Page 349 - Masonry and Concrete

Retaining Walls
348 CHAPTER TEN

10.2.1 Footings
The bottom of the footing must be below
the winter frost line to avoid displacement
by frost heave. The soil must be of suffi-
cient strength to withstand significant
pressure under the front edge of the foot-
ing, since the tendency of a cantilever
retaining wall is to tip forward. Figure 10-4
shows allowable soil-bearing pressures
from the CABO One and Two Family
Dwelling Code. If the soil under the footing
FIGURE 10-3 is soft or unstable, an engineer should
Segmental retaining wall units. (from National Con- design the wall and its footing. If the
crete Masonry Association, Design Manual for Seg- ground slopes, the footing should be
mental Retaining Walls, NCMA, Herndon, VA).
stepped as described in Chapter 6. The
frost depth map in Figure 10-5 shows long
lines of equal frost depth in the central and southern states, but in the
west and north shows local frost depths that can vary widely within a
small area. Consult the local building official if you need more infor-
mation about the frost depth in your area. In warm climates, the frost
depth is very shallow, but the footing should still be set about 12 in.
below finish grade so that it is supported on firm, undisturbed soil.
Retaining wall footings are a little dif-
ferent from foundation wall or garden wall
Soil-Bearing footings. The soil pressure pushes against
Class of Material Pressure, psf
the wall, so to keep the footing from slid-
Crystalline bedrock 12,000 ing, a bottom projection is added which
Sedimentary rock 6,000 sits further down in the soil. The bottom
Sandy gravel or gravel 5,000 trench can be cut directly into the soil and
Sand, silty sand, clayey the upper portion of the footing formed
sand, silty gravel, and with wood in the usual way. The bottom of
clayey gravel 3,000 the main footing section as well as the pro-
Clay, sandy clay, silty clay jection should be below the frost line.
and clayey silt 2,000
Reinforcing bars from the footing must
FIGURE 10-4 stick up into the wall. For walls that are
relatively short, they will extend the full
Allowable bearing pressures for various types of soil.
(from Council of American Building Officials, One and height of the wall. For taller retaining
Two-Family Dwelling Code, Falls Church, VA). walls, the bars from the footing will be

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