Page 359 - Moving the Earth_ The Workbook of Excavation

ROADWAYS

ROADWAYS 8.21

BANK SLOPES

Angle. The angle at which bank slopes will stand in cuts and on fills is an important factor in
the cost, and sometimes in the feasibility, of sidehill construction. It is also a limiting factor in the
depth of through cuts.
There are two approaches to determining how steep a cut slope may be left. One is the behavior
of the same or similar material in cuts and on natural slopes, the other is soil analysis and calcu-
lations. They are frequently used in conjunction, a tentative slope being determined by field
observation and then checked by engineering research.
Natural slopes are seldom steeper than those that can be used in the same material in a highway
cut. The exceptions usually involve groundwater problems or the binding effect of vegetation.
However, natural slopes may not be nearly as steep as the soil qualities permit. In general, a
hill whose foot is being vigorously eroded by a fast-flowing stream will approach maximum
steepness, and one rising above meadows will tend to have a flatter slope.
Old cuts give a much more accurate indication. However, before dependence can be placed on them,
it would have to be ascertained that the material is actually the same, that it is subject to the same weath-
er conditions (freezing and thawing loosen faces more actively on the shaded side than on the sunny side
walls of canyons), to the same dip of strata, as in Fig. 8.15, and that groundwater conditions are similar.
It is desirable to cut back to entirely safe slopes, but this may not be possible. In notching along
the side of a mountain, the cut wall must be substantially steeper than the natural slope to avoid
excavating tremendous yardages. Also, a fill slope must be steeper than that of the sidehill on
which it rests if it is to support a road.
Some soils, such as loess in the midwest and lightly cemented gravels in the southwest, will
stand for long periods with almost vertical faces. In general, slopes can be steeper in arid climates
than in wet ones.
There are varieties of clay that will stand steeply when first cut, but under influence of surface
freezing and thawing, groundwater pressure, and vibration will slump to a 10 percent (1-on-10)
grade unless stabilized by topsoiling and planting.

FIGURE 8.15 Slope stability.