Page 351 - Masonry and Concrete

Retaining Walls
350 CHAPTER TEN

those shown in the design tables should also be designed by an engi-
neer. The taller the wall, the greater the pressures exerted on it and the
greater the possibility of wall failure or collapse if the design does not
provide adequate strength. In situations where the embankment is high
and steep, consider using a series of shorter walls with flat terraces in
between so that the loads on the retaining walls are minimized. When-
ever there is any doubt about the adequacy of a retaining wall to with-
stand the soil pressure or if there is a significant amount of soil
moisture, hire an engineer. The design fees are much more affordable
than the liability associated with property damage, personal injury, or
life safety in the event of a failure.
Concrete retaining walls are built the same as the concrete founda-
tion walls described in Chapter 6. Since the wall will not have the ben-
efit of the house framing to brace it along the top edge, a retaining wall
actually has to be stronger than a foundation or basement wall. Figure
10-6 illustrates typical reinforced concrete retaining walls for 3- and 4-
ft. heights. Minimum concrete strength should be 2,500 psi in mild cli-
mates and 3,000 psi in moderate and severe weathering climates. Taller
concrete retaining walls typically have a stem which increases in thick-
ness from top to bottom, but the stem in shorter walls such as these is
the same thickness throughout its height. Control joints should be
located every 40 ft. on center. They must be saw-cut into the face of the
concrete wall after the forms are removed and should be about 2 in.
deep. If the wall is longer than 60 ft., it should be separated into sec-
tions with an expansion joint which fully separates the adjacent sec-
tions. The joint should be filled with a compressible material or
caulked with a high-performance exterior sealant. Horizontal reinforc-
ing bars should stop on either side of an expansion joint but may con-
tinue through a control joint.
The table and drawings for concrete block retaining walls in Figure
10-7 provide design dimensions for walls up to 4 ft., 8 in. in height. The
reinforcing dowels in the footing must be located in the center of the
wall and spaced in conjunction with the coursing layout of the block so
that the dowels will line up with the hollow block cores. Using a two-
core rather than a three-core type of block provides larger core spaces
that are easier to align with the steel dowels. Two-core block walls also
make it easier to align the cores throughout the height of the wall so
that the vertical reinforcing steel fits properly. Control joint spacing in

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