Civil Engineering Reference
In-Depth Information
backfill with
free draining
soil
backfill with
free draining
soil
weep holes
(4 in. or
larger)
granular
material of
sufficient
size to avoid
plugging
weep holes
perforated pipe
covered with
granular material
(cut hole in
counterforts if
necessary)
weep
holes
(a) Weep holes
(b) Drain pipe and perhaps
weep holes too
Figure 13.3
Retaining wall drainage.
13.4
FAILURES OF RETAINING WALLS
The number of failures or partial failures of retaining walls is rather alarming. The truth of
the matter is that if large safety factors were not used, the situation would be even more
severe. One reason for the large number of failures is the fact that designs are so often
based on methods that are suitable only for certain special situations. For instance, if a
wall that has a saturated clay behind it (never a good idea) is designed by a method that is
suitable for a dry granular material, future trouble will be the result.
13.5
LATERAL PRESSURES ON RETAINING WALLS
The actual pressures that occur behind retaining walls are quite difficult to estimate be-
cause of the large number of variables present. These include the kinds of backfill materi-
als and their compactions and moisture contents, the types of materials beneath the
footings, the presence or absence of surcharge, and other items. As a result, the detailed
estimation of the lateral forces applied to various retaining walls is clearly a problem in
theoretical soil mechanics. For this reason the discussion to follow is limited to a rather
narrow range of cases.
If a retaining wall is constructed against a solid rock face, there will be no pressure
applied to the wall by the rock. But if the wall is built to retain a body of water, hydrosta-
tic pressure will be applied to the wall. At any point the pressure (
p
) will equal
wh
, where
w
is the unit weight of the water and
h
is the vertical distance from the surface of the
water to the point in question.
If a wall is built to retain a soil, the soil's behavior will generally be somewhere be-
tween that of rock and water (
but as we will learn, the pressure caused by some soils is
much higher than that caused by water
). The pressure exerted against the wall will in-
crease, as did the water pressure, with depth but usually not as rapidly. This pressure at
any depth can be estimated with the following expression:
p
Cwh
