Engineering Rock Mechanics

16 Geological sethg

Figure 2.6 Tunnel in mudstone which has deteriorated over a period of several

years after excavation.

approach already shown in Fig. 1.12. Moreover, the explicit site conditions
will be taken into account in the project design and analysis. So let us
consider now what can be said generally about the influence of geological
history on rocks and rock masses, ideas that will apply to all sites and all
proposed site investigations, whether for civil or mining engineering.

2.3 The influence of geological factors on
rocks and rock masses
Five main subjects are discussed below in terms of the influence of
geological factors on rocks and rock masses. In the context of the mechanics
problem, we should consider the material and the forces applied to it.
We have the intact rock which is itself divided by discontinuities (the
latter word being a generic term for all rock fractures) to form the rock
structure. We find then that the rock is already subjected to an in situ
stress. Superimposed on this fundamental mechanics circumstance are
the influences of pore fluiddwater flow and time. In all of these
subjects, the geological history has played its part, altering the rock and
the applied forces, and the engineer should be aware that these will
have been significantly affected by the geological processes, as explained
below.

2.3. I lntact rock

Intact rock is defined in engineering terms as rock containing no signifi-

cant fractures. However, on the small scale it is composed of grains with

the form of the microstructure being governed by the basic rock forming

processes. Subsequent geological events may affect its mechanical