ETL 1110-2-352
31 Mar 94
5. Procedures for Calculating the Vertical
backfill and wall causes a greater portion of the
Shear Force
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 Kv decreases as the backfill stiffness
increases.
a.
Simplified procedure.
(c) The value of Kv 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 Kv is greater for a wall with a
1
stepped back side than for a wall with a smooth back
1
Kv γt (D1)2 γt (D1D2)
γb (D2)2
(2)
Fv
side at the same average slope.
2
2
d. Shear force - instrumented field and model
where
wall measurements.
(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
D2 = thickness of submerged backfill above the base
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 Kv 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 D1 and D2:
(2) In a recent research program conducted at
Virginia Polytechnic Institute and State University
(Filz and Duncan 1992), both the horizontal earth
(3)
H
D1
D2
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 Kv. In the
ing wall were measured. The research program
included 16 tests using compacted fine sand (Unified
simplified procedure, the value of Kv 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 Kv
ranged from 0.11 to 0.23 (Table 8.9 in Filz and
(4)
Kv
CθCs)
(Kv)vert
(1
Duncan (1992)). The more compressible backfills
exhibited higher Kv values. The compacted backfills
where
were left in place for periods ranging from 1 to
14 days after completion of backfilling. Values of Kv
tended to increase with time.
3
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