Page 48 - Geotechnical Engineering Soil and Foundation Principles and Practice

Special Problems with Sedimentary Rocks
Special Problems with Sedimentary Rocks 43

Question: In Fig. 3.1 identify (a) a dendritic drainage pattern; (b) a base level
for the dendritic drainage; (c) slope retreat and a critical distance; (d) soil creep;
(e) a narrow, flat-topped hill that is capped with a harder rock.

Shale as a Construction Material
The hardness of dense shale can be deceiving, because when it is broken up and used
as fill it may slowly absorb water and soften into clay. The moisture content of
unweathered shale can be as low as 2 or 3 percent compared with 25 to 50 percent or
more in more ordinary clayey soils. Gradual softening of shale can result in slope
failures mysteriously occurring years after an embankment is completed. Consider-
able caution should be exercised before using an untried shale in construction.

3.3.2 Sandstone

Sandstone is less abundant and less troubling than shale, being composed of sand
grains that are cemented by clay, carbonates, iron oxides, or silica. Most sandstone
is relatively soft, but some is hard enough that it can be used as coarse aggregate.

Feldspathic Sandstone
Sandstone that is close to a granitic source can have a granite-like composition
that is dominated by pink feldspars. These are referred to as ‘‘arkose.’’ An
example is rocks of the Garden of the Gods in Colorado.

Quartz Sandstone
Far more common is sand that has been transported sufficiently to wear down
feldspars and leave quartz, which is a much harder mineral. This occurs regardless
of whether the sand is carried by wind or by water, as it moves by saltation,or
bouncing along. The dominant mineral in most sandstone is quartz, and extensive
reworking as a beach sand in geological time eventually can result in a nearly pure
quartz sandstone that is used for manufacture of glass. ‘‘Ottawa sand’’ is from
a sandstone of Ordovician age that has highly rounded grains and is used as
a glass sand and as a reference sand in engineering tests.

Graywacke
This is gray, fine-grained sandstone that originally was deposited in deltas.
Being in relatively thick, deep deposits it is much more likely to be encountered
in deep borings for petroleum than in geotechnical engineering.

Geomorphology of Sandstone
Sandstone, being more permeable than shale, has a lower runoff and a lower
drainage density, or total stream length per unit area (Fig. 3.2). Sandstone layers
usually have variable cementation, such that harder layers are more resistant to
erosion. Horizontal layers of harder rock therefore cap and create flat-topped hills
or mesas.

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