Structural Concrete - Hassoun

502 Chapter 14 Retaining Walls
the exterior surface of the wall is normally exposed to different weather conditions and temperature
changes.
For interior wall surfaces, the balance of the required reinforcement in each direction should
have a minimum concrete cover of 3 in. but not more than 1 of the wall thickness.
4 3
The minimum steel area in the wall footing (heel or toe), according to the ACI Code,
Section 7.6.1, is that required for shrinkage and temperature reinforcement, which is 0.0018bh
when f y = 60 ksi and 0.0020bh when f y = 40 ksi or 50 ksi. Because this minimum steel area
is relatively small, it is a common practice to increase it to that minimum A s required for
flexure:
( √ )
3 f
′
( )
c 200
A s,min = bd ≥ bd (14.16)
f y f y
14.10 DRAINAGE
The earth pressure discussed in the previous sections does not include any hydrostatic pressure. If
water accumulates behind the retaining wall, the water pressure must be included in the design.
Surface or underground water may seep into the backfill and develop the case of submerged soil.
To avoid hydrostatic pressure, drainage should be provided behind the wall. If well-drained cohesionless
soil is used as a backfill, the wall can be designed for earth pressure only. The drainage
system may consist of one or a combination of the following:
1. Weep holes in the retaining wall of 4 in. or more in diameter and spaced about 5 ft on centers
horizontally and vertically (Fig. 14.9a).
2. Perforated pipe 8 in. in diameter laid along the base of the wall and surrounded by gravel
(Fig. 14.9b).
3. Blanketing or paving the surface of the backfill with asphalt to prevent seepage of water from
the surface.
4. Any other method to drain surface water.
Figure 14.9
Drainage systems.