Environmental Engineering Reference
In-Depth Information
the dry season when movements will not close trenches, break off drains, or result in even
larger failures when cuttings are made.
In general, the time required for various treatment measures are as follows:
Sealing surface cracks and constructing interceptor ditches upslope are per-
formed within several days, at most.
●
Excavation at the head of a slide or the removal of loose blocks may require 1 to
2 weeks.
●
Relief of internal water pressures may require 1 to 4 weeks for toe drains and
trenches and 1 to 2 months for the installation of horizontal or vertical drains.
●
Counterberms and buttresses at the toe require space, but can be constructed
within 1 to 2 weeks.
●
Retention with concrete walls can require 6 months or longer.
●
9.4.2
Changing Slope Geometry
Natural Slope Inclinations
Significance
In many cases, the natural slope represents the maximum long-term inclination, but in
other cases the slope is not stable. The inclination of existing slopes should be noted dur-
ing field reconnaissance, since an increase in inclination by cutting may result in failure.
Some examples of natural slope inclinations
Hard, massive rocks
: Maximum slope angle and height is controlled by the con-
centration and orientation of joints and by seepage. The critical angle for high
slopes of hard, massive rock with random joint patterns and no seepage acting
along the joints is about 70° (Terzaghi, 1962).
●
Interbedded sedimentary rocks
: Extremely variable, depending upon rock type, cli-
mate, and bedding thickness as well as joint orientations and seepage conditions.
Along the river valleys, natural excavation may have reduced stresses suffi-
ciently to permit lateral movement along bedding planes and produce bedding-
plane mylonite shear zones. On major projects such shears should be assumed to
exist until proven otherwise. The slickensides illustrated in
Figure 6.54
are prob-
ably shale mylonite.
●
Clay shales
: 8 to 15°, but often unstable. When interbedded with sandstones, 20 to
45° (see
Section 6.7.3).
●
Residual soils
: 30 to 40°, depending upon parent rock type and seepage.
●
Colluvium:
10 to 20°, and often unstable.
●
Loess
: Often stands vertical to substantial heights.
●
Sands:
dry and “clean,” are stable at the angle of repose (
i
φ
).
●
Clays
: Depends upon consistency, whether intact or fissured, and the slope
height.
●
Sand-clay mixtures
: Often stable at angles greater than repose as long as seepage
forces are not excessive.
●
Cut Slopes in Rock
Excavation
The objective of any cut slope is to form a stable inclination without retention. Careful
blasting procedures are required to avoid excessive rock breakage resulting in numerous