Civil Engineering Reference
In-Depth Information
Chapter 1
Classification and Physical
Properties of Soils
In the field of civil engineering, nearly all projects are built on to, or into, the ground. Whether the project
is a structure, a roadway, a tunnel, or a bridge, the nature of the soil at that location is of great importance
to the civil engineer.
Geotechnical engineering
is the term given to the branch of engineering that is
concerned with aspects pertaining to the ground. Soil mechanics is the subject within this branch that
looks at the behaviour of soils in civil engineering.
Geotechnical engineers are not the only professionals interested in the ground; soil physicists, agricul-
tural engineers, farmers and gardeners all take an interest in the types of soil with which they are working.
These workers, however, concern themselves mostly with the organic topsoils found at the soil surface.
In contrast, geotechnical engineers are mainly interested in the engineering soils found beneath the
topsoil. It is the engineering properties and behaviour of these soils which are their concern.
1.1 Agricultural and engineering soil
If an excavation is made through previously undisturbed ground the following materials are usually encoun-
tered (Fig.
1.1)
.
Topsoil
A layer of organic soil, usually not more than 500 mm thick, in which humus (highly organic partly decom-
posed vegetable matter) is often found.
Subsoil
The portion of the Earth's crust affected by current weathering, and lying between the topsoil and the
unweathered soil below.
Hardpan
In humid climates humic acid can be formed by rainwater causing decomposition of humus. This acid
leaches out iron and alumina oxides down into the lower layers where they act as cementation agents to
form a hard, rock-like material. Hardpan is difficult to excavate and, as it does not soften when wet, has
a high resistance to normal soil drilling methods. A hardpan layer is sometimes found at the junction of
the topsoil and the subsoil.
