Engineering Rock Mechanics

20 Geological setting

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Figure 2.12 Complete stress-strain for chalk-low stiffness, low strength, quite

brittle.

During the lithification process and throughout geological history, there

have been orogenic periods and other less severe loading processes applied

to the rock. The result in terms of the rock fracturing is to produce a

geometrical structure (often very complex) of fractures forming rock blocks.

An example of such a structure is shown in Fig. 2.14.

Because in the general uses of mechanics and stress analysis it is assumed

that a material is continuous, these geological features such as faults, joints,

bedding planes and fissures-all of which can be significant mechanical

breaks in the continuum-are termed 'discontinuities' for engineering

purposes. These discontinuities have many geometrical and mechanical

features which often govern the total behaviour of the rock mass. The

discontinuities will have certain shapes, certain sizes and be orientated in

certain directions. The overall geometrical configuration of the discontinui-

ties in the rock mass is termed rock structure. For engineering purposes,

it is vital that we understand this geometrical structure, as will be explained

further in Chapter 7.

Although the rock engineer is primarily concerned with the mechanical

behaviour of the rock, it is very helpful to understand the way in which

the discontinuities were formed and hence to have an initial idea of their

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Figure 2.13 Complete stressstrain curve for rock salt-low stiffness, low strength,

ductile.