meeting filtration requirements of chapter 3

z = vertical distance from load to point where

should be placed to separate the fines from the

stress is being calculated

free draining backfills, thus preventing fouling of

the higher quality material. Since the retained soil

y = horizontal distance from load to wall, and

and backfill may have an effect on the external

parallel to the wall

stability of the reinforced wall, the properties of

A typical live load pressure distribution is shown

both materials are needed. The unit weight should

in figure 7-1b. Figure 7-2 illustrates live load

be estimated as for the retained soil; use the

stress calculations.

maximum density at zero air voids. The strength

parameters should be determined using drained

equal to the lateral stress at the depth of the layer

direct shear tests (ASTM D 3080) for the perme-

times the face area that the fabric must support.

able backfill. The backfill and the retained soil

For a vertical fabric spacing of X , a unit width of

must have similar gradation at their interface so

fabric at depth d must support a force of

,

as to minimize the potential for lateral migration

is the average total lateral pressure

where

of soil particles. If such requirement is not practi-

(composite of dead plus live load) over the vertical

cal, then a conventional soil filter should be

interval X .

designed, or a geotextile filter used along the

interface.

geotextile must be embedded behind the failure

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

the length, Le, of fabric behind the failure plan

The design method recommended for retaining

AB would be used to resist pullout. Pullout resis-

walls reinforced with geotextiles is basically the

tance can be calculated from:

U.S. Forest Service method as developed by Stew-

(eq 7-4)

ard, Williamson, and Mahoney (1977) using the

Rankine approach. The method considers the earth

where

pressure, line load pressure, fabric tension, and

= pullout resistance

pullout resistance as the primary design parame-

d = depth of retained soil below top of retain-

ters.

ing wall

= unit weight of retained soil

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

= angle of internal friction of retained soil

by:

= length of embedment behind the failure

plane

(eq 7-1)

It can be seen from this expression that pullout

where

resistance is the product of overburden pressure,

= lateral earth pressure acting on the wall

, and the coefficient of friction between retained

= at rest pressure coefficient

soil and fabric which is assumed to be TAN

= soil unit weight

This resistance is in pounds per square foot which

d = depth below the top of the wall

is multiplied by the surface area of

for a unit

A typical earth pressure distribution is shown in

width. Where different soils are used above and

figure 7-1b. Use of the "at rest" pressure coeffi-

below the fabric layer, the expression is modified

cient, Ko , is recommended and is determined by

to account for different coefficients of friction for

the following equation:

each soil:

(eq 7-2)

(eq 7-5)

where is the angle of internal friction of the soil.

The failure surface, AB in figure 7-1a, slopes

upward at an angle of

The recommended design procedure is discussed in

the following steps. The calculations for the fabric

live loads are calculated for a point load acting on

dimensions for overlap, embedment length and

the surface of the backfill using the following

vertical spacing should include a safety factor of

equation:

1.5 to 1.75 depending upon the confidence level in

the strength parameters.

(eq 7-3)

where

draining granular materials should be used as

P = vertical load

retained soil. The friction angle,

, will be

x = horizontal distance from load to wall and

determined using the direct shear (ASTM D 3080)

perpendicular to the wall

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