backfill and wall causes a greater portion of the

interface strength to be mobilized. This process

approaches a limiting condition for high walls as the

Two procedures for computing the magnitudes of

interface strength becomes fully mobilized over most

shear loads along the backs of gravity walls are

of the wall-backfill contact area.

described in this section: a simplified procedure, and

(b) As the stiffness of the backfill increases,

a complete soil-structure interaction analysis proce-

backfill compression decreases, and the wall height

dure using the finite element method. These proce-

necessary to mobilize the full interface strength

dures are intended only as guidelines and are not

increases. For low walls with vertical back sides, the

intended to replace judgment by the engineers respon-

sible for the project.

value of *K*v decreases as the backfill stiffness

increases.

(c) The value of *K*v decreases as the back side of

(1) Inclusion of a shear force on a vertical plane

the wall becomes inclined away from the backfill and

through the heel of the wall, as shown in Figure 2,

towards the front of the wall.

can be computed by use of the following equation:

(d) The value of *K*v is greater for a wall with a

1

stepped back side than for a wall with a smooth back

1

γb (*D*2)2

(2)

side at the same average slope.

2

2

where

(1) Shear loads have been reported for several

table

instrumented walls (Duncan, Clough, and Ebeling

1990), including a lock wall 30.2 m in height and

founded on rock (Hilmer 1986). Measurements at the

of the wall

lock wall are reported over a 6-year period. Mobi-

lized interface friction at the lock wall fluctuates

γb = buoyant unit weight of submerged backfill,

seasonally and with changes in water level inside the

γt - γw

locks. However, the data indicate that the shear force

is persistent over the 6-year period, and does not

γw = unit weight of water

decay with time. According to a conservative inter-

pretation of the data, the minimum value of *K*v during

(2) As indicated in Figure 2, the total height of

the 6-year period is about 0.18.

the backfill against the wall is the sum of the thick-

nesses *D*1 and *D*2:

(2) In a recent research program conducted at

Virginia Polytechnic Institute and State University

(Filz and Duncan 1992), both the horizontal earth

(3)

pressure force and the vertical shear force along the

vertical back side of a 2.1-m- (7-ft-) high rigid retain-

(3) Equation 2 requires a value for *K*v. In the

ing wall were measured. The research program

included 16 tests using compacted fine sand (Unified

simplified procedure, the value of *K*v is obtained from

Soil Classification SP) and compacted non-plastic

Figures 3 through 5 and Equation 4:

silty sand (SM) as backfill. Measured values of *K*v

ranged from 0.11 to 0.23 (Table 8.9 in Filz and

(4)

(*K*v)vert

(1

Duncan (1992)). The more compressible backfills

exhibited higher *K*v values. The compacted backfills

where

were left in place for periods ranging from 1 to

14 days after completion of backfilling. Values of *K*v

tended to increase with time.

3