TM 5-818-1 / AFM 88-3, Chap. 7
CHAPTER 14
RETAINING WALLS AND EXCAVATION SUPPORT SYSTEMS
exceed φ'/2 and for passive earth pressures should not
14-1.
Design considerations for retaining walls.
exceed φ'/3. The angle of wall friction for walls subjected
a. General. Retaining walls must be designed
so that foundation pressures do not exceed allowable
to vibration should be assumed to be zero.
bearing pressures, wall settlements are tolerable, safety
e. Distribution of earth pressure.
A
factors against sliding and overturning are adequate, and
presentation of detailed analyses is beyond the scope of
the wall possesses adequate structural strength.
this manual. However, it is sufficiently accurate to
Methods for evaluating earth pressures on retaining walls
assume the following locations of the earth pressure
and design procedures are summarized herein for
resultant:
cohesionless backfill materials, which should be used
(1) For walls on rock:
whenever practicable.
0.38H above base for horizontal or
b. Forces acting on retaining walls. Forces
downward sloping backfill
include earth pressures, seepage and uplift pressures,
0.45H above base for upward sloping
surcharge loads, and weight of the wall. Typical load
backfill
diagrams for principal wall types are shown in figure 14-
(2) For walls on soil:
1. The magnitude and distribution of active and passive
0.33H above base of horizontal backfill
earth pressures are developed from the earth theory for
0.38H above base of upward sloping
walls over 20 feet high and from semiempirical curves for
backfill
lower walls. The subgrade reaction along the base is
Water pressures are handled separately.
assumed linearly distributed.
f.
Surcharge
loads.
Equations
for
concentrated point and line load are presented in figure
14-2.
Earth pressures.
14-5. For uniform or nonuniform surcharge pressure
a. Earth pressure at rest. For cohesionless
acting on an irregular area, use influence charts based
soils, with a horizontal surface, determine the coefficient
on the Boussinesq equations for horizontal loads and
of earth pressure at rest, K>, from the following:
double the horizontal pressures obtained.
g. Dynamic loads. The effects of dynamic
K0 = 1 - sin φ
(14-1)
loading on earth pressures are beyond the scope of this
manual. Refer to geotechnical engineering textbooks
b. Active earth pressure.
Formulas for
dealing with the subject.
calculating the coefficient of active earth pressure for a
cohesionless soil with planar boundaries are presented
14-3. Equivalent fluid pressures. The equivalent
in figure 14-2.
fluid method is recommended for retaining walls less
c. Passive earth pressure.
Formulas for
than 20 feet high. Assign available backfill material to a
calculating the coefficient of passive earth pressure for a
category listed in figure 14-6. If the wall must be
cohesionless soil with planar boundaries are presented
designed without knowledge of backfill properties,
in figure 14-3.
estimate backfill pressures on the basis of the most
d. Earth pressure charts.
Earth pressure
unsuitable material that may be used. Equivalent fluid
coefficients based on planar sliding surfaces are
pressures are shown in figure 14-6 for the straight slope
presented in figure 14-4. The assumption of a planar
backfill and in figure 14-7 for the broken slope backfill.
sliding surface is sufficiently accurate for the majority of
Dead load surcharges are included as an equivalent
practical problems. A logarithmic spiral failure surface
weight of backfill. If the wall rests on a compressible
should be assumed when passive earth pressure is
foundation and moves downward with respect to the
calculated and the angle of wall friction, δ, exceeds φ'/3.
backfill, pressures should be increased 50 percent for
backfill types 1, 2, 3, and 5. Although equivalent fluid
Earth pressure coefficients based on a logarithmic spiral
pressures include seepage effects and time-conditioned
sliding surface are presented in textbooks on
changes in the backfill material, adequate drainage
geotechnical engineering. Passive pressure should not
should be provided.
be based on Coulomb theory since it overestimates
passive resistance. Because small movements mobilize
δ and concrete walls are relatively rough, the wall friction
can be considered when estimating earth pressures. In
general, values of δ for active earth pressures should not
14-1
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