Environmental Engineering Reference
In-Depth Information
Benching
Benching is common practice in high cuts in rock slopes but there is disagreement among
practitioners as to its value. Some consider benches as undesirable because they provide
takeoff points for falling blocks (Chassie and Goughnor, 1975). To provide for storage they
must be of adequate width. Block storage space should always be provided at the slope
toe to protect the roadway from falls and topples.
Cut Slopes in Soils
Typical Inclinations
Thin soil cover over rock (
Figure 9.103)
:
The soil should be removed or retained as the con-
dition is unstable.
Soil-rock transition (strong residual soils to weathered rock) such as in
Figures 9.100
and 9.104 are often excavated to between 1H:1V to 1H:2V (45-63°), although potential fail-
ure along relict discontinuities must be considered. Saprolite is usually cut to 1.5H:1V
(Figure 9.100).
Most soil formations are commonly cut to an average inclination of 2H:1V (26°) (Figure
9.100), but consideration must be given to seepage forces and other physical and environ-
mental factors to determine if retention is required. Slopes between benches are usually
steeper.
Benching and Surface Drainage
Soil cuts are normally designed with benches, especially for cuts over 25-30 ft (8-10 m)
high. Benches reduce the amount of excavation necessary to achieve lower inclinations
because the slope angle between benches may be increased.
FIGURE 9.103
Cuts in colluvium over inclined rock are potentially highly unstable and require either removal of soil or
retention (Rio de Janeiro, Brazil).
