Page 10 - Geotechnical Engineering Soil and Foundation Principles and Practice

Introduction
Introduction 5

Figure 1.3
This could have
been avoided.
Fill soil was
identified by
drilling, so part of
the house was
supported on piles.
What was not
recognized was
that the fill was on
top of an old
landslide that was
reactivated by
weight of the fill.

1.4.2 Site Walkabout
A site visit by a geotechnical engineer may be somewhat bewildering to spectators
as the engineer pokes around in adjacent hillsides or ravines, or even goes across the
street or around the corner to observe existing structures. Such a seemingly casual
inspection of nearby areas can provide important clues to site conditions and geo-
logy. For example, soil or rock layers exposed in a nearby ravine may be the same as
those underlying the site—but don’t count on it without other confirmation.

Often the most important clues to landslides or expansive clays are signs of
distress in existing structures. Such clues can be as subtle as a pavement heaved
up in the middle, or a scarp or step that may be devoid of vegetation or may
support a line of younger trees extending laterally across a hillside. A series of
shallow depressions can warn of the existence of subterranean caverns, or a mine
or tunnel. Such observations can be extremely important because the causes can
be missed by routine pattern boring; borings can reveal only what is encountered
by drilling, not what is in between. Regardless of how many holes are bored,
soil conditions between the borings are interpolated based on circumstantial
evidence, a knowledge of the local geology, and general observations.

1.4.3 Drilling Program

The purpose of exploration drilling is to identify, sample, and test the soils.
Drilling programs sometimes are laid out without a preliminary site visit but only
if the engineer is familiar with and experienced in the area. Sometimes an initial
investigation is made to select or evaluate different possible building sites and give
information relative to design and construction. A design not only must be safe,
it must be ‘‘buildable’’ within the economic and time constraints of a project.

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