Page 46 - Geotechnical Engineering Soil and Foundation Principles and Practice

Special Problems with Sedimentary Rocks
Special Problems with Sedimentary Rocks 41

Expansive Shale
Geologically young shale, younger than Paleozoic, often contains expansive
clay minerals, or minerals that expand on wetting and shrink on drying. Tests
described later in this book can determine the presence and expansive potential
of expansive clay.

Expansive clays constitute one of the most difficult engineering problems,
inflicting extensive damages on highways, floors, walls, and foundations. Several
control measures may be used. A common approach is to try and keep the water
out, but this may be difficult because of the high affinity of an expansive clay
for water. The attraction can be reduced with chemical stabilization, which is most
commonly accomplished using hydrated chemical lime, Ca(OH) 2 .

Weathering of Shale
Shale weathers by simply disintegrating back into clay. As weathering is most
intense near the ground surface, several distinctive layers may form that are called
a ‘‘soil profile.’’ A typical soil profile consists of an organic-rich topsoil that
is weak and should not be used in engineering construction, over a subsoil in
which clay has been concentrated. The details of soil profiles depend on many
factors including climate, moisture and temperature environment, erosion rate,
and time, and are treated in Chapter 5.

Geomorphology of Shale
Geomorphology is the study of landforms as they relate to geological materials
and processes. Streams cutting headward into shale are like fingers randomly
reaching out for water, and create a dendritic drainage pattern. Shale soils have
a low permeability and are readily erodible, which results in a finely sculptured
drainage pattern. An example is shown in Fig. 3.1. The total stream length per
unit of area is defined as the ‘‘drainage density,’’ which is high for shale.

Why Are Stream Valleys Wider at the Top?
Unless they are cut into hard rock, valleys of small tributary streams normally
are V-shaped, that is, wider at the top. It is unrealistic to suppose that streams
initially were wide and became narrower as they cut downward. Instead, other
processes must kick in to widen the valleys. The processes depend on slopes of the
valley walls and can be sequential:

1. A vertical cut in weak soil is unstable, so the cut develops a landslide that tends
to level things out.
2. A flattened slope gathers more rain so it is subjected to increased slope wash
erosion. Eroded slopes tend to be concave as the elevation at the lower end
reaches a local base level such as a stream or river that restricts downcutting.
3. Weathered or loose soil on a slope tends to move downhill by gravity, a slow
process that is called soil creep. Soil creep gradually rounds the edges of
unprotected hilltops.

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