Page 11 - Geotechnical Engineering Soil and Foundation Principles and Practice

Introduction
6 Geotechnical Engineering

1.5 SOIL TESTS

Testing is required to evaluate soils at a particular site, and normally are con-
ducted as part of the boring program. Relatively undisturbed samples are taken
for laboratory testing, and in recent years there has been a rapid increase in the
use of rapid tests that are performed in situ in order to save time and money.

The footprint and complexity of a structure help to define appropriate depths and
patterns of borings, and the tests that should be performed. Statistical methods
are used to evaluate the results, and show that the more variable the soil, the more
data are required for an accurate characterization of its properties. It also is
important to define and identify different soils at a site in order to evaluate the
properties of each instead of mixing apples and random dog turds.

One challenge for geotechnical engineers is to not only convince clients and their
representatives of the need for a proper investigation, but also to heed the results.
For example, when a pile foundation was recommended for an elaborate marble-
faced building, the architect objected that he was not going to approve putting
money into the ground ‘‘where it would not show.’’ An argument was made that
without an adequate foundation the consequences indeed would show, and the
building was put on piles.

1.6 SOIL WATER

Architects or structural engineers who are not accustomed to dealing with the
vagaries and inconsistencies of nature may regard soil as perverse and lacking in
manners. What other material is so variable and so dependent on its environment?
Even a change in the elevation of the groundwater table can have adverse effects.
An important clue that may be overlooked is the soil color, gray indicating
seasonal wet conditions.

1.7 SOIL VARIABILITY AND THE ‘‘FACTOR OF SAFETY’’

Factor of safety is defined as a ratio between a design value and a failure value.
For example, if a rope breaks under load X, the allowable load must be
reduced by a factor of safety or the rope will break. Factors of safety in
geotechnical engineering tend to be generous, often 3 to 5, to ensure that
everything remains on the safe side. The more variable the soil, the less able one
is to accurately define its average or most critical properties, and the higher
the factor of safety. Thus ‘‘factor of safety’’ therefore in part is a ‘‘factor of
ignorance,’’ although it seldom is stated in this way, at least to a client. The less

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