Page 47 - Geotechnical Engineering Soil and Foundation Principles and Practice

Special Problems with Sedimentary Rocks
42 Geotechnical Engineering

Figure 3.1
Airphoto of shale
area showing a
fine, dendritic
drainage pattern.
A river cuts
across the corner
at the left. (USDA
photo.)

Why Does Erosion Stop Near the Crest of a Hill?
Whereas downcutting at the toe of a slope is controlled by a local base level,
the width at the crest of a hill also is subject to a different kind of control.
As slopes on opposing flanks of a hill work headward and approach one another,
the area of hilltop collecting rain diminishes so slope retreat slows down and
may come to a stop at some ‘‘critical distance’’ that defines the widths of ridges.
The explanation was first proposed by a hydraulic engineer, Robert Horton,
who used his background in engineering to become a pioneer in the field of
quantitative geomorphology. As the critical distance is relatively constant for
a given soil under given conditions, this leads to a fairly constant width of hilltops,
or interfluves. The edges of the interfluves are rounded by weathering and by
soil creep.

Why Are Some Hills Flat on Top?
As previously mentioned, a flat-topped hill or mesa indicates a protective layer
of harder rock lying horizontal across the top. The situation changes if the rock
layer is inclined, in which case it outcrops along the high side, creating landforms
that are discussed later in this chapter.

Observations and explanations from geology can be quite helpful in geotechnical
engineering, as a steep slope should be investigated for landslides, and a shallower
slope for soil creep. Creep, although slow, eventually can multiply the soil
pressure against a retaining wall, and explains why older low retaining walls that
were not engineered gradually tilt and eventually may fall over.

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