Introduction - Geotechnical Properties of Soil

The design of foundations of structures such as buildings, bridges, and dams generally
requires a knowledge of such factors as (a) the load that will be transmitted by the superstructure
to the foundation system, (b) the requirements of the local building code, (c) the
behavior and stress-related deformability of soils that will support the foundation system,
and (d) the geological conditions of the soil under consideration. To a foundation engineer,
the last two factors are extremely important because they concern soil mechanics.
The geotechnical properties of a soil—such as its grain-size distribution, plasticity,
compressibility, and shear strength—can be assessed by proper laboratory testing. In addition,
recently emphasis has been placed on the in situ determination of strength and deformation
properties of soil, because this process avoids disturbing samples during field
exploration. However, under certain circumstances, not all of the needed parameters can
be or are determined, because of economic or other reasons. In such cases, the engineer
must make certain assumptions regarding the properties of the soil. To assess the accuracy
of soil parameters—whether they were determined in the laboratory and the field or
whether they were assumed—the engineer must have a good grasp of the basic principles
of soil mechanics. At the same time, he or she must realize that the natural soil deposits on
which foundations are constructed are not homogeneous in most cases. Thus, the engineer
must have a thorough understanding of the geology of the area—that is, the origin and
nature of soil stratification and also the groundwater conditions. Foundation engineering
is a clever combination of soil mechanics, engineering geology, and proper judgment
derived from past experience. To a certain extent, it may be called an art.
When determining which foundation is the most economical, the engineer must consider
the superstructure load, the subsoil conditions, and the desired tolerable settlement.
In general, foundations of buildings and bridges may be divided into two major categories:
(1) shallow foundations and (2) deep foundations. Spread footings, wall footings, and mat
foundations are all shallow foundations. In most shallow foundations, the depth of embedment
can be equal to or less than three to four times the width of the foundation. Pile and
drilled shaft foundations are deep foundations. They are used when top layers have poor load-bearing capacity and when the use of shallow foundations will cause considerable
structural damage or instability. The problems relating to shallow foundations and mat
foundations are considered in Chapters 3, 4, 5, and 6. Chapter 11 discusses pile foundations,
and Chapter 12 examines drilled shafts.
This chapter serves primarily as a review of the basic geotechnical properties of soils.
It includes topics such as grain-size distribution, plasticity, soil classification, effective stress,
consolidation, and shear strength parameters. It is based on the assumption that you have
already been exposed to these concepts in a basic soil mechanics course.