Geoscience Reference
In-Depth Information
Introduction
1.1 SOIL ENGINEERING
Soil engineering or ground engineering, more commonly called geotechnical engineer-
ing, is a branch of civil engineering that deals with soil, rocks and underground water,
and their relation to the design, construction and operation of engineering projects.
Nearly all civil engineering projects must be supported by the ground, and thus require
at least some form of ground/soil or geotechnical engineering.
1.2 TYPES AND FORMATION OF SOILS
To begin with, let us look at the various types of soils, their composition and how they
are formed. In term of composition, soils in engineering are generally considered to
comprise four main constituents: gravel, sand, silt and clay. Soils are therefore con-
sidered as particulate materials that consist of individual particles, and not as a solid
mass. Soils are also considered as three-phase materials, whereby they can simultane-
ously contain solid, liquid and gas phases. The liquid and gas phases are contained in
the voids or pores between the solid particles.
In their natural environment, soils rarely consist of one constituent only, but are
often a mixture of two or more constituents. For example, we have gravelly sand,
which means sand with some gravel, or sandy-silty clay, which means clay with some
sand and silt. Soils may also contain organic matter, which is essentially the remains of
plants. These soils are called 'organic soil' if their organic content exceeds 20% of their
dry mass. Organic soils in turn are termed 'peat' once their organic content exceeds
75%. The main reason for this definition is that when the organic content of the soil
exceeds 20%, its mechanical behaviour will start to depart from that of mineral soils
(i.e. gravel, sand, silt and clay). This will be explained in more detail in later chapters.
Soils are also described based on their formation, transportation and deposition.
The categories include residual soils, glacial soils, alluvial soils, lacustrine and marine
soils, aeolian soils and colluvial soils, as well as organic soils and peat.
1.2.1 Residual soils
If the rate of rock weathering is faster than the water, gravity and wind transport
processes, much of the resulting soil will remain in place. Such soil is known as residual
