Retaining Walls - Design of Reinforced Concrete - page 395

front or back of the stem. If the cross walls are behind the stem (that is, inside the soil)
and not visible, the retaining walls are called counterfort walls . Should the cross walls be
visible (that is, on the toe side), the walls are called buttress walls . These walls are illus-
trated in parts (d) and (e) of Figure 13.1. The stems for these walls are continuous mem-
bers supported at intervals by the buttresses or counterforts. Counterforts or buttresses are
usually spaced at distances approximately equal to one-half (or a little more) of the retain-
ing wall heights.
The counterfort type is more commonly used because it is normally thought to be
more attractive as the cross walls or counterforts are not visible. Not only are the but-
tresses visible on the toe side, but their protrusion on the outside or toe side of the wall
will use up valuable space. Nevertheless, buttresses are somewhat more efficient than
counterforts because they consist of concrete that is put in compression by the overturning
moments, whereas the counterforts are concrete members used in a tension situation and
they need to be tied to the wall with stirrups. Occasionally, high walls are designed with
both buttresses and counterforts.
Figure 13.2 presents a few other retaining wall variations. When a retaining wall is
placed at a property boundary or next to an existing building, it may be necessary to use a
wall without a toe, as shown in part (a) of the figure, or without a heel, as shown in part
(b). Another type of retaining wall very often encountered is the bridge abutment shown
in part (c) of the figure. Abutments may very well have wing wall extensions on the sides
to retain the soil in the approach area. The abutment, in addition to other loads, will have
to support the end reactions from the bridge.
The use of precast retaining walls is becoming more common each year. The walls
are built with some type of precast units, and the footings are probably poured in place.
The results are very attractive, and the units are high-quality concrete members made
under “plant controlled” conditions. Less site preparation is required and the erection of
the walls is much quicker than cast-in-place ones. The precast units can later be disas-
sembled and the units used again. Other types of precast retaining walls consist of walls
or sheeting actually driven into the ground before excavation. Also showing promise
are the gabions or wire baskets of stone used in conjunction with geotextile reinforced
embankments.

13.3 DRAINAGE

One of the most important items in designing and constructing successful retaining walls
is the prevention of water accumulation behind the walls. If water is allowed to build up
there, the result can be great lateral water pressure against the wall and perhaps an even
worse situation in cold climates due to frost action.
The best possible backfill for a retaining wall is a well-drained and cohesionless soil.
Furthermore, this is the condition for which the designer normally plans and designs. In
addition to a granular backfill material, weep holes of 4 in. or more in diameter (the large
sizes are used for easy cleaning) are placed in the walls approximately 5 to 10 ft on center,
horizontally and vertically, as shown in Figure 13.3(a). If the backfill consists of a coarse
sand, it is desirable to put a few shovels of pea gravel around the weep holes to try to pre-
vent the sand from stopping up the holes.