Civil Engineering Reference
In-Depth Information
The structure of the Earth
4.1 Introduction
Soils occur very near the surface of the Earth and are essentially the products of the
action of the weather and the climate on rocks. Weathering of rock
in situ
leads to
the formation of residual soils. These may be eroded, transported and laid down as
deposited soils. The engineering properties of soils and how they occur in the ground
depend to a great extent on their geological origins and so geotechnical engineers need
to know something about geology.
In this one chapter I cannot possibly cover the whole of geology, or even all the
parts related to engineering. You will find a number of simple and easy-to-read topics
on geology for engineers and on engineering geology and you will probably attend
lectures on the subject. What I want to do here is set down what I consider to be the
most interesting and important aspects of geology related to geotechnical engineering
in soils. This is my personal list and other geotechnical engineers and geologists will
probably want you to know about other things. This does not really matter because if
you want to be a good geotechnical engineer you will need to study geology in some
detail.
4.2 The Earth's crust
The Earth has a radius of about 8000 km and a crust of soils and rocks about 25
to 50 km thick (see Fig. 4.1(a)). The ratio of the thickness of the crust to its radius
of curvature is about the same as that of an eggshell. Below the crust is a mantle of
hot plastic material and plates of crust move about on the mantle. This drift of the
continental crust accounts for mountain building, earthquakes and volcanic activity
at boundaries between the plates. It also accounts for evidence of glacial deposits
in Australia and tropical soils in Antarctica. In a single core of rock taken almost
anywhere on Earth, there will be rocks deposited in conditions that were like all the
known present-day environments.
The surface of the crust (the land surface and the sea bed) has altitudes and depths
above and below mean sea level of the order of 8 km (see Fig. 4.1(b)). Materials near
the surface are soils and rocks although there is not a very clear distinction between
the two; at low stresses soils fracture like rocks while at high stresses rocks will deform
plastically like soils. For engineering purposes soils rarely occur below a depth of
about 300 m (see Fig. 4.1(c)). Geologically old soils (older than about 2 millions years)
