Retaining Walls - Design of Reinforced Concrete - page 416

Figure 13.18
shown by P p in the figure. The actual theory involved, and thus the design of keys, is still a
question among geotechnical engineers. As a result, many designers select the sizes of keys
by rules of thumb. One common practice is to give them a depth between two-thirds and the
full depth of the footing. They are usually made approximately square in cross section and
have no reinforcing provided other than perhaps the dowels mentioned in the next paragraph.
Keys are often located below the stem so that some dowels or extended vertical rein-
forcing may be extended into the key. If this procedure is used, the front face of the key
needs to be at least 5 or 6 in. in front of the back face of the stem to allow room for the
dowels. From a soil mechanics view, keys may be a little more effective if they are placed
a little further toward the heel.
If the key can be extended down into a very firm soil or even rock, the result will be a
greatly increased sliding resistance—that resistance being equal to the force necessary to
shear the key off from the footing, that is, a shear friction calculated as described in Sec-
tions 8.12 and 12.13 of this text.
Heel Design
The lateral earth pressure tends to cause the retaining wall to rotate about its toe. This ac-
tion tends to pick up the heel into the backfill. The backfill pushes down on the heel can-
tilever, causing tension in its top. The major force applied to the heel of a retaining wall is
the downward weight of the backfill behind the wall. Although it is true that there is some
upward soil pressure, many designers choose to neglect it because it is relatively small.
The downward loads tend to push the heel of the footing down, and the necessary upward
reaction to hold it attached to the stem is provided by the vertical tensile steel in the stem,
which is extended down into the footing.
Because the reaction in the direction of the shear does not introduce compression into
the heel part of the footing in the region of the stem, it is not permissible to determine V u
at a distance d from the face of the stem, as provided in Section 11.1.3.1 of the ACI Code.
The value of V u is determined instead at the face of the stem due to the downward loads.
This shear is often of such magnitude as to control the thickness, but the moment at the