Civil Engineering Reference
In-Depth Information
It has been estimated that the cost of constructing retaining walls varies directly with
the square of their heights. Thus as retaining walls become higher, the accuracy of the
computed lateral pressures becomes more and more important in providing economical
designs. Since the Coulomb equation does take into account friction on the wall, it is
thought to be the more accurate one and is often used for walls of over 20 ft. The Rankine
equation is commonly used for ordinary retaining walls of 20 ft or less in height. It is in-
teresting to note that the two methods give identical results if the friction of the soil on the
wall is neglected.
The Rankine expressions for the active and passive pressure coefficients are given at
the end of this paragraph, with reference being made to Figure 13.7. In these expressions
is the angle of internal fric-
tion of the soil. For well-drained sand or gravel backfills, the angle of internal friction is
often taken as the angle of repose of the slope. One common slope used is 1 vertically to
horizontally (33
is the angle the backfill makes with the horizontal, while
1
2
40
).
cos
2
cos
2
cos
C
a
cos
cos
2
cos
2
cos
cos
2
cos
2
cos
C
p
cos
cos
2
cos
2
cos
Should the backfill be horizontal—that is, should
be equal to zero—the expressions
become
1
sin
C
a
1
sin
1
sin
C
p
1
sin
One trouble with using these expressions is in the determination of
. It can be as
small as 0
for some granular materials. As a
result, the values of
C
a
can vary from perhaps 0.30 for some granular materials up to
about 1.0 for some wet clays.
to 10
for soft clays and as high as 30
or 40
Figure 13.7
