Page 44 - Geotechnical Engineering Soil and Foundation Principles and Practice

Special Problems with Sedimentary Rocks
Special Problems with Sedimentary Rocks 39

will occupy valleys in between. The relative hardness and softness therefore is
indicated by the landform and the geomorphology.

3.2 THE GEOLOGICAL TIME SCALE

The major subdivisions of geological time is into eras, periods, and epochs.
These separations are based on fossil evidence and on extinctions. For example,
the Mesozoic era ended with an abrupt termination of the dinosaurs, now usually
attributed to a severe climate change following the impact of a meteor. Breaks
in deposition of sediments create erosional surface ‘‘unconformities.’’

The ‘‘Paleozoic era’’ literally means ‘‘ancient-life-time’’ and was dominated
by invertebrates including many varieties of now-extinct shellfish. The ‘‘Mesozoic
era,’’ or ‘‘middle-life-time,’’ was a time of dinosaurs and the earliest mammals.
The ‘‘Cenozoic era,’’ or ‘‘recent-life-time,’’ is dominated by mammals.

Unconformities are very common and subdivide a rock column into ‘‘formations’’
of consistent geological age. ‘‘Formation’’ is not a time designation but a rock
designation, and a single formation may incorporate a variety of rock layers.
Formations for the most part are identified from fossils, which is very important
in exploration drilling for oil.

Radioactive dating depends on constant rates of radioactive decay of certain
isotopes, and indicates that the majority of geological time was without life
except for single-celled plants, or algae. These rocks generally have been buried
under younger rocks and constitute part of the ‘‘basement complex.’’ They may
be lumped together in age as ‘‘pre-Cambrian,’’ which means that they originated
prior to the earliest, Cambrian, period of the Paleozoic era. Granitic shield areas
often are referred to as ‘‘pre-Cambrian shield.’’

3.3 CLASSIFICATION OF SEDIMENTARY ROCKS

The three major groups of sedimentary rocks are shale, sandstone, and limestone.
A fourth less common group is called evaporites and includes rock salt and gypsum.

3.3.1 Shale
Shale is by far the most abundant sedimentary rock. Although strictly speaking
shale is a rock, it is relatively weak and soft, and engineers consider it to be a
soil. However, shale differs from most soils in a number of ways that affect
its engineering uses. Shale typically is dense and has a very low content of voids,
causing it to be relatively impermeable to water. Second, shale typically has

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