TM 5-818-8/AFJMAN 32-1030
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
c. Fabric Tension. Tension in any fabric layer is
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
d. Pullout Resistance. A sufficient length of
interface.
geotextile must be embedded behind the failure
plane to resist pullout. Thus, in Figure 7-1a, only
7-7. Design Method
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
a. Earth Pressure. Lateral earth pressure at any
= 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.
7-8.Design Procedure
The failure surface, AB in figure 7-1a, slopes
upward at an angle of
The recommended design procedure is discussed in
b. Live Load Pressure. Lateral pressures from
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)
a. Retained Soil Properties
and . Only free-
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
7-3
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