exceed φ'/2 and for passive earth pressures should not

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

14-5. 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

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

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