Civil Engineering Reference
In-Depth Information
Gang-formed panels. (Courtesy of Burke Concrete Accessories, Inc.)
In this equation,
w
is the unit weight of the soil,
h
is the distance from the surface to
the point in question, and
C
is a constant that is dependent on the characteristics of the
backfill. Unfortunately, the value of
C
can vary quite a bit, being perhaps as low as 0.3 or
0.4 for loose granular soils and perhaps as high as 0.9 or even 1.0 or more for some clay
soils. Figure 13.4 presents charts that are sometimes used for estimating the vertical and
horizontal pressures applied by soil backfills of up to 20-ft heights. Several different types
of backfill materials are considered in the figure.
Unit weights of soils will vary roughly as follows: 90 to 100 lb/ft
3
for soft clays, 100
to 120 lb/ft
3
for stiff clays, 110 to 120 lb/ft
3
for sands, and 120 to 130 lb/ft
3
for sand and
gravel mixes.
If you carefully study the second chart of Figure 13.4, you will probably be amazed
to see how high lateral pressures can be, particularly for clays and silts. As an illustration,
a 1 ft-wide vertical strip is considered for a 15-ft-high retaining wall backfilled with soil
number (4) with an assumed
of 10
(6
1 slope). The total estimated horizontal pressure
on the strip is
2
k
h
h
2
(
2
)(102)(15)
2
P
h
11,475 lb
