Civil Engineering Reference
In-Depth Information
Figure 13.5
Internal friction is greater for a cohesive soil than for a noncohesive one, but the wet-
ter such a soil becomes, the smaller will be its cohesiveness and thus the flatter the plane
of rupture. The flatter the plane of rupture, the greater is the volume of earth tending to
slide and push against the wall. Once again it can be seen that good drainage is of the ut-
most importance. Usually the designer assumes that a cohesionless granular backfill will
be placed behind the walls.
Due to lateral pressure, the usual retaining wall will give or deflect a little because it
is constructed of elastic materials. Furthermore, unless the wall rests on a rock foundation,
it will tilt or lean a small distance away from the soil due to the compressible nature of the
supporting soils. For these reasons, retaining walls are frequently constructed with a slight
batter, or inclination, toward the backfill so that the deformations described are not obvi-
ous to the passerby.
Under the lateral pressures described, the usual retaining wall will move a little
distance and
active soil pressure
will develop, as shown in Figure 13.6. Among the
many factors that affect the pressure applied to a particular wall are the kind of back-
fill material used, the drainage situation, the level of the water table, the seasonal con-
ditions such as dry or wet or frozen, the presence of trucks or other equipment on the
backfill, and so on.
For design purposes it is usually satisfactory to assume that the active pressure varies
linearly with the depth of the backfill. In other words, it is just as though (so far as lateral
pressure is concerned) there is a liquid of some weight behind the wall that can vary from
considerably less than the weight of water to considerably more. The chart of Figure 13.4
shows this large variation in possible lateral pressures. The assumed lateral pressures are
often referred to as
equivalent fluid pressures
.
Values from 30 to 50 pcf are normally as-
sumed but may be much too low for clay and silt materials.
If the wall moves away from the backfill and against the soil at the toe, a passive
soil pressure will be the result. Passive pressure, which is also assumed to vary linearly
with depth, is illustrated in Figure 13.6. The inclusion or noninclusion of passive pres-
sure in the design calculations is a matter of judgment on the designer's part. For effec-
