Engineering Rock Mechanics

26 Geological setting

Figure 2.24 (a) Locations of the largest surface mine and largest underground mine
in the world. (b) Subduction zone off the coast of Chile.

rock stress environment, it would be impossible to understand the
mechanics of this rockburst.
Similar, although not so extreme, circumstances exist throughout the
world due to the tectonic activity that is currently underway. Of course,
there have been past orogenic events so that the rock has a stress history.
Moreover, there are also factors such as surface topography and erosion
which affect the in situ stress state.
There are ways of estimating the in situ stress state from geological
indicators and there are ways of measuring the stress directly by engi-
neering techniques. The subject of stress will be explained in detail in
Chapter 3. The distribution of in situ stress values throughout the
world will be discussed in Chapter 4. We cannot over-emphasize the
importance of in situ stress because of its natural origin, ubiquity and
because it is one of the boundary conditions for our mechanics
considerations.

2.3.4 Pore fluids and water flow

In soil mechanics, the concept of pore fluid is fundamental to the whole

subject. This is because most soils have been formed by the transportation

and depostion of discrete particles with significant space around them for

water to move through the soil. The water can be under pressure and hence

reduce the effect of the applied stresses described in Section 2.3.3. This

leads to the concept of effective stresses which have proved to be so

important in soil mechanics, both from the theoretical and applied points

of view.

However, rock masses have not been reconstituted in the same way as

soils, although rock masses are all fractured to a greater or lesser extent.

This means that accounting for pore fluids and water flow is much more

difficult in rock mechanics than in soil mechanics. Many rocks in their intact

state have a very low permeability compared to the duration of the engi-

neering construction, but the main water flow is usually via the secondary

permeability, i.e. through the pre-existing fractures. Thus the study of water