Page 9 - Geotechnical Engineering Soil and Foundation Principles and Practice

Introduction
4 Geotechnical Engineering

Figure 1.2
Pressure in
pore water can
be positive
or negative,
respectively
decreasing or
increasing
frictional strength.

shown at the left in Fig. 1.2. As pore water pressure increases, friction
between the grains decreases and the soil becomes weaker. Measurement and
control of pore water pressure therefore can be a major factor influencing
construction.
11. Certain soil clay minerals blow up like an accordion when wet with water,
and can exert enough pressure to lift pavements, floors, and foundations, and
push in walls. This may come as a surprise, but the cost of repairs of damages
caused by expansive clays exceeds that from landslides and earthquakes,
and runs to billions of dollars per year.

This list is not complete, partly because geotechnical engineering is relatively
new and the era of discovery is not over. Soils are complex materials, and tests are
required to identify, evaluate, and effectively use them. Curtailing an investigation
to save money up front will increase overall cost as the design then must be overly
conservative to allow for any unrecognized problems. A competent geotechnical
investigation is essential if problems are to be avoided, and it is axiomatic that
the investigation that is not conducted prior to construction probably will
be conducted later, when repairs are needed or the matter goes to court. The
problems depicted in Fig. 1.3 went to court.

1.4 SITE INVESTIGATION

1.4.1 Site Geology
The first step in a site investigation is to identify the geological conditions, which
can profoundly influence the plan and methods of the investigation. For example,
borings performed on a hill whose flanks reveal a persistent rock outcrop will be
far different from those performed in a river floodplain where rock may be tens or
hundreds of meters deep. A preliminary assessment is made through recognition
of surface features, reference to geological and agricultural soil maps, and records
from prior borings in the vicinity.

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