Civil Engineering Reference
In-Depth Information
TABLE 14.8
Foundations Adjacent Slopes (
Continued
)
Topic
Discussion
Regulations
(
Continued
)
Section 1803.5.10 Alternate setback and clearance. Where setbacks or
clearances other than those required in Sec. 1808.7 are desired, the building official
shall be permitted to require a geotechnical investigation by a registered design
professional to demonstrate that the intent of Sec. 1808.7 would be satisfied. Such
an investigation shall include consideration of material, height of slope, slope
gradient, load intensity and erosion characteristics of slope material.”
Comments
As previously mentioned, the
International Building Code
(2012) requires that
both fill and cut slopes have maximum inclinations of one unit vertical in two
units horizontal (50-percent slope). Thus for most slopes, the discussion in
Sections 1808.7.1 and 1808.7.2 dealing with slopes steeper than 1 unit vertical in
1 unit horizontal (100-percent slope) will not be applicable. For the usual situation
of a 2:1 (50-percent slope) or flatter slope, the setback requirement for structures
at the toe of the slope is easy to determine and simply consists of a horizontal
distance “at least the smaller of
H
/2 and 15 ft [4.6 m],” where
H
height of
the slope. At the top of the slope, the required setback is more complicated. The
horizontal setback is measured from the face of the footing to the face of the slope
and must be “at least the smaller of
H
/3 and 40 ft [12 m].” If, because of property
size constraints, the building must be close to the top of slope, then the perimeter
footing can be deepened in order to meet this regulation.
As mentioned in the earlier discussion, there is no building code regulation
for a minimum factor of safety for slope stability. Nevertheless, the geotechnical
engineer should evaluate the static and seismic stability of slopes that will poten-
tially impact the proposed development.
14.2.6 Retaining Walls
Retaining walls are covered in Chap. 10. Regulations for the geotechnical engineering
aspects of retaining walls are presented in Tables 14.9 and 14.10. Although the code (see
Table 14.10) allows the use of clayey soils (i.e., GC, SM-SC, SC, ML, ML-CL, and CL)
as backfill materials, clayey soils should generally not be used as retaining wall backfill
material because of the following reasons:
1.
Predictable behavior.
Import granular backfill generally has a more predictable
behavior in terms of the earth pressure exerted on the wall.
2.
Expansive soil forces.
Expansive soil related forces would not be generated by clean
granular soil. However, if clay backfill is used, the seepage of water into the backfill
could cause swelling pressures well in excess of the at-rest values listed in Table 14.10.
3.
Excessive rotation of the top of wall.
The rotation
Y
H
(where
Y
wall displace-
ment and
H
height of wall) to reach the active state for dense cohesionless soil is
about 0.0005, while the value of
Y
H
is around 0.02 for soft cohesive soil. Hence,
given a wall of the same height, the top of the wall will need to move horizontally
about 40 times more for soft cohesive backfill as compared to dense granular backfill
in order to reach the active state.
4.
Drainage system
. Retaining walls usually are constructed with drainage systems to
prevent the buildup of hydrostatic water pressure on the retaining wall. The drainage
system will be more effective if highly permeable soil, such as clean granular soil, is
used instead of clayey backfill.
