long term clogging potential of geotextile
drains. They are known to be effective in short
duration applications.

d. Erosion Control.

In erosion control, the geo-

textile protects soil surfaces from the tractive
forces of moving water or wind and rainfall ero-
sion. Geotextiles can be used in ditch linings to
protect erodible fine sands or cohesionless silts.
The geotextile is placed in the ditch and is secured
in place by stakes or is covered with rock or gravel
to secure the geotextile, shield it from ultraviolet
light, and dissipate the energy of the flowing
water. Geotextiles are also used for temporary
protection against erosion on newly seeded slopes.
After the slope has been seeded, the geotextile is

anchored to the slope holding the soil and seed
in-place until the seeds germinate and vegetative
cover is established. The erosion control function

can be thought of as a special case of the combina-
tion of the filtration and separation functions.

e. Sediment Control.

A geotextile serves to con-

trol sediment when it stops particles suspended in
surface fluid flow while allowing the fluid to pass
through. After some period of time, particles accu-
mulate against the geotextile, reducing the flow of
fluid and increasing the pressure against the
geotextile. Examples of this application are silt
fences placed to reduce the amount of sediment
carried off construction sites and into nearby

1-5

TM 5-818-8/AFJMAN 32-1030

CHAPTER 2

GEOTEXTILES IN PAVEMENT APPLICATIONS

2-1. Applications

This chapter discusses the use of geotextiles for

asphalt concrete (AC) overlays on roads and air-

fields and the separation and reinforcement of

materials in new construction. The functions per-
formed by the geotextile and the design consider-
ations are different for these two applications. In
an AC pavement system, the geotextile provides a
stress-relieving interlayer between the existing
pavement and the overlay that reduces and re-
tards reflective cracks under certain conditions
and acts as a moisture barrier to prevent surface
water from entering the pavement structure.
When a geotextile is used as a separator, it is
placed between the soft subgrade and the granular
material. It acts as a filter to allow water but not

fine material to pass through it, preventing any

mixing of the soft soil and granular material
under the action of the construction equipment or
subsequent traffic.

2-2. Paved Surface Rehabilitation

a. General.

Old and weathered pavements con-

tain transverse and longitudinal cracks that are
both temperature and load related. The method
most often used to rehabilitate these pavements is
to overlay the pavement with AC. This tempo-
rarily covers the cracks. After the overlay has
been placed, any lateral or vertical movement of
the pavement at the cracks due to load or ther-
mal effects causes the cracks from the existing
pavement to propagate up through the new AC
overlay (called reflective cracking). This movement
causes raveling and spalling along the reflective
cracks and provides a path for surface water to
reach the base and subgrade which decreases the
ride quality and accelerates pavement deteriora-

tion.

b. Concept.

Under an AC overlay, a geotextile

may provide sufficient tensile strength to relieve
stresses exerted by movement of the existing
pavement. The geotextile acts as a stress-relieving
interlayer as the cracks move horizontally or

vertically. A typical pavement structure with a
geotextile interlayer is shown in figure 2-1. Im-
pregnation of the geotextile with a bitumen pro-
vides a degree of moisture protection for the
underlying layers whether or not reflective crack-

ing occurs.

2-3. Reflective Crack Treatment for Pave-
ments

a. General.

Geotextiles can be used successfully

in pavement rehabilitation projects. Conditions
that are compatible for the pavement applications
of geotextiles are AC pavements that may have
transverse and longitudinal cracks but are rela-
tively smooth and structurally sound, and PCC

pavements that have minimum slab movement.
The geographic location and climate of the project
site have an important part in determining

whether or not geotextiles can be successfully used
in pavement rehabilitation. Geotextiles have been
successful in reducing and retarding reflective
cracking in mild and dry climates when tempera-
ture and moisture changes are less likely to
contribute to movement of the underlying pave-
ment; whereas, geotextiles in cold climates have
not been as successful. Figure 2-2 gives guidance
in using geotextiles to minimize reflective crack-
ing on AC pavements. Geotextiles interlayers are
recommended for use in Areas I and II, but are not
recommended for use in Area III. Since geotextiles
do not seem to increase the performance of thin
overlays, minimum overlay thicknesses for Areas I
and II are given in figure 2-2. Even when the

climate and thickness requirements are met, there
has been no consistent increase in the time it
takes for reflective cracking to develop in the

overlay indicating that other factors are influenc-
ing performance. Other factors affecting perfor-
mance of geotextile interlayers are construction

techniques involving pavement preparation, as-
phalt sealant application, geotextile installation,

and AC overlay as well as the condition of the
underlying pavement.

b. Surface Preparation.

Prior to using geotex-

tiles to minimize reflective cracks, the existing
pavement should be evaluated to determine pave-
ment distress. The size of the cracks and joints in
the existing pavement should be determined. All
cracks and joints larger than ¼ inch in width

should be sealed. Differential slab movement
should be evaluated, since deflections greater than

0.002 inch cause early reflective cracks. Areas of

the pavement that are structurally deficient

should be repaired prior to geotextile installation.

Placement of a leveling course is recommended
when the existing pavement is excessively cracked
and uneven.

c. Geotextile Selection.

2-1

TM 5-818-8/AFJMAN 32-1030

ASPHALT CONCRETE

OVERLAY

BASE

COURSE

S U B G R A D E

Figure 2-1. Geotextile in AC Overlay.

(1) Geotextile interlayers are used in two dif-

ferent capacities-the full-width and strip methods.
The full-width method involves sealing cracks and

joints and placing a nonwoven material across the

entire width of the existing pavement. The mate-
rial should have the properties shown in table 2-1.
Nonwoven materials provide more flexibility and
are recommended for reflective crack treatment of
AC pavements.

(2) The strip method is primarily used on PCC

pavements and involves preparing the existing
cracks and joints and placing a 12 to 24 inch wide

geotextile and sufficient asphalt directly on the
cracks and joints. The required physical properties
are shown in table 2-1, however nonwoven geotex-

tiles are not normally used in the strip method.
Membrane systems have been developed for strip
repairs.

d. Asphalt Sealant.

The asphalt sealant is used

to impregnate and seal the geotextile and bond it
to both the base pavement and overlay. The grade

of asphalt cement specified for hot-mix AC pave-
ments in each geographic location is generally the
most acceptable material. Either anionic or catio-
nic emulsion can also be used. Cutback asphalts

and emulsions which contain solvents should not
be used.

AC Overlay.

The thickness of the AC overlay

should be determined from the pavement struc-
tural requirements outlined in TM 5-822-5/

A F J M A N 3 2 - 1 0 1 8 , T M 5 - 8 2 5 - 2 / A F J M A N
32-1014 and TM 5-825-3/AFJMAN 32-1014,
Chap. 3 or from minimum requirements, which-

2-2

ever is greater. For AC pavements, Area I shown
in figure 2-2 should have a minimum overlay
thickness of 2 inches; whereas, Area II should
have a minimum overlay thickness of 3 inches.
The minimum thickness of an AC overlay for
geotextile application on PCC pavements is 4
inches.

f. Spot Repairs.

Rehabilitation of localized dis-

tressed areas and utility cuts can be improved
with the application of geotextiles. Isolated dis-
tressed areas that are excessively cracked can be
repaired with geotextiles prior to an AC overlay.
Either a full-width membrane strip application can
be used depending on the size of the distressed
area. Localized distressed areas of existing AC

pavement that are caused by base failure should
be repaired prior to any pavement rehabilitation.
Geotextiles are not capable of bridging structur-
ally deficient pavements.

2-4. Separation and Reinforcement

Soft subgrade materials may mix with the granu-
lar base or subbase material as a result of loads
applied to the base course during construction

and/or loads applied to the pavement surface that
force the granular material downward into the soft
subgrade or as a result of water moving upward

into the granular material and carrying the sub-
grade material with it. A sand blanket or filter
layer between the soft subgrade and the granular

material can be used in this situation. Also, the
subgrade can be stabilized with lime or cement or
the thickness of granular material can be in-

TM 5-818-8/AFJMAN 32-1030

2-5. Design for Separation

When serving as a separator, the geotextile pre-
vents fines from migrating into the base course

and/or prevents base course aggregate from pene-
trating into the subgrade. The soil retention prop-
erties of the geotextile are basically the same as
those required for drainage or filtration. Therefore,
the retention and permeability criteria required
for drainage should be met. In addition, the geo-
textile should withstand the stresses resulting
from the load applied to the pavement. The nature
of these stresses depend on the condition of the
subgrade, type of construction equipment, and the
cover over the subgrade. Since the geotextile
serves to prevent aggregate from penetrating the
subgrade, it must meet puncture, burst, grab and
tear strengths specified in the following para-
graphs.

2-6. Geotextile Survivability

Table 2-2 has been developed for the Federal
Highway Administration (FHWA) to consider sur-
vivability requirements as related to subgrade

conditions and construction equipment; whereas,
table 2-3 relates survivability to cover material

and construction equipment. Table 2-4 gives mini-
mum geotextile grab, puncture, burst, and tear
strengths for the survivability required for the

conditions indicated in tables 2-2 and 2-3.

2-7. Design for Reinforcement

Use of geotextiles for reinforcement of gravel

surfaced roads is generally limited to use over soft
cohesive soils (CBR

4). One procedure for

determining the thickness requirements of aggre-
gate above the geotextile was developed by the US

Forest Service (Steward, et al. 1977) and is as
follows:

a. Determine In-Situ Soil Strength.

Determine

the in-situ soil strength using the field California
Bearing Ratio (CBR), cone penetrometer, or Vane

Shear device. Make several readings and use the
lower quartile value.

b. Convert Soil Strength.

Convert the soil

strength to an equivalent cohesion (C) value using
the correlation shown in figure 2-3. The shear
strength is equal to the C value.

Table 2-2. Construction Survivability Ratings (FHWA 1989).

Site Soil CBR
at Installation

1-2

Equipment Ground

>50

<50

>50

<50

>50

<50

Contact Pressure

(psi)

Cover Thickness

(in.)

(Compacted)

2 , 3

H = High, M = Medium, NR = Not recommended.

Maximum aggregate size not to exceed one half the compacted cover

thickness.

For low volume unpaved road (ADT 200 vehicles).

The four inch minimum cover is limited to existing road bases and

not intended for use in new construction.

2-4

TM 5-818-8/AFJMAN 32-1030

Table 2-3. Relationship of Construction Elements to Severity of Loading Imposed on Geotextile in Roadway Construction.

Variable

Equipment

Subgrade
Condition

Subgrade
Strength

(CBR)

Aggregate

Lift
Thickness

(in.)

LOW

Light weight
dozer (8 psi)

Cleared

<0.5

Rounded sandy

gravel

Severity Category

Moderate

High to Very High

Medium weight

dozer;

lig t

Heavy weight dozer;

wheeled equipment

loaded dump truck

(8-40 psi)

(>40 psi)

Partially cleared Not cleared

1-2

Coarse angular

Cobbles, blasted

gravel

rock

Table 2-4. Minimum Geotextile Strength Properties for Survivability

Required

Degree

of Geotextile
Survivability

Very high

High

Grab Strength'

270

180

Puncture

Burst

Trap

Strength

Tear

psi

l b

110

430

290

Moderate

130

210

Low

145

Note:

All values represent minimum average roll values (i.e., any roll in a

lot should meet or exceed the minimum values in this table).

These

values are normally 20 percent lower than manufacturers reported

typical values.

ASTM D 4632.

ASTM D 4833.

ASTM D 3786.

ASTM D 4533, either principal direction.

2-5

TM 5-818-8/AFJMAN 32-1030

Figure 2-3. Relationship Between Shear Strength, CBR,

and Cone Index.

c. Select Design Loading.

Select the desired de-

sign loading, normally the maximum axle loads.

d. Determine Required Thickness of Aggregate.

Determine the required thickness of aggregate
above the geotextile using figures 2-4, 2-5, and
2-6. These figures relate the depth of aggregate
above the geotextile to the cohesion of the soil (C)
and to a bearing capacity factor (N

) The product

of C and N

is the bearing capacity for a rapidly

loaded soil without permitting drainage. The sig-
nificance of the value used for N

as it relates to

the design thickness using figures 2-4, 2-5, and

2-6 is as follows:

(1) For thickness design without using geotex-

tile.

(a)

A value of 2.8 for N

would result in a

thickness design that would perform with very
little rutting (less than 2 inches) at traffic volumes
greater than 1,000 equivalent 18-kip axle loadings.

(b)

A value of 3.3 for N

would result in a

thickness design that would rut 4 inches or more
under a small amount of traffic (probably less than

100 equivalent 18-kip axle loadings).

(2) For thickness design using geotextile.

(a)

A value of 5.0 for N

would result in a

thickness design that would perform with very
little rutting (less than 2 inches) at traffic vol-
umes greater than 1,000 equivalent 18-kip axle
loadings.

(b)

A value of 6.0 for N

would result in a

thickness design that would rut 4 inches or more
under a small amount of traffic (probably less than

100 equivalent 18-kip axle loadings).

e. Geotextile reinforced gravel road design exam-

ple.

Design a geotextile reinforced gravel road for

a 24,000-pound-tandem-wheel load on a soil having
a CBR of 1. The road will have to support several

thousand truck passes and very little rutting will
be allowed.

(1) Determine the required aggregate thick-

ness with geotextile reinforcement.

(a)

From figure 2-3 a 1 CBR is equal to a C

value of 4.20.

(b)

Choose a value of 5 for N

since very

little rutting will be allowed.

(c)

Calculate CN

as: CN

= 4.20(5) = 21.

(d)

Enter figure 2-6 with CN

of 21 to

obtain a value of 14 inches as the required
aggregate thickness above the geotextile.

(e)

Select geotextile requirements based on

survivability requirements in tables 2-2 and 2-3.

(2) Determine the required aggregate thick-

ness when a geotextile is not used.

(a)

Use a value of 2.8 for N

since a geotex-

tile is not used and only a small amount of rutting

will be allowed.

(b)

Calculate CN

as: CN

= 4.20(2.8) =

11.8.

(c)

Enter figure 2-6 with CN

of 11.8 to

obtain a value of 22 inches as the required
aggregate thickness above the subgrade without

the geotextile.

(3) Compare cost and benefits of the alterna-

tives. Even with nearby economical gravel sources,

the use of a geotextile usually is the more econom-
ical alternative for constructing low volume roads
and airfields over soft cohesive soils. Additionally,
it results in a faster time to completion once the

geotextiles are delivered on site.

2-6

TM 5-818-8/AFJMAN 32-1030

CHAPTER 3

FILTRATION AND DRAINAGE

3-1. Water Control

Control of water is critical to the performance of
buildings, pavements, embankments, retaining
walls, and other structures. Drains are used to
relieve hydrostatic pressure against underground

and retaining walls, slabs, and underground tanks
and to prevent loss of soil strength and stability in
slopes, embankments, and beneath pavements. A

properly functioning drain must retain the sur-
rounding soil while readily accepting water from
the soil and removing it from the area. These
general requirements apply to granular and geo-
textile filters. While granular drains have a long
performance history, geotextile use in drains is

relatively recent and performance data are limited
to approximately 25 years. Where not exposed to

sunlight or abrasive contact with rocks moving in
response to moving surface loads or wave action,
long-term performance of properly selected geotex-

tiles has been good. Since long-term experience is
limited, geotextiles should not be used as a substi-
tute for granular filters within or on the upstream
face of earth dams or within any inaccessible
portion of the dam embankment. Geotextiles have
been used in toe drains of embankments where

they are easily accessible if maintenance is re-
quired and where malfunction can be detected.
Caution is advised in using geotextiles to wrap
permanent piezometers and relief wells where they
form part of the safety system of a water retaining

structure. Geotextiles have been used to prevent

infiltration of fine-grained materials into piezo-
meter screens but long-term performance has not
been measured.

3-2. Granular Drain Performance

To assure proper performance in granular drains,

the designer requires drain materials to meet

grain-size requirements based on grain size of the
surrounding soil. The two principal granular filter

criteria, piping and permeability, have been devel-
oped empirically through project experience and
laboratory testing. The piping and permeability
criteria are contained in TF 5-820-2/ AFJMAN

32-1016, Chap. 2.

3-3. Geotextile Characteristics Influencing Fil-

ter Functions

The primary geotextile characteristics influencing
filter functions are opening size (as related to soil

retention), flow capacity, and clogging potential.

These properties are indirectly measured by the
apparent opening size (AOS) (ASTM D 4751),
permittivity (ASTM D 4491), and gradient ratio
test (ASTM D 5101). The geotextile must also have
the strength and durability to survive construction
and long-term conditions for the design life of the
drain. Additionally, construction methods have a
critical influence on geotextile drain performance.

3-4. Piping Resistance

a. Basic Criteria.

Piping resistance is the ability

of a geotextile to retain solid particles and is
related to the sizes and complexity of the openings

or pores in the geotextile. For both woven and
nonwoven geotextiles, the critical parameter is the
AOS. Table 3-1 gives the relation of AOS to the
gradation of the soil passing the number 200 sieve
for use in selecting geotextiles.

Table 3-1. Geotextile Filter Design Criteria.

Protected Soil

Permeability

(Percent Passing
No. 200 Sieve)

Piping

Woven

Nonwoven

Less than 5%

AOS (mm) < 0.6

POA

>10% k

> 5 k

(mm)

(Greater than #30
US Standard

Sieve)

5 to 50%

AOS (mm) < 0.6

POA> 4% k

> 5 k

(mm)

(Greater than #30
US Standard

Sieve)

50 to 85%

A O S ( m m ) < 0 . 2 9 7 P O A>4% k

>5k

(mm)
(Greater than #50
US Standard

Sieve)

Greater than 85% AOS (mm) < 0.297

>5k

(mm)

(Greater than #50
US Standard

Sieve)

When the protected soil contains appreciable quantities of

material retained on the No. 4 sieve use only the soil passing
the No. 4 sieve in selecting the AOS of the geotextile.

is the permeability of the nonwoven geotextile and k

the permeability of the protected soil.

POA = Percent Open Area.

b. Percent Open Area Determination Procedure

for Woven Geotextiles.

(1) Installation of geotextile. A small section

of the geotextile to be tested should be installed in

3-1

TM 5-818-8/AFJMAN 32-1030

a standard 2 by 2 inch slide cover, so that it can

be put into a slide projector and projected onto a
screen. Any method to hold the geotextile section

and maintain it perpendicular to the projected
light can be used.

(2) Slide projector. The slide projector should

be placed level to eliminate any distortion of the
geotextile openings. After placing the slide in the
projector and focusing on a sheet of paper approxi-
mately 8 to 10 feet away, the opening outlines can
be traced.

(3) Representative area. Draw a rectangle of

about 0.5 to 1 square foot area on the “projection
screen” sheet of paper to obtain a representative
area to test; then trace the outline of all openings
inside the designated rectangle.

(4) Finding the area. After removing the

sheet, find the area of the rectangle, using a
planimeter. If necessary, the given area may be
divided to accommodate the planimeter.

(5) Total area of openings. Find the total area

of openings inside rectangle, measuring the area of
each with a planimeter.

(6) Compute percent. Compute POA by the

equation:

POA=

Total Area Occupied by Openings

x 100

Total Area of Test Rectangle

c. Flow Reversals.

Piping criteria are based on

granular drain criteria for preventing drain mate-
rial from entering openings in drain pipes. If flow
through the geotextile drain installation will be

reversing and/or under high gradients (especially
if reversals are very quick and involve large
changes in head), tests, modeling prototype condi-

tions, should be performed to determine geotextile
requirements.

d. Clogging.

There is limited evidence (Giroud

1982) that degree of uniformity and density of

granular soils (in addition to the D

size) influ-

ence the ability of geotextiles to retain the drained
soil. For very uniform soils (uniformity coefficient

2 to 4), the maximum AOS may not be as critical
as for more well graded soils (uniformity coeffi-
cient greater than 5). A gradient ratio test with
observation of material passing the geotextile may
be necessary to determine the adequacy of the

material. In normal soil- geotextile filter systems,
detrimental clogging only occurs when there is
migration of fine soil particles through the soil
matrix to the geotextile surface or into the geotex-
tile. For most natural soils, minimal internal
migration will take place. However, internal mi-
gration may take place under sufficient gradient if

one of the following conditions exists:

(1) The soil is very widely graded, having a

coefficient of uniformity C

, greater than 20.

(2) The soil is gap graded. (Soils lacking a

range of grain sizes within their maximum and
minimum grain sizes are called “gap graded” or

“skip graded” soils.) Should these conditions exist

in combination with risk of extremely high repair
costs if failure of the filtration system occurs the
gradient ratio test may be required.

e. Clogging Resistance.

Clogging is the reduc-

tion in permeability or permittivity of a geotextile
due to blocking of the pores by either soil particles
or biological or chemical deposits. Some clogging
takes place with all geotextiles in contact with
soil. Therefore, permeability test results can only

be used as a guide for geotextile suitability. For
woven geotextiles, if the POA is sufficiently large,
the geotextiles will be resistant to clogging. The

POA has proved to be a useful measure of clogging
resistance for woven textiles but is limited to

woven geotextiles having distinct, easily measured
openings. For geotextiles which cannot be evalu-
ated by POA, soil- geotextile permeameters have
been developed for measuring soil-geotextile per-

meability and clogging. As a measure of the
degree to which the presence of geotextile affects

the permeability of the soil- geotextile system, the

gradient ratio test can be used (ASTM D 5101).
The gradient ratio is defined as the ratio of the
hydraulic gradient across the geotextile and the 1
inch of soil immediately above the geotextile to
the hydraulic gradient between 1 and 3 inches
above the geotextile.

3-5. Permeability

a. Transverse Permeability.

After installation,

geotextiles used in filtration and drainage applica-

tions must have a flow capacity adequate to
prevent significant hydrostatic pressure buildup in
the soil being drained. This flow capacity must be
maintained for the range of flow conditions for
that particular installation. For soils, the indicator
of flow capacity is the coefficient of permeability
as expressed in Darcy’s Law (TM 5-820-2/
AFSMAN 32-1016 ). The proper application of

Darcy’s Law requires that geotextile thickness be
considered. Since the ease of flow through a
geotextile regardless of its thickness is the prop-
erty of primary interest, Darcy’s Law can be
modified to define the term permittivity,

, with

units of sec

-1

, as follows:

(eq 3-1)

3-2

TM 5-818-8/AFJMAN 32-1030

where

k = Darcy coefficient of permeability, L/T
L

= length of flow path (geotextile thickness) over which

occurs, L

q = hydraulic discharge rate, L

= hydraulic head loss through the geotextile, L

A = total cross-sectional area available to flow, L

L = units of length
T = units of time

The limitation of directly measuring the perme-
ability and permittivity of geotextiles is that
Darcy’s Law applies only as long as laminar flow
exists. This is very difficult to achieve for geotex-
tiles since the hydraulic heads required to assure

laminar flow are so small that they are difficult to
accurately measure. Despite the fact that Darcy’s
equation does not apply for most measurements of

permeability, the values obtained are considered
useful as a relative measure of the permeabilities
and permittivities of various geotextiles. Values of
permeability reported in the literature, or obtained
from testing laboratories, should not be used with-
out first establishing the actual test conditions
used to determine the permeability value. ASTM
Method D 4491 should be used for establishing the
permeability and permittivity of geotextiles. The
permeability of some geotextiles decreases signifi-
cantly when compressed by surrounding soil or
rock. ASTM D 5493 can be used for measuring the
permeabilities of geotextiles under load.

b. In-plane Permeability.

Thick nonwoven geo-

textiles and special products as prefabricated
drainage panels and strip drains have substantial
fluid flow capacity in their plane. Flow capacity in
a plane of a geotextile is best expressed indepen-
dently of the material’s thickness since the thick-

ness of various materials may differ considerably,
while the ability to transmit fluid under a given
head and confining pressure is the property of

interest. The property of in-plane flow capacity of
a geotextile is termed “transmissivity,”

, and is

expressed as:

(eq 3-2)

where

= in-plane coefficient of permeability (hydraulic conduc-

tivity), L/T

t = thickness of geotextile, L (ASTM D 5199)
q = hydraulic discharge rate, L

3/T

1 = length of geotextile through which liquid is flowing, L

= hydraulic head loss, L

w = width of geotextile, L
L = units of length, length between geotextile grips
T = units of time

Certain testing conditions must be considered if
meaningful values of transmissivity are to be

acquired. These conditions include the hydraulic

gradients used, the normal pressure applied to the

product being tested, the potential for reduction of
transmissivity over time due to creep of the drain-
age material, and the possibility that intermittent
flow will result in only partial saturation of the
drainage material and reduced flow capacity.

ASTM D 4716 may be used for evaluating the
transmissivity of drainage materials.

c. Limiting Criteria.

Permeability criteria for

nonwoven geotextiles require that the permeabil-
ity of the geotextile be at least five times the
permeability of the surrounding soil. Permeability

criteria for woven geotextiles are in terms of the
POA. When the protected soil has less than 0.5
percent passing the No. 200 sieve, the POA should
be equal to or greater than 10 percent. When the
protected soil has more than 5 percent but less
than 85 percent passing the No. 200 sieve, the
POA should be equal to or greater than 4 percent.

3-6. Other Filter Considerations

To prevent clogging or blinding of the geotex-

tile, intimate contact between the soil and geotex-
tile should be assured during construction. Voids
between the soil and geotextile can expose the

geotextile to a slurry or muddy water mixture
during seepage. This condition promotes erosion of
soil behind the geotextile and clogging of the
geotextile.

Very fine-grained noncohesive soils, such as

rock flour, present a special problem, and design of

drain installations in this type of soil should be
based on tests with expected hydraulic conditions
using the soil and candidate geotextiles.

As a general rule slit-film geotextiles are

unacceptable for drainage applications. They may
meet AOS criteria but generally have a very low

POA or permeability. The wide filament in many
slit films is prone to move relative to the cross
filaments during handling and thus change AOS
and POA.

The designer must consider that in certain

areas an ochre formation may occur on the geotex-
tile. Ochre is an iron deposit usually a red or tan
gelatinous mass associated with bacterial slimes.

It can, under certain conditions, form on and in
subsurface drains. The designer may be able to
determine the potential for ochre formation by
reviewing local experience with highway, agricul-

tural, embankment, or other drains with local or

state agencies. If there is reasonable expectation
for ochre formation, use of geotextiles is discour.

aged since geotextiles may be more prone to clog
Once ochre clogging occurs, removal from geotex-

tiles is generally very difficult to impossible, since
chemicals or acids used for ochre removal car

3-3

TM 5-818-8/AFJMAN 32-1030

damage geotextiles, and high pressure jetting
through the perforated pipe is relatively ineffec-
tive on clogged geotextiles.

3-7. Strength Requirements

Unless geotextiles used in drainage applications
have secondary functions (separation, reinforce-
ment, etc.) requiring high strength, the require-
ments shown in table 3-2 will provide adequate

strength.

Table 3-2. Geotextile Strength Requirements for Drains.

Strength Type

Test Method

Class A

Class B

Grab Tensile

ASTM D 4632

180

Seam

ASTM D 4632

160

Puncture

ASTM D 4833

Burst

ASTM D 3786

290

130

Trapezoid Tear ASTM D 4533

Class A Drainage applications are for geotextile installation

where applied stresses are more severe than Class B applica-
tions; i.e., very coarse shape angular aggregate is used, compac-
tion is greater than 95 percent of ASTM D 1557 of maximum
density or depth of trench is greater than 10 feet.

Class B Drainage applications are for geotextile installations

where applied stresses are less severe than Class A applica-
tions; i.e., smooth graded surfaces having no sharp angular
projections, and no sharp angular aggregate, compaction is less
than or equal to 95 percent of ASTM D 1557 maximum density.

3-8. Design and Construction Considerations

a. Installation Factors.

In addition to the re-

quirement for continuous, intimate geotextile con-

tact with the soil, several other installation factors

strongly influence geotextile drain performance.

These include:

(1) How the geotextile is held in place during

construction.

(2) Method of joining consecutive geotextile

elements.

(3) Preventing geotextile contamination.
(4) Preventing geotextile deterioration from

exposure to sunlight. Geotextile should retain 70
percent of its strength after 150 hours of exposure

to ultraviolet sunlight (ASTM D 4355).

b. Placement.

Pinning the geotextile with long

nail-like pins placed through the geotextile into
the soil has been a common method of securing the
geotextile until the other components of the drain

have been placed; however, in some applications,

this method has created problems. Placement of
aggregate on the pinned geotextile normally puts
the geotextile into tension which increases poten-

tial for puncture and reduces contact of the geotex-
tile with soil, particularly when placing the geo-
textile against vertical and/or irregular soil

surfaces. It is much better to keep the geotextile
loose but relatively unwrinkled during aggregate

placement. This can be done by using small
amounts of aggregate to hold the geotextile in
place or using loose pinning and repinning as

necessary to keep the geotextile loose. This method
of placement will typically require 10 to 15 per-
cent more geotextile than predicted by measure-
ment of the drain’s planer surfaces.

c. Joints.

(1) Secure lapping or joining of consecutive

pieces of geotextile prevents movement of soil into
the drain. A variety of methods such as sewing,
heat bonding, and overlapping are acceptable

joints. Normally, where the geotextile joint will

not be stressed after installation, a minimum

12-inch overlap is required with the overlapping

inspected to ensure complete geotextile-to-geo-

textile contact. When movement of the geotextile
sections is possible after placement, appropriate
overlap distances or more secure joining methods
should be specified. Field joints are much more
difficult to control than those made at the factory
or fabrication site and every effort should be made
to minimize field joining.

(2) Seams are described in chapter 1. Strength

requirements for seams may vary from just
enough to hold the geotextile sections together for
installation to that required for the geotextile.
Additional guidance for seams is contained in
AASHTO M 288. Seam strength is determined
using ASTM 4632.

d. Trench Drains.

(1) Variations of the basic trench drain are

the most common geotextile drain application.

Typically, the geotextile lines the trench allowing
use of a very permeable backfill which quickly

removes water entering the drain. Trench drains
intercept surface infiltration in pavements and
seepage in slopes and embankments as well as

lowering ground-water levels beneath pavements
and other structures. The normal construction
sequence is shown in figure 3-1. In addition to
techniques shown in figure 3-1, if high compactive
efforts are required (e.g., 95 percent of ASTM D

1557 maximum density), the puncture strength

requirements should be doubled. Granular backfill
does not have to meet piping criteria but should be
highly permeable, large enough to prevent move-
ment into the pipe, and meet durability and

structural requirements of the project. This allows
the designer to be much less stringent on backfill

requirements than would be necessary for a totally
granular trench drain. Some compaction of the
backfill should always be applied.

(2) Wrapping of the perforated drain pipe with

a geotextile when finer grained filter backfill is
used is a less common practice. Normally not used

3-4

TM 5-818-8/AFJMAN 32-1030

CHAPTER 4

GEOTEXTILE REINFORCED EMBANKMENT ON SOFT FOUNDATION

4-1. General

Quite often, conventional construction techniques
will not allow dikes or levees to be constructed on
very soft foundations because it may not be cost
effective, operationally practical, or technically
feasible. Nevertheless, geotextile-reinforced dikes
have been designed and constructed by being made
to float on very soft foundations. Geotextiles used
in those dikes alleviated many soft-ground founda-

tion dike construction problems because they per-
mit better equipment mobility, allow expedient
construction, and allow construction to design ele-
vation without failure. This chapter will address
the potential failure modes and requirements for

design and selection of geotextiles for reinforced
embankments.

4-2. Potential Embankment Failure Modes

The design and construction of geotextile-rein-
forced dikes on soft foundations are technically
feasible, operationally practical, and cost effective

when compared with conventional soft foundation
construction methods and techniques. To success-

fully design a dike on a very soft foundation, three
potential failure modes must be investigated (fig
4-1).

a. Horizontal sliding, and spreading of the em-

bankment and foundation.

b. Rotational slope an&or foundation failure.

c. Excessive vertical foundation displacement.

The geotextile must resist the unbalanced forces
necessary for dike stability and must develop

moderate-to-high tensile forces at relatively low-to-
moderate strains. It must exhibit enough soil-
fabric resistance to prevent pullout. The geotextile

tensile forces resist the unbalanced forces, and its
tensile modulus controls the vertical and horizon-
tal displacement of dike and foundation. Adequate
development of soil-geotextile friction allows the
transfer of dike load to the geotextile. Developing

geotextile tensile stresses during construction at

small material elongations or strains is essential.

d. Horizontal Sliding and Spreading.

These

types of failure of the dike and/or foundation may
result from excessive lateral earth pressure (fig

4-1

). These forces are determined from the dike

height, slopes, and fill material properties. During

conventional construction the dikes would resist
these modes of failure through shear forces devel-
oped along the dike-foundation interface. Where

geotextiles are used between the soft foundation

and the dike, the geotextile will increase the
resisting forces of the foundation. Geotextile-
reinforced dikes may fail by fill material sliding
off the geotextile surface, geotextile tensile failure,
or excessive geotextile elongation. These failures
can be prevented by specifying the geotextiles that
meet the required tensile strength, tensile modu-
lus, and soil-geotextile friction properties.

e. Rotational Slope and/or Foundation Failure.

Geotextile-reinforced dikes constructed to a given
height and side slope will resist classic rotational

failure if the foundation and dike shear strengths
plus the geotextile tensile strength are adequate
(fig 4-lb). The rotational failure mode of the dike
can only occur through the foundation layer and

geotextile. For cohesionless fill materials, the dike
side slopes are less than the internal angle of

friction. Since the geotextile does not have flexural

strength, it must be placed such that the critical
arc determined from a conventional slope stability

analysis intercepts the horizontal layer. Dikes
constructed on very soft foundations will require a
high tensile strength geotextile to control the
large unbalanced rotational moments.

f. Excessive Vertical Foundation Displacements.

Consolidation settlements of dike foundations,

whether geotextile-reinforced or not, will be simi-
lar. Consolidation of geotextile-reinforced dikes
usually results in more uniform settlements than
for non-reinforced dikes. Classic consolidation

analysis is a well-known theory, and foundation
consolidation analysis for geotextile-reinforced
dikes seems to agree with predicted classical con-

solidation values. Soft foundations may fail par-
tially or totally in bearing capacity before classic

foundation consolidation can occur. One purpose of
geotextile reinforcement is to hold the dike to-
gether until foundation consolidation and strength
increase can occur. Generally, only two types of
foundation bearing capacity failures may occur-
partial or center-section foundation failure and

rotational slope stability/foundation stability. Par-
tial bearing failure, or “center sag” along the dike

alignment (fig 4-1

), may be caused by improper

construction procedure, like working in the center

of the dike before the geotextile edges are covered
with fill materials to provide anchorage. If this

procedure is used, geotextile tensile forces are not
developed and no benefit is gained from the geo-
textile used. A foundation bearing capacity failure

may occur as in conventional dike construction.

4-1

TM 5-818-8/AFJMAN 32-1030

Center sag failure may also occur when low-tensile
strength or low-modulus geotextiles are used, and
embankment spreading occurs before adequate

geotextile stresses can be developed to carry the
dike weight and reduce the stresses on the founda-

tion. If the foundation capacity is exceeded, then
the geotextile must elongate to develop the re-
quired geotextile stress to support the dike weight.
Foundation bearing-capacity deformation will oc-
cur until either the geotextile fails in tension or
carries the excess load. Low modulus geotextiles

generally fail because of excessive foundation dis-

placement that causes these low tensile strength
geotextiles to elongate beyond their ultimate

strength. High modulus geotextiles may also fail if
their strength is insufficient. This type of failure
may occur where very steep dikes are constructed,

and where outside edge anchorage is insufficient.

4-3. Recommended Criteria

The limit equilibrium analysis is recommended for

design of geotextile-reinforced embankments.
These design procedures are quite similar to con-
ventional bearing capacity or slope stability analy-
sis. Even though the rotational stability analysis
assumes that ultimate tensile strength will occur
instantly to resist the active moment, some geotex-
tile strain, and consequently embankment dis-
placement, will be necessary to develop tensile

stress in the geotextile. The amount of movement

within the embankment may be limited by the use
of high tensile modulus geotextiles that exhibit
good soil-geotextile frictional properties. Conven-

tional slope stability analysis assumes that the

geotextile reinforcement acts as a horizontal force
to increase the resisting moment. The following
analytical procedures should be conducted for the

design of a geotextile-reinforced embankment: (1)
overall bearing capacity, (2) edge bearing capacity

or slope stability, (3) sliding wedge analysis for
embankment spreading/splitting, (4) analysis to
limit geotextile deformation, and (5) determine
geotextile strength in a direction transverse to the
longitudinal axis of the embankment or the longi-
tudinal direction of the geotextile. In addition,

embankment settlements and creep must also be
considered in the overall analysis.

a Overall Bearing Capacity.

The overall bearing

capacity of an embankment must be determined
whether or not geotextile reinforcement is used. If

the overall stability of the embankment is not

satisfied, then there is no point in reinforcing the

embankment. Several bearing capacity procedures
are given in standard foundation engineering text-

books. Bearing capacity analyses follow classical
limiting equilibrium analysis for strip footings,

using assumed logarithmic spiral or circular fail-
ure surfaces. Another bearing capacity failure is
the possibility of lateral squeeze (plastic flow) of
the underlying soils. Therefore, the lateral stress
and corresponding shear forces developed under
the embankment should be compared with the
sum of the resisting passive forces and the product
of the shear strength of the soil failure plane area.
If the overall bearing capacity analysis indicates
an unsafe condition, stability can be improved by
adding berms or by extending the base of the
embankment to provide a wide mat, thus spread-
ing the load to a greater area. These berms or
mats may be reinforced by properly designing
geotextiles to maintain continuity within the em-

bankment to reduce the risk of lateral spreading.
Wick drains may be used in case of low bearing
capacity to consolidate the soil rapidly and achieve
the desired strength. The construction time may
be expedited by using geotextile reinforcement.

b. Slope Stability Analysis.

If the overall bear-

ing capacity of the embankment is determined to
be satisfactory, then the rotational failure poten-
tial should be evaluated with conventional limit
equilibrium slope stability analysis or wedge anal-

ysis. The potential failure mode for a circular arc

analysis is shown in figure 4-2. The circular arc
method simply adds the strength of the geotextile
layers to the resistance forces opposing rotational
sliding because the geotextile must be physically
torn for the embankment to slide. This analysis
consists of determining the most critical failure
surfaces, then adding one or more layers of geotex-

tile at the base of the embankment with sufficient
strength at acceptable strain levels to provide the
necessary resistance to prevent failure at an ac-

ceptable factor of safety. Depending on the nature
of the problem, a wedge-type slope stability analy-

sis may be more appropriate. The analysis may be
conducted by accepted wedge stability methods,
where the geotextile is assumed to provide hori-
zontal resistance to outward wedge sliding and

solving for the tensile strength necessary to give
the desired factor of safety. The critical slip circle
or potential failure surfaces can be determined by
conventional geotechnical limited equilibrium
analysis methods. These methods may be simpli-
fied by the following assumptions:

(1) Soil shear strength and geotextile tensile

strength are mobilized simultaneously.

(2) Because of possible tensile crack forma-

tions in a cohesionless embankment along the
critical slip surface, any shear strength developed

by the embankment (above the geotextile) should
be neglected.

4-3

TM 5-818-8/AFJMAN 32-1030

Figure 4-2. Concept Used for Determining Geotextile Tensile Strength Necessary to Prevent Slope Failure.

(3) The conventional assumption is that criti-

cal slip circles will be the same for both the
geotextile-reinforced and nonreinforced embank-
ments although theoretically they may be differ-
ent. Under these conditions, a stability analysis is
performed for the no-geotextile condition, and a
critical slip circle and minimum factor of safety is
obtained. A driving moment or active moment
(AM) and soil resistance moment (RM) are deter-
mined for each of the critical circles. If the factor
of safety (FS) without geotextile is inadequate,
then an additional reinforcement resistance mo-

ment can be computed from the following equa-
tion:

TR + RM/FS = AM

where

(eq 4-1)

T = geotextile tensile strength

R = radius of critical slip circle

RM = soil resistance moment

FS = factor of safety

AM = driving or active moment

This equation can be solved for T so that the
geotextile reinforcement can also be determined to
provide the necessary resisting moment and re-

quired FS.

c. Sliding Wedge Analysis.

The

forces

involved

in an analysis for embankment sliding are shown

in figure 4-3. These forces consist of an actuating
force composed of lateral earth pressure and a
resisting force created by frictional resistance be-
tween the embankment fill and geotextile. To
provide the adequate resistance to sliding failure,
the embankment side slopes may have to be
adjusted, and a proper value of soil-geotextile

friction needs to be selected. Lateral earth pres-
sures are maximum beneath the embankment
crest. The resultant of the active earth pressure

per unit length (P

) for the given cross section

may be calculated as follows:

(eq 4-2)

where

= embankment fill compacted density-force

per length cubed

H = maximum embankment height

= coefficient of active earth pressure (di-

mensionless)

For a cohesionless embankment fill, the equation
becomes:

(eq 4-3)

Resistance to sliding may be calculated per unit

length of embankment as follows:

(eq 4-4

4-4

TM 5-818-8/AFJMAN 32-1030

with the soil and geotextile chosen, then the
embankment side slopes must be flattened, or
additional berms may be considered. Most high-

strength geotextiles exhibit a fairly high soil-
geotextile friction angle that is equal to or greater
than 30 degrees, where loose sand-size fill material
is utilized. Assuming that the embankment sliding
analysis results in the selection of a geotextile
that prevents embankment fill material from slid-
ing along the geotextile interface, then the result-
ant force because of lateral earth pressure must be
less than the tensile strength at the working load
of the geotextile reinforcement to prevent spread-

ing or tearing. For an FS of 1, the tensile strength

would be equal to the resultant of the active earth
pressure per unit length of embankment. A mini-
mum FS of 1.5 should be used for the geotextile to
prevent embankment sliding. Therefore, the mini-
mum required tensile strength to prevent sliding
is:

as the average strain, then the maximum strain
which would occur is 5 percent.

e. Potential Embankment Rotational Displace-

ment.

It is assumed that the geotextile ultimate

tensile resistance is instantaneously developed to
prevent rotational slope/foundation failure and is

inherently included in the slope stability limit
equilibrium analysis. But for the geotextile to
develop tensile resistance, the geotextile must
strain in the vicinity of the potential failure plane.
To prevent excessive rotational displacement, a

high-tensile-modulus geotextile should be used.

The minimum required geotextile tensile modulus
to limit or control incipient rotational displace-

ment is the same as for preventing spreading

failure.

= 1.5 P

(eq 4-6)

where T

= minimum geotextile tensile strength.

d. Embankment Spreading Failure Analysis.

Geotextile tensile forces necessary to prevent lat-
eral spreading failure are not developed without
some geotextile strain in the lateral direction of

the embankment. Consequently, some lateral

movement of the embankment must be expected,
Figure 4-3 shows the geotextile strain distribution

that will occur from incipient embankment spread-

ing if it is assumed that strain in the embankment

varies linearly from zero at the embankment toe
to a maximum value beneath embankment crest.
Therefore, an FS of 1.5 is recommended in deter-

mining the minimum required geotextile tensile
modulus. If the geotextile tensile strength (T

)

determined by equation 4-6 is used to determine
the required tensile modulus (E

), an FS of 1.5

will be automatically taken into account, and the

minimum required geotextile tensile modulus may
be calculated as follows:

(eq 4-7)

f. Longitudinal Geotextile Strength Require-

ments.

Geotextile strength requirements must be

evaluated and specified for both the transverse
and longitudinal direction of the embankment.

Stresses in the warp direction of the geotextile or

longitudinal direction of the embankment result
from foundation movement where soils are very

soft and create wave or a mud flow that drags on
the underside of the geotextile. The mud wave not
only drags the geotextile in a longitudinal direc-

tion but also in a lateral direction toward the

embankment toes. By knowing the shear strength
of the mud wave and the length along which it

drags against the underneath portion of the geo-
textile, then the spreading force induced can be
calculated. Forces induced during construction in
the longitudinal direction of the embankment may

result from the lateral earth pressure of the fill
being placed. These loads can be determined by

the methods described earlier where T

= 1.5 P

and E

= 20 T

at 5 percent strain. The geotextile

strength required to support the height of the

embankment in the direction of construction must
also be evaluated. The maximum load during
construction includes the height or thickness of

the working table, the maximum height of soil and
the equipment live and dead loads. The geotextile

strength requirements for these construction loads

must be evaluated using the survivability criteria
discussed previously.

where

= maximum strain which the geotex-

g. Embankment Deformation.

One of the pri-

tile is permitted to undergo at the embankment

mary purposes of geotextile reinforcement in an

center line. The maximum geotextile strain is

embankment is to reduce the vertical and horizon-

equal to twice the average strain over the embank-

tal deformations. The effect of this reinforcement

ment width. A reasonable average strain value of

on horizontal movement in the embankment

2.5 percent for lateral spreading is satisfactory

spreading modes has been addressed previously.

from a construction and geotextile property stand-

One of the more difficult tasks is to estimate the

point. This value should be used in design but

deformation or subsidence caused by consolidation

depending on the specific project requirements

and by plastic flow or creep of very soft foundation

larger strains may be specified. Using 2.5 percent

materials. Elastic deformations are a function of

4-6

TM 5-818-8/AFJMAN 32-1030

the subgrade modulus. The presence of a geotextile
increases the overall modulus of the reinforced
embankment. Since the lateral movement is mini-

mized by the geotextile, the applied loads to the
soft foundation materials are similar to the ap-
plied loads in a laboratory consolidation test.
Therefore, for long-term consolidation settlements
beneath geotextile-reinforced embankments, the

compressibility characteristics of the foundation
soils should not be altered by the presence of the

reinforcement. A slight reduction in total settle-
ment may occur for a reinforced embankment but
no significant improvement. Other studies indicate
that very high-strength, high-tensile modulus geo-
textiles can control foundation displacement dur-
ing construction, but the methods of analysis are
not as well established as those for stability

analysis. Therefore, if the embankment is designed
for stability as outlined previously, then the lat-
eral and vertical movements caused by subsidence

NOTE:

NATURAL GROUND SURFACE COVERED
WITH GRASS AND VOID OF OTHER THAN
SMALL DEBRIS, HUMPS, DEPRESSIONS,
ETC. MAY OR MAY NOT HAVE A CRUST

from consolidation settlements, plastic creep, and
flow of the soft foundation materials will be
minimized. It is recommended that a conventional
consolidation analysis be performed to determine
foundation settlements.

4-4. Example Geotextile-Reinforced Embank-

ment Design

a. The Assumption.

(1) An embankment, fill material consisting of

clean sand with

= 100 pounds per cubic foot,

and

= 30 degrees (where

is the angle of

internal friction).

(2) Foundation properties (unconsolidated, un-

determined shear strength) as shown in figure 4-4
(water table at surface).

(3) Embankment dimensions (fig 4-4).

(a)

Crest width of 12 feet.

(b)

Embankment height (H) of 7 feet.

(c)

Embankment slope, 10 Horizontal on 1

Vertical (i.e., x = 10).

Figure 4-4. Embankment Section and Foundation Conditions of Embankment Design Example Problem.

4-7

TM 5-818-8/AFJMAN 32-1030

b. Factor of Safety.

This design example will

consider an FS of 1.3 against rotational slope
failure, 1.5 against spreading, 2.0 against sliding
failure, and 1.3 against excessive rotational dis-
placement for the geotextile fabric requirements.
Determine minimum geotextile requirements.

c. Calculate Overall Bearing Capacity.

(1) Ultimate bearing capacity qult for strip

footing on clay.

ult

= cN

= (75)(5.14) = 385 pounds per

square foot (with
surface crust)

ult

= cN

= (75)(3.5) = 263 pounds per

square foot (without
surface crust)

Values shown for N

are standard values for

0. It has been found from experience that excessive
mud wave formation is minimized when a dried
crust has formed on the ground surface.

(2)

Applied stress.

(3)

Determine FS.

The bearing capacity was

not sufficient for an unreinforced embankment,
but for a geotextile-reinforced embankment, the
lower portion of its base will act like a mat
foundation, thus distributing the load uniformly
over the entire embankment width. Then, the
average vertical applied stress is:

where L = width of embankment slope. If a dried
crust is available on the soft foundation surface,

then the FS is about 1. If no surface crust is
available, the FS is less than 1.0, and the embank-

ment slopes or crest height would have to be
modified. Since the embankment is very wide and
the soft clay layer is located at a shallow depth,

failure is not likely because the bearing-capacity
analysis assumes a uniform soil twice the depth of
the embankment width.

4-5. Bearing-Capacity Consideration

A second bearing-capacity consideration is
chance of soft foundation material squeezing

t h e

out.

Therefore, the lateral stress and corresponding
shear forces below the embankment, with respect
to resisting passive forces and shear strength of
soil, are determined.

Plastic flow method for overall squeeze-

squeeze between two plates.

(eq 4-8)

where

c = cohesion (shear strength) of soil

next higher strength foundation soil layer

L = width of embankment slope

For the conditions in previous example:

Cohesion available is 75 pounds per square foot,

which is greater than 32.2 pounds per square foot
required and is therefore satisfactory.

Toe squeeze of soft foundation materials is a

common problem that requires investigating.
Therefore, the passive resistance for toe squeeze is
as follows:

(eq 4-9)

Then, the difference:

(eq 4-10)

(eq 4-11)

(eq 4-12)

For the example:

is greater than P

; therefore, foundation

squeeze may occur. Solutions would be to either
allow squeezing to occur or construct shallow

berms to stabilize the embankment toe or use
plastic strip drains.

c. Slope Stability Analysis.

Perform a slope sta-

bility analysis to determine the required geotextile
tensile strength and modulus to provide an FS of

4 - 8

TM 5-818--8/AFJMAN 32-1030

1.3 against rotational slope failure. There are

many slope stability procedures available in the
literature for determining the required tensile

strength T . Computer programs are also available
that will determine the critical slip surface with a
search routine. Assume that an analysis was
conducted on the example embankment and an
active moment of 840,000 foot-pounds per foot of
width was calculated and a resisting moment of
820,000 foot-pounds per foot of width calculated for
a slip circle having a radius of 75 feet. This would

result in a safety factor of 0.98 which is not
satisfactory. Using equation 4-1, the tensile
strength of a geotextile necessary to provide an FS
of 1.3 can be calculated as follows:

= 2,800 pounds per

foot of width

d. Pullout Resistance.

Pullout resistance of the

geotextile from the intersection of the potential
failure plane surface is determined by calculating
the resistance and necessary geotextile embedment
length. There are two components to geotextile
pullout resistance-one below and one above the
geotextile. Resistance below the geotextile in this
example is 50 pounds per square foot, and resis-
tance above the geotextile is determined by the
average height of fill above the geotextile in the
affected areas. In this example, the resistance
above and below the geotextile is determined as

follows:

where

(eq 4-13)

= moist weight of sand fill, 100 pounds per

cubic foot

h = average height of sand fill above geotex-

tile in the affected area, 6.5 feet

= sand-geotextile friction equal to

= remolded strength of foundation clay

soil beneath the geotextile, 50 pounds
per square foot

The required pullout length is determined from
the ultimate tensile strength requirement of 2,800

pounds per foot width. Therefore,

L = 9.8 ft; approximately

10 ft

e. Prevention of Sliding.

Calculate

to pro-

vide an FS of 2 against sliding failure across the
geotextile.

(1) Calculate lateral earth pressure, P

;

817

(2) Calculate

FS = Resisting Force

Active Force

where X = ratio of the vertical and horizonal slope
(i.e., 10 horizontal to 1 vertical).

f. Prevention of Geotextile Splitting.

Calculate

required geotextile tensile strength (T

) to provide

an FS of 1.5 against splitting.

FS = 1.5 against splitting
P

= 817 pounds per foot width

Calculate To :

1.5

(1.5)(817)

= 1,226 pounds per foot width or

= 102 pounds per inch width

g. Limiting Spreading and Rotation.

Calculate

the tensile modulus E

required to limit embank-

ment average spreading and rotation to 5 percent
geotextile elongation.

(1) Spreading analysis:

4-9

TM 5-818-8/AFJMAN 32-1030

(20)(102)

2,040

pounds per inch width

(2) Rotational slope stability analysis:

= 20

(20)

(T =

233

pounds per inch width)

G R

4,670

pounds per inch width

h. Tensile Seam Strength and Fill Require-

ments.

Determine geotextile tensile strength re-

quirements in geotextile fill (cross machine direc-
t i o n ) a n d a c r o s s s e a m s . T e n s i l e s t r e n g t h

requirement in this direction depends on the

amount of squeezing out and dragging loads on the
underside of the geotextile and the amount of
shoving or sliding that the 2 to 3 feet of sand fill
material causes during initial placement. If three
panels 16 feet wide are in place and the founda-

tion material moves longitudinally along the em-
bankment alignment because of construction activ-
ities when establishing a working platform, then

the loads in the geotextile fill direction can be
calculated as follows:

(1) Geotextile fill and seam tensile strength

requirement:

GRF

panels)(

feet wide) C

where

GRF

= (3)(16 feet)(50 pounds per square

foot)

GRF

= 2,400 pounds per foot width

GRF

= 200 pounds per inch width

G R F

at FS of 1.5 = 300 pounds per inch

width

= remolded shear strength of foundation

materials

(2) Geotextile fill and seam tensile modulus of

10 percent elongation:

i. Summary of Minimum Geotextile Require-

ments.

If the geotextile chosen is a woven polyes-

ter yarn and only 50 percent of the ultimate
geotextile load is used, then the minimum ulti-
mate strength is 2 times the required working
tensile strength 233, or 466 pounds per inch width
to compensate for possible creep.

(1) Soil-geotextile friction angle,

equals

3.9 degrees.

(2) Ultimate tensile strength T

ULT

in the geo-

textile warp directions working tensile strength

equals 466 pounds per inch width.

(3) Ultimate tensile strength T

GRF

in the geo-

textile fill and cross seams directions equals 300

pounds per inch width.

(4) Tensile modulus (slope of line drawn

through zero load and strain and through load at 5
percent elongation) at 5 percent geotextile elonga-

tion in geotextile warp direction is 4,670 pounds
per inch width, (based on working tensile strength)

and 10 percent geotextile elongation in the fill and

cross seam directions is 3,000 pounds per inch
width.

(5) Contractor survivability and constructabil-

ity requirements are included in tables 2-3, 2-4,
and 2-5. Geotextile specifications must meet or

exceed these requirements.

4-10

TM 5-818-8/AFJMAN 32-1030

CHAPTER 5

RAILROAD TRACK CONSTRUCTION AND REHABILITATION

5-1. General

The use of geotextiles in a railroad track structure

is dependent upon many factors including the
traffic, track structure, subgrade conditions, drain-
age conditions, and maintenance requirements” In
railroad applications, geotextiles are primarily
used to perform the functions of separation, filtra-
tion, and lateral drainage. Based on current

knowledge, little is known of any reinforcement
effect geotextiles have on soft subgrades under
railroad track. Therefore, geotextiles should not be
used to reduce the ballast or subballast design
thickness. Geotextiles have found their greatest
railroad use in those areas where a large amount
of track maintenance has been required on an
existing right-of-way as a result of poor drainage

conditions, soft conditions, and/or high-impact
loadings. Geotextiles are normally placed between
the subgrade and ballast layer or between the
subgrade and subballast layers if one is present. A

common geotextile application is found in what is
commonly known as “pumping track” and “ballast
pocket areas.” Both are associated with fine-
grained subgrade soil and difficult drainage condi-
tions. Under traffic, transient vertical stresses are

sufficient to cause the subgrade and ballast or
subballast materials to intermix if the subgrade is
weak (i.e. wet). As the intermixing continues, the
ballast becomes fouled by excessive fines contami-

nation, and a loss of free drainage through the
ballast occurs as well as a loss of shear strength.

The ballast is pulled down into the subgrade. As
this process continues, ballast is forced deeper and
deeper into the subgrade, forming a pocket of
fouled and ineffective ballast and loss of track

grade control. Ballast pockets tend to collect wa-
ter, further reducing the strength of the roadbed
around them and result in continual track mainte-

nance problems. Installation of geotextiles during
rehabilitation of these areas provides separation,

filtration, and drainage functions and can prevent
the reoccurrence of pumping track. Common loca-
tions for the installation of a geotextile in railroad
track are locations of excessive track maintenance
resulting from poor subgrade/drainage conditions,
highway-railroad grade crossing, diamonds (rail-

road crossings), turnouts, and bridge approaches. If

a geotextile is installed in track without provisions
made for adequate drainage, water will be re-
tained in the track structure and the instability of

the track will be worsened. In any track construc-
tion or rehabilitation project, adequate drainage
must be incorporated in the project design.

5-2. Material Selection

Based on current knowledge, woven geotex-

tiles are not recommended for use in railroad track
applications. Test installations have shown that

woven geotextiles tend to clog with time and act
almost as a plastic sheet preventing water from
draining out of the subgrade.

Geotextiles selected for use in the track struc-

ture of military railroads should be nonwoven,

needle-punched materials that meet the require-

ments listed in table 5-1.

ASTM D 4886 is used to measure the abra-

sion resistance of a geotextile for use in a railroad
application. Indications are that abrasion is
greater for geotextiles placed during track rehabil-
itations where the rail remains in-place than for
geotextiles placed during new construction or reha-

bilitations where the existing rail, ties, and ballast

are removed and the subgrade reworked. This may
be due to the differences in the surface upon which

the geotextile is placed. In new construction the
subgrade surface is normally graded, compacted

and free from large stone. During in-place rehabil-

itations the old ballast may be removed by under-
cutting or ploughing which leave ballast particles

loose on, or protruding from, the surface, creating
a rough surface for placement of the geotextile.

5-3. Application

Geotextiles should be used to separate the ballast
or subballast from the subgrade (or ballast from
subballast) in a railroad track in cut sections
where the subgrade soil contains more than 25
percent by weight of particles passing the No. 200

sieve. Geotextiles are also used in embankment

sections consisting of such material where there is
less than 4 feet from the bottom of the tie to the

ditch invert or original ground surface.

5-4. Depth of Placement

Technical Manual TM 5-850- 2/AFM 88-7, chap.

2 specifies a minimum ballast thickness of 12

inches. An additional minimum of 6 inches of

subballast may be used in areas where drainage is
difficult. The actual total ballast/ subballast thick-
ness required is a function of the maximum wheel

load, rail weight, size, tie spacing, and allowable

5-1

TM 5-818-8/AFJMAN 32-1030

Property

(1)

Weight

ounce per square yard

Structure

Grab tensile strength, pounds

Elongation at failure, percent

Burst strength, pounds per

square inch

Puncture strength, pounds

Trapezoidal tear strength, pounds

Apparent opening size (AOS),

millimeter

Normal permeability, (k)

, centimeters

per second

Permittivity, seconds

-1

Planar water flow/transmissivity

square feet per minute x 10

-3

Ultraviolet degradation at 150 hours

percent strength retained

Seam strength, pounds

Minimum Requirement

Needle-punched nonwoven

350

620

185

150

<0.22

(No.

70 sieve)

0.1

0.2

350

Test Method

(3)

ASTM D 3776

option B

ASTM D 4632

ASTM D 3786

ASTM D 4833

ASTM D 4533

ASTM D 4751

ASTM D 4491

ASTM D 4716

ASTM D 4355

ASTM D 1683

Value in weaker principal direction.

All numerical values represent minimum

average roll value.

The minimum weight listed herein is based on the experience that geotextiles

with weights less than 15 oz/yd tend to show greater abrasion and wear than do
heavier weight materials.

It is recommended that the selection of geotextile

be based on the minimum physical property requirements of this table and not

solely on weight.

The k of the geotextile should be at least five times greater than the k

value of the soil.

Planar water flow/transmissivity determined at normal stress of 3.5 psi and

i = 1.0.

Seam strength applies to both field and manufactured seams, if geotextile is

seamed.

Table 5-1. Recommended Geotextile Property Requirements for Railroad Applications.

subgrade bearing pressure. In the design of new

5-5. Protective Sand Layer

track construction or track rehabilitation using

a Although not normally required, a a-inch-

geotextiles, the geotextile should be placed at the

thick layer placed over the geotextile may assist in

deeper of the following:

reducing the abrasion forces caused by the ballast

At least 12 inches below the cross tie.

as well as provide an additional filtration layer. In

At the bottom of the ballast layer in the case

track rehabilitation where undercutting or plow-

of rehabilitation by plowing.

ing type of ballast removal operation is used, there

may be many large aggregate pieces remaining on

At the bottom of the subballast in new con-

the surface of the subgrade prior to the placement

struction or rehabilitation where the track is

of the geotextile. A 2-inch-thick layer of sand

removed.

placed on the subgrade provides a smooth surface

5-2

TM 5-818-8/AFJMAN 32-1030

for the placement of the geotextile and protects the
geotextile from punctures and abrasion due to the
large aggregate pieces that are on the subgrade.

While the use of protective clean sand (less

than 5 percent passing the No. 200 sieve) extends

service life of a geotextile, there are also several
disadvantages. These disadvantages include the
extra cost of the sand, the increase in rail height
(which results from the extra thickness in the

track structure), and the difficulty and cost of

placing the sand layer during construction or
rehabilitation.

5-6. Drainage

Adequate drainage is the key to a stable railroad

track structure. During the design of a new track
or a track rehabilitation project, provisions for

improving both internal and external track drain-
age should be included. Drainage provisions that

should be considered include adequate (deep) side
ditches to handle surface runoff, sufficient crown
in both the subgrade and subballast layers to

prevent water from ponding on the top of the
subballast or subgrade, installation of perpendicu-

lar drains to prevent water accumulation in the
track, and French drains where required to assist
in the removal of water from the track structure.

During track rehabilitation, the creation of bath-

tub or canal effects should be avoided by having
the shoulders of the track below the level of the
ballast/geotextile/subgrade interface. Geotextiles

should not be placed in a railroad track structure
until existing drainage problems are corrected.
Proper maintenance of railroad drainage facilities

is described in TM 5-627.

5-7. Typical Sections

Figure 5-1 presents typical cross sections of the
railroad track structure showing the recommended

use of a geotextile in the track.

5-8. Special Applications

a. Installation of Geotextiles Below Natural

Ground Level.

In some locations, the elevation of

the track structure may be such that the geotex-
tile is placed below the level of the natural

ground. Where the natural ground surface is ele-
vated above the geotextile, steps should be taken
to prevent the inflow of water. A French drain
installed along the edge of the track and lined or

completely encapsulated in a geotextile to filter
the inflow of surface water may be used to direct
water away from the track structure. In extremely

flat areas it may be necessary to construct perpen-

dicular side ditches and soak-away pits from the

track structure to allow the water to drain out of
the French drains. Slotted drain pipes can be

placed in the trenches to facilitate movement of
the water from the track.

b. Highway Grade Crossings.

(1) Drainage in a grade crossing is generally

parallel to the rails until the pavement and road
shoulder have been cleared. Once clear of the
crossing itself, the drainage should be turned

perpendicular to the track and discharged away
from the track structure. A perforated drain pipe,
either wrapped with a geotextile during installa-
tion or prewrapped, may be placed in the trench to

assist the flow of water from within the crossing to
the ditches outside of the crossing area. Such

drainpipes should be placed in the trench with the
line of perforations facing downward. The ends of

the perforated drainpipes and the geotextile under
the crossing should be laid with sufficient fall
toward the side ditches to prevent water from

ponding in the crossing area. Whether perforated
pipes are used or not, the shoulders at the corner
of the crossing should be removed, and the ends of

the geotextile turned down so that the geotextile
facilitates drainage under gravity toward the side

ditches.

(2) In cold climates it is common to salt and

sand highways, including grade crossings, which
can lead to ballast fouling in the grade crossing.
One method of preventing or minimizing this

ballast fouling is to encapsulate the ballast in a

geotextile. The provision for drainage in this type

of installation would be the same as discussed
above.

c. Turnout Applications.

(1) The installation of a geotextile under a

turnout is basically the same as installation in
any other segment of track. In the vicinity of a

switch, drainage of ballast or subballast to ditches
is more difficult to achieve because horizontal
distances for subsurface flow are about doubled

and gradients are about halved. Thus, there are
reasons for using geotextiles to promote lateral
drainage under a turnout where none is used in

adjacent straight sections. If this is done, it should

extend at least 25 feet away from the turnout
itself to provide a transition section. As with road
crossings, particular attention should be given to

the removal of surface water from the turnout
area.

(2) Many geotextile manufacturers produce

specially packaged units ready-made for quick
application under turnouts varying from No. 8 to
No. 20.

d. Rail Crossings (Diamonds).

The use of a

geotextile in the track under a rail crossing is very

similar to the road crossing application. The de-
sign and installation process must provide ade-
quate drainage.

5-3

TM 5-818-8/AFJMAN 32-1030

CHAPTER 6

EROSION AND SEDIMENT CONTROL

6-1. Erosion Control

Erosion is caused by a group of physical and
chemical processes by which the soil or rock
material is loosened, detached, and transported
from one place to another by running water,
waves, wind, moving ice, or other geological sheet
and bank erosion agents. Clayey soils are less

erodible than fine sands and silts. See figure 6-1.
This chapter covers the use of geotextiles to
minimize erosion caused by water.

6-2. Bank Erosion

Riprap is used as a liner for ditches and channels

subjected to high-velocity flow and for lake, reser-
voir and channel banks subject to wave action.
Geotextiles are an effective and economical alter-
native to conventional graded filters under stone
riprap. However, for aesthetic or economic reasons,
articulated concrete mattresses, gabions, and pre-

cast cellular blocks have also been used to cover
the geotextile. The velocity of the current, the
height and frequency of waves and the erodibility
of the bank determine whether bank protection is
needed. The geotextiles used in bank protection
serve as a filter. Filter design is covered in chapter
3.

a. Special Design Considerations.

(1)

Durability.

The term includes chemical,

biological, thermal, and ultraviolet (UV) stability.
Streams and runoff may contain materials that
can be harmful to the geotextile. When protected
from prolonged exposure to UV light, the common

synthetic polymers do not deteriorate or rot in
prolonged contact with moisture. All geotextile
specifications must include a provision for covering

the geotextile to limit its UV radiation exposure to

30 days or less.

(2)

Strength and abrasion resistance.

The re-

quired properties will depend on the specific appli-
cation-the type of the cover material to be used

(riprap, sand bags, concrete blocks, etc.), the size,
weight, and shape of the armor stone, the han-
dling placement techniques (drop height), and the

severity of the conditions (stream velocity, wave
height, rapid changes of water level, etc.). Abra-
sion can result from movement of the cover mate-

rial as a result of wave action or currents.
Strength properties generally considered of pri-
mary importance are tensile strength, dimensional
stability, tearing, puncture, and burst resistance.

Table 6-1 gives recommended minimum strength
values.

(3)

Cover material.

The cover material (gravel,

rock fragments, riprap, armor stone, concrete
blocks, etc.) is a protective covering over the
geotextile that minimizes or dissipates the hydrau-
lic forces, protects the geotextile from extended
exposure to UV radiation, and keeps it in intimate
contact with the soil. The type, size, and weight of
cover material placed over the geotextile depends
on the kinetic energy of water. Cover material

that is lightweight in comparison with the hydrau-
lic forces acting on it may be moved. By removing
the weight holding the geotextile down, the
ground-water pressure may be able to separate the

geotextile from the soil. When no longer con-
strained, the soil erodes. The cover material must

be at least as permeable as the geotextile. If the

cover material is not permeable enough, a layer of
fine aggregate (sand, gravel, or crushed stone)

should be placed between it and the geotextile. An
important consideration in designing cover mate-

rial is to keep the void area between stones
relatively small. If the void area is excessively

large, soils may move from areas weighted by
stones to unweighted void areas between the

stones, causing the geotextile to balloon or eventu-
ally rupture. The solution in this case is to place a
graded layer of smaller stones below the large

stones that will prevent the soil from moving. A
layer of aggregate may also be needed if a major
part of the geotextile is covered as for example by

concrete blocks. The layer will act as a pore water
dissipator.

(4)

Anchorage.

At the toe of the streambank,

the geotextile and cover material should be placed
along the bank to an elevation below mean low
water level to minimize erosion at the toe. Place-

ment to a vertical distance of 3 feet below mean
low water level, or to the bottom of the streambed
for streams shallower than 3 feet, is recommended
At the top of the bank, the geotextile and cover

material should either be placed along the top of
the bank or with 2 feet vertical freeboard above
expected maximum water stage. If strong water

movements are expected, the geotextile needs to be
anchored at the crest and toe of the streambank
(fig 6-2).

(5) If the geotextile must be placed below low

water, a material of a density greater than that of
water should be selected.

6-1

TM 5-818-8/AFJMAN 32-1030

Table 6-1. Recommended Geotextile Mininmum Strength Re-

quirements.

Type Strength Test Method

Class A

Class B

Grab Tensile

ASTM D 4632

200

Elongation (%) ASTM D 4632

Puncture

ASTM D 4833

Tear

ASTM D 4533

Abrasion

ASTM D 3884

Seam

ASTM D 4632

180

Burst

ASTM D 3786

320

140

Fabrics are used under conditions more severe than Class B

such as drop height less than 3 feet and stone weights should
not exceed 250 pounds.

Fabric is protected by a sand cushion or by zero drop height.

b. Construction Considerations.

(1)

Site preparation.

The surface should be

cleared of vegetation, large stones, limbs, stumps,

trees, brush, roots, and other debris and then
graded to a relatively smooth plane free of obstruc-
tions, depressions, and soft pockets of materials.

(2)

Placement of geotextiles.

The geotextile is

unrolled directly on the smoothly graded soil

surface. It should not be left exposed to

deterioration for more than 1 week in case of
untreated geotextiles, and for more than 30 days

in case of

protected and low UV susceptible

polymer geotextiles. The geotextile should be
loosely laid, free of tension, folds, and wrinkles.
When used for streambank protection, where cur-
rents acting parallel to the bank are the principal
erosion forces, the geotextile should be placed with
the longer dimension (machine direction) in the
direction of anticipated water flow. The upper
strips of the geotextile should overlap the lower
strips (fig 6-3). When used for wave attack or cut
and fill slope protection, the geotextile should be
placed vertically down the slope (fig 6-3), and the
upslope strips should cover the downslope strips.
Stagger the overlaps at the ends of the strips at
least 5 feet. The geotextile should be anchored at
its terminal ends to prevent uplift or undermining.
For this purpose, key trenches and aprons are used
at the crest and toe of the slope.

(3)

Overlaps, seams, securing pins.

Adjacent

geotextile strips should have a minimum overlap
of 12 inches along the edges and at the end of
rolls. For underwater placement, minimum over-

lap should be 3 feet. Specific applications may
require additional overlaps. Sewing, stapling, heat

LIQUID LIMIT

EXPLANATION

SUGGESTED TREND OF EROSION CHARACTERISTICS

FOR FINE-GRAINED COHESIVE SOILS WITH

RESPECT TO PLASTICITY

Figure 6-1. Relationship between Atterberg Limits and Expected Erosion Potential.

6-2

Table 6-2. Pin Spacing Requirements in Erosion Control

Appli-

cations.

Slope

Pin Spacing

feet

Steeper than 1V on 3H

1V on 3H to 1V on 4H

Flatter than 1V on 4H

V = vertical; H = horizontal.

2
3
5

welding, or gluing adjacent panels, either in the
factory or on site, are preferred to lapping only.
Sewing has proved to be the most reliable method
of joining adjacent panels. It should be performed

using polyester, polypropylene, kevlar or nylon

thread. The seam strength for both factory and
field seams should not be less than 90 percent of
the required tensile strength of the unaged geotex-
tile in any principal direction. Geotextiles may be
held in place on the slope with securing pins prior

to placing the cover material. These pins with
washers should be inserted through both strips of
the overlapped geotextile along a line through the

midpoint of the overlap. The pin spacing, both
along the overlaps or seams, depends on the slope,
as specified in table 6-2. Steel securing pins, 3/16

inch in diameter, 18 inches long, pointed at one
end, and fitted with a 1.5-inch metal washer on
the other have performed well in rather firm soils.
Longer pins are advisable for use in loose soils.
The maximum slope on which geotextiles may be
placed will be determined by the friction angles

between the natural-ground and geotextile and
cover- material and geotextile. The maximum al-
lowable slope in no case can be greater than the

lowest friction angle between these two materials
and the geotextile.

TM 5-818-8/AFJMAN 32-1030

(4) Placement of cover material on geotextile.

For sloped surfaces, placement of the cover stone
or riprap should start from the base of the slope
moving upward and preferably from the center
outward to limit any partial movement of soil
because of sliding. In no case should drop heights
which damage the geotextile be permitted. Testing

may be necessary to establish an acceptable drop
height.

6-3. Precipitation Runoff Collection and Diver-

sion Ditches

A diversion ditch is an open, artificial, gravity
flow channel which intercepts and collects precipi-

tation runoff, diverts it away from vulnerable
areas, and directs it toward stabilized outlets. A

geotextile or revegetation mat can be used to line
the ditch. It will retard erosion in the ditch, while
allowing grass or other protective vegetation
growth to take place. The mat or geotextile can
serve as additional root anchoring for some time
after plant cover has established itself if UV
resistant geotextiles are specified. Some materials

used for this purpose are designed to degrade after
grass growth takes place. The geotextile can be

selected and specified using physical properties

indicated in table 6-1 and the filter criteria of
chapter 3. Figure 6-4 shows a typical example.

6-4. Miscellaneous Erosion Control

Figures 6-5 and 6-6 show examples of geotextile
applications in erosion control at drop inlets and

culvert outlets and scour protection around

bridges, piers, and abutments. Design criteria sim-

ilar to that used for bank protection should be
used for these applications.

Figure 6-2. Pin Spacing Requirements

Erosion Control Applications.

6-3

TM 5-818-8/AFJMAN 32-1030

a. ORIENTATION FOR CURRENT ACTING PARALLEL TO BANK

b. ORIENTATION FOR WAVE ATTACK NORMAL TO BANK

Figure 6-3. Geotextile Placement for Currents Acting Parallel to Bank or for Wave Attack on the Bank.

6-5. Sediment Control

Silt fences and silt curtains are sediment control
systems using geotextiles.

a. Silt Fence.

A silt fence is a temporary vertical

barrier composed of a sheet of geotextile supported
by fencing or simply by posts, as illustrated in
figure 6-5. The lower end of the geotextile is

buried in a trench cut into the ground so that
runoff will not flow beneath the fence. The purpose

of the permeable geotextile silt fence is to inter-
cept and detain sediment from unprotected areas
before it leaves the construction site. Silt fence are

sometimes located around the entire downslope

portion or perimeter of urban construction sites.
Short fences are often placed across small drainage
ditches (permanent or temporary) constructed on

the site. Both applications are intended to function
for one or two construction seasons or until grass
sod is established. The fence reduces water veloc-

ity allowing the sediment to settle out of suspen-
sion.

(1) Design concepts.

A silt fence consists of a

sheet of geotextile and a support component. The

support component may be a wire or plastic mesh
support fence attached to support posts or in some
cases may be support posts only. The designer has
to determine the minimum height of silt fences,
and consider the geotextile properties (tensile
strength, permeability) and external factors (the
slope of the surface, the volume of water and
suspended particles which are delivered to the silt

fence, and the size distribution of the suspended
particles). Referring to figure 6-7, the total height
of the silt fence must be greater than h

+ h

; where h

is the height of geotextile necessary to

allow water flowing into the basin to flow through

the geotextile, considering the permeability of the
geotextile; h

is the height of water necessary to

overcome the threshold gradient of the geotextile
and to initiate flow. For most expected conditions,
h

+ h

is about 6 inches or less. The silt fence

accomplishes its purpose by creating a pond of

relatively still water which serves as a sedimenta-
tion basin and collects the suspended solids from
the runoff. The useful life of the silt fence is the
time required to fill the triangular area of height

6-4

TM 5-818-8/AFJMAN 32-1030

CROSS SECTION OF GEOTEXTILE-LINED DITCH

CENTER LINE PROFILE OF GEOTEXTILE-LINED DITCH

Figure 6-4. Ditch Liners.

h (fig 6-7) behind the silt fence with sediment. The

height of the silt fence geotextile should not
exceed 3 feet.

(2)

Design for maximum particle retention.

Geotextiles selected for use in silt fences should
have an AOS that will satisfy the following equa-
tion with a limiting value equal to the No. 120

sieve size.

(eq 6-1)

A minimum of 90-pound tensile strength (ASTM D

4632 Grab Test Method) is recommended for use
with support posts spaced a maximum of 8 feet
apart.

(3)

Design for filtration efficiency.

The geotex-

tile should be capable of filtering most of the soil
particles carried in the runoff from a construction

site without unduly impeding the flow. ASTM D
5141 presents the laboratory test used to deter-
mine the filtering efficiency and the flow rate of

the sediment-filled water through the geotextile.

(4) Required geotextile properties.

The geotex-

tile used for silt fence must also have:

(a)

Reasonable puncture and tear resistance

to prevent damage by floating debris and to limit
tearing where attached to posts and fence.

(b)

Adequate resistance to UV deterioration

and biological, chemical, and thermal actions for
the desired life of the fence.

(5)

Construction considerations.

(a)

Silt fences should be constructed after

the cutting of trees but before having any sod

disturbing construction activity in the drainage
area.

(b)

It is a good practice to construct the silt

fence across a flat area in the form of a horseshoe.

This aids in the ponding of the runoff, and in-
creases the strength of the fence. Prefabricated silt

fence sections containing geotextile and support
posts are commercially available. They are gener-
ally manufactured in heights of 18 and 36 inches.

6-5

TM 5-818-8/AFJMAN 32-1030

CHAPTER 7

REINFORCED SOIL WALLS

7-1. Geotextile-Reinforced Soil Walls

Soil, especially granular, is relatively strong under
compressive stresses. When reinforced, significant
tensile stresses can be carried by the reinforce-

ment, resulting in a composite structure which
possesses wider margins of strength. This extra
strength means that steeper slopes can be built.

Geotextiles have been utilized in the construction
of reinforced soil walls since the early 1970’s.
Geotextile sheets are used to wrap compacted soil
in layers producing a stable composite structure.
Geotextile-reinforced soil walls somewhat resemble
the popular sandbag walls which have been used

for some decades. However, geotextile- reinforced
walls can be constructed to significant height
because of the geotextile’s higher strength and a

simple mechanized construction procedure.

7-2. Advantages of Geotextile-Reinforced
Walls

Some advantages of geotextile-reinforced walls

over conventional concrete walls are the following:

a. They are economical.

Construction usually is easy and rapid. It

does not require skilled labor or specialized equip-
ment. Many of the components are prefabricated

allowing relatively quick construction.

c. Regardless of the height or length of the wall,

support of the structure is not required during
construction as for conventional retaining walls.

They are relatively flexible and can tolerate

large lateral deformations and large differential

vertical settlements. The flexibility of geotextile-
reinforced walls allows the use of a lower factor of

safety for bearing capacity design than for conven-

tional more rigid structures.

e. They are potentially better suited for earth-

quake loading because of the flexibility and inher-
ent energy absorption capacity of the coherent

earth mass.

7-3. Disadvantages of Geotextile-Reinforced

Walls

Some disadvantages of geotextile-reinforced walls
over conventional concrete walls are the following:

Some decrease in geotextile strength may

occur because of possible damage during construc-
tion.

Some decrease in geotextile strength may

occur with time at constant load and soil tempera-
ture.

The construction of geotextile-reinforced walls

in cut regions requires a wider excavation than

conventional retaining walls.

Excavation behind the geotextile-reinforced

wall is restricted.

7-4.

7-4. Uses

Geotextile-reinforced walls can be substantially

more economical to construct than conventional
walls. However, since geotextile application to
walls is relatively new, long term effects such as
creep, aging, and durability are not known based
on actual experience. Therefore, a short life, seri-

ous consequences of failure, or high repair or
replacement costs could offset a lower first cost.
Serious consideration should be given before utili-

zation in critical structures. Applications of
geotextile-reinforced walls range from construction
of temporary road embankments to permanent

structures remedying slide problems and widening

highways effectively. Such walls can be con-
structed as noise barriers or even as abutments for

secondary bridges. Because of their flexibility,

these walls can be constructed in areas where poor
foundation material exists or areas susceptible to
earthquake activity.

7-5. General Considerations

The wall face may be vertical or inclined.

This can be because of structural reasons (internal

stability), ease of construction, or architectural
purposes. All geotextiles are equally spaced so that
construction is simplified. All geotextile sheets,

except perhaps for the lowest one, usually extend
to the same vertical plane.

Geotextiles exposed to UV light may degrade

quite rapidly. At the end of construction, a protec-

tive coating should be applied to the exposed face

of the wall. An application of 0.25 gallon per

square yard of CSS-1 emulsified asphalt or spray-
ing with a low viscosity water-cement mixture is
recommended. This cement mixture bonds well

and provides satisfactory protection even for
smooth geotextiles. To protect the face of the wall

from vandalism, a 3-inch layer of gunnite can be

applied. This can be done by projecting concrete
over a reinforcing mesh manufactured from No. 12

wires, spaced 2 inches in each direction, supported
by No. 3 rebars inserted between geotextile layers
to a depth of 3 feet.

7-1

TM 5-818-8/AFJMAN 32-1030

When aesthetic appearance is important, a

low-cost solution like the facing system comprised
of used railroad ties or other such materials can be
used.

No weepholes are specified, although after

UV and vandal protection measures the wall face
may be rather impermeable. To ensure the fast
removal of seeping water in a permanent struc-
ture, it is recommended to replace 1 to 2 feet of
the natural foundation soil (in case it is not

free-draining) with a crushed-stone foundation

layer to facilitate drainage from within and behind
the wall. The crushed rock may be separated from
the natural soil by a heavy weight geotextile
meeting filter criteria of chapter 3.

7-6. Properties of Materials

a. Retained Soil.

The soil wrapped by the geo-

textile sheets is termed “retained soil.” This soil
must be free-draining and nonplastic. The ranking
(most desirable to less desirable) of various re-
tained soils for permanent walls using the Unified
Soil Classification System is as follows: SW, SP,
GW, GP, and any of these as a borderline classifi-
cation which is dual designated with GM or SM.
The amount of fines in the soil is limited to 12
percent passing sieve No. 200. This restriction is

imposed because of possible migration of fines

being washed by seeping water. The fines may be
trapped by geotextile sheets, thus eventually creat-

ing low permeability liners. Generally, the perme-

ability of the retained soil must be more than 10-3

centimeters per second. The ranking order indi-
cates that gravels are not at the top. Although

they posses high permeability and, possibly, high

strength, their utilization requires special atten-

tion. Gravel, especially if it contains angular

grains, can puncture the geotextile sheets during
construction. Consequently, consideration must be
given to geotextile selection so as to resist possible
damage. If a geotextile possessing high puncture

resistance is available, then GP and GW should
replace SP and SW, respectively, in their ranking
order. The retained soil unit weight should be
specified based on conventional laboratory compac-
tion tests. A minimum of 95 percent of the maxi-

mum dry unit weight, as determined by ASTM D
698 should be attained during construction. Since

the retained soil will probably be further densified
as additional layers are placed and compacted, and
may be subjected to transitional external sources
of water, such as rainfall, it is recommended for
design purposes that the saturated unite weight be

used.

b. Backfill Soil.

The soil supported by the rein-

forced wall (the soil to the right of L in figure 7-1)
is termed “backfill soil.” This soil has a direct

effect on the external stability of the wall. There-
fore, it should be carefully selected. Generally,

backfill specifications used for conventional retain-

ing walls should be employed here as well. Clay,
silt, or any other material with low permeability

should be avoided next to a permanent wall. If low

quality materials are used, then a geotextile filter

(a)

EARTH

PRESSURE

LATERAL PRESSURE

(b)

Figure 7-1. General Configuration of a Geotextile Retained Soil Wall and Typical Pressure Diagrams

7-2

TM 5-818-8/AFJMAN 32-1030

meeting filtration requirements of chapter 3
should be placed to separate the fines from the
free draining backfills, thus preventing fouling of

the higher quality material. Since the retained soil

and backfill may have an effect on the external
stability of the’ reinforced wall, the properties of

both materials are needed. The unit weight should
be estimated as for the retained soil; use the

maximum density at zero air voids. The strength
parameters should be determined using drained

direct shear tests (ASTM D 3080) for the perme-
able backfill. The backfill and the retained soil
must have similar gradation at their interface so

as to minimize the potential for lateral migration
of soil particles. If such requirement is not practi-
cal, then a conventional soil filter should be

designed, or a geotextile filter used along the
interface.

7-7. Design Method

The design method recommended for retaining
walls reinforced with geotextiles is basically the
U.S. Forest Service method as developed by Stew-

ard, Williamson, and Mahoney (1977) using the
Rankine approach. The method considers the earth

pressure, line load pressure, fabric tension, and
pullout resistance as the primary design parame-

ters.

a. Earth Pressure.

Lateral earth pressure at any

depth below the top of the wall (fig 7-1a) is given

by:

(eq 7-1)

where

A typical live load pressure distribution is shown
in figure 7-1

Figure 7-2 illustrates live load

stress calculations.

c. Fabric Tension.

Tension in any fabric layer is

equal to the lateral stress at the depth of the layer
times the face area that the fabric must support.
For a vertical fabric spacing of X , a unit width of
fabric at depth d must support a force of

where

is the average total lateral pressure

(composite of dead plus live load) over the vertical

interval X .

d. Pullout Resistance.

A sufficient length of

geotextile must be embedded behind the failure

plane to resist pullout. Thus, in Figure 7-1

only

the length, Le, of fabric behind the failure plan
AB would be used to resist pullout. Pullout resis-

tance can be calculated from:

where

= pullout resistance

(eq 7-4)

d = depth of retained soil below top of retain-

ing wall

= unit weight of retained soil

= angle of internal friction of retained soil

= length of embedment behind the failure

plane

It can be seen from this expression that pullout

resistance is the product of overburden pressure,

, and the coefficient of friction between retained

soil and fabric which is assumed to be TAN 2/3

This resistance is in pounds per square foot which
is multiplied by the surface area of 2L

for a unit

width. Where different soils are used above and
below the fabric layer, the expression is modified
to account for different coefficients of friction for
each soil:

(eq 7-5)

7-8. Design Procedure

The recommended design procedure is discussed in

the following steps. The calculations for the fabric

dimensions for overlap, embedment length and

vertical spacing should include a safety factor of

1.5 to 1.75 depending upon the confidence level in

the strength parameters.

a. Retained Soil Properties

and

Only free-

draining granular materials should be used as
retained soil. The friction angle,

, will be

determined using the direct shear (ASTM D 3080)

7-3

TM 5-818-8/AFJMAN 32-1030

strength and solve for L

e , the length of geotextile

This can be solved for the length of overlap

required. Thus, the expression would be:

required:

(eq 7-7)

where

= fabric tensile strength

F.S. = safety factor of 1.5 to 1.75

The minimum length of the fabric required is 3
feet.

Length of Fabric Overlap for the Folded

Portion of Fabric at the Face. The overlap, L

o ,

must be long enough to transfer the stress from

the lower section of geotextile to the longer layer

above. The pullout resistance of the geotextile is
given by:

(eq 7-8)

where d

= depth to overlap. Tension in the

geotextile is:

(eq 7-9)

Since the factor of safety can be expressed as:

(eq 7-10)

(eq 7-11)

The minimum length of overlap should be 3 feet to
ensure adequate contact between layers.

h. External Wall Stability.

Once the internal

stability of the structure is satisfied, the external
stability against overturning, sliding and founda-

tion bearing capacity should be checked. This is

accomplished in the same manner as for a retain-
ing wall without a geotextile. Overturning loads

are developed from the lateral pressure diagram
for the back of the wall. This may be different
from the lateral pressure diagram used in check-
ing internal stability, particularly due to place-
ment of live loads. Overturning is checked by
summing moments of external forces about the
bottom at the face of the wall. Sliding along the
base is checked by summing external horizontal
forces. Bearing capacity is checked using standard
foundation bearing capacity analysis. Theoreti-
cally, the fabric layers at the base could be shorter
than at the top. However, because of external
stability considerations, particularly sliding and
bearing capacity, all fabric layers are normally of
uniform width.

7-5

TM 5-818-8/AFJMAN 32-1030

APPENDIX A

REFERENCES

Government Publications

Departments of the Army and the Air Force

TM 5-820-2/AFJMAN 32-1016

Drainage and Erosion Control Subsurface Drainage Facil-

ities for Airfield Pavements

TM 5-822-5/AFJMAN 32-1018

Pavement Design for Roads, Streets, Walks, and Open

Storage Areas

TM 5-850-2/AFJMAN 32-1014

Flexible Pavement Design for Airfields

TM 5-825-3/AFJMAN 32-1014

Rigid Pavements for Airfields

TM 5-850-2/AFJMAN 32-1046

Railroad Design and Construction at Army and Air Force

Installations

TM 5-627

Maintenance of Trackage

Federal Highway Administration,

400 Seventh Street, SW, Washington, DC

FHWA-HI-90-001

Geotextile Design and Construction Guidelines October,

1989

United States Department of Agriculture Forest Service,

Portland, Oregon

Guidelines for Use of Fabrics in Construction and Maintenance of Low Volume Roads (June 1977)

Nongovernment Publications

American Society for Testing and Materials (ASTM),

1916 Race St., Philadelphia, PA 19103

D 276-93

Identification of Fibers in Textiles

D 698-91

Laboratory Compaction Characteristics of Soil Using Stan-

dard Effort

D 1557-91

Laboratory Compaction Characteristics of Soil Using Modi-

fied Effort

D 1683-90

Failure in Sewn Seams of Woven Fabrics

D 2850-87

Unconsolidated, Undrained Strength of Cohesive Soils in

Triaxial Compression

D 3080-90

Direct Shear Test of Soils Under Consolidated Drained

Conditions

D 3776-85 (1990)

Mass per Unit Area (Weight) of Woven Fabric

D 3786-87

Hydraulic Bursting Strength of Knitted Goods and Non-

woven Fabrics-Diaphragm Bursting Strength Tester
Method

D 4355-92

Deterioration of Geotextiles from Exposure to Ultraviolet

Light and Water (Xenon-Arc Type Apparatus)

D 4491-92

Water Permeability of Geotextiles by Permittivity

D 4533-91

Trapezoid Tearing Strength of Geotextiles

D 4595-93

Tensile Properties of Geotextiles by the Wide Strip Method

D 4632-91

Breaking Load and Elongation of Geotextiles (Grab Method)

D 4716-87

Constant Head Hydraulic Transmissivity (In-Plane Flow) of

Geotextiles and Geotextile Related Products

D 4751-87

Determining the Apparent Opening Size of a Geotextile

D 4833-88

Index Puncture Resistance of Geotextiles, Geomembranes,

and Related Products

D 5101-90

Measuring the Soil Geotextile System Clogging Potential by

the Gradient Ratio

D 5141-91

Determining Filtering Efficiency and Flow Rate of a Geo-

textile for Silt Fence Application Using Site Specific Soil

A - l

TM 5-818-8/AFJMAN 32-1030

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