Civil Engineering Reference
In-Depth Information
Fig. 8.1
Types of reinforced concrete retaining walls.
stresses within its stem and base are resisted by steel reinforcement. If the face of the wall is to be exposed
then general practice is to provide it with a small backward batter of about 1 in 50 in order to compensate
for any slight forward tilting of the wall.
Relieving platforms
A retaining wall is subjected to both shear and bending stresses caused by the lateral pressures induced
from the soil that it is supporting. A mass construction gravity wall can take such stresses in its stride but
this is not so for the vertical stem of a reinforced concrete retaining wall. If structural failure of the stem
is to be avoided then it must be provided with enough steel reinforcement to resist the bending moment
and to have a sufficient thickness to withstand the shear stresses, for all sections throughout its height.
It is this situation that imposes a practical height limitation of about 7 m on the wall stem of a conven-
tional retaining wall. As a wall is increased in dimensions it becomes less flexible and the lateral pressures
exerted on it by the soil will tend to be higher than the active values assumed in the design. It is possible
therefore to enter a sort of upwards spiral - if a wall is strengthened to withstand increased lateral pres-
sures then its rigidity is increased and the lateral pressures are increased - and so on.
A way out of the problem is the provision of one or more horizontal concrete slabs, or platforms, placed
within the backfill and rigidly connected to the wall stem. A platform carries the weight of the material
above it (up as far as the next platform if there are more than one). This vertical force exerts a cantilever
moment on to the back of the wall in the opposite direction to the bending moment caused by the lateral
soil pressure. The resulting bending moment diagram becomes a series of steps and the wall is subjected
to a maximum bending moment value that is considerably less than the value when there are no platforms
(Fig.
8.2)
.
With the reduction of bending moment values to a manageable level, the wall stem can be kept slim
enough for the assumption of active pressure values to be realistic, with a consequential more economical
construction.
Counterfort wall
This wall can be used for heights greater than about 6 m. Its wall stem acts as a slab spanning between
the counterfort supports which are usually spaced at about 0.67 H but not less than 2.5 m, because of
construction considerations. Details of the wall are given in Fig.
8.1b
.
