The influence of geological factors on rocks and rock masses 27flow in rock masses will be a function of the discontinuities, their
connectivity and the hydrogeological environment.
Both the stress and the water flow are significantly affected by engineer-
ing activity. As will be explained later in greater detail, all excavation affects
the stress state because all the unsupported excavation surfaces are principal
stress planes and all excavations act as sinks because the in situ hydraulic
pressure is reduced to atmospheric pressure on the excavation boundary.
Thus, in the present context we need to understand the nature of perme-
ability and water flow in fractured rock mass systems. Moreover, as will be
discussed in Chapter 14, there can be interactions between the stress and
the permeability, both naturally and as they are affected by engineering
activities.
Depending on the engineering objective, the above considerations may
be enough-but there are some applications in which an understanding of
fluid flow through intact rock can be critical, e.g. in reservoir engineering
for the petroleum industry. Alternatively, a case where the water flow
through the fractures is particularly important is in hot dry rock geothermal
energy projects where the success of the whole project depends on achieving
the required borehole-to-borehole water flow. An extreme example is the
case of radioactive waste disposal where the engineer can only state that the
design of the waste repository is valid if the radionuclide dosage back to the
biosphere can be calculated, and this can only be done if the three-dimen-
sional flow of water through fractured rock masses can be accurately
modelled. Because of the long operational life in this latter application, the
permeabilities of both the intact rock and the fractured rock mass must be
understood as well as other factors such as sorption of radionuclides onto
rock fracture surfaces. Many groups have studied the age of the water
present in the rock to assist in the approach to this problem, again reinforcing
the point that an understanding of the geological setting is fundamental.
There are several other aspects of pore fluids and water flow that may
be important in specific cases, such as groundwater chemistry, the
formation of caves and rock alteration by fluid movement. The subject of
water flow recurs throughout this book.
2.3.5 Influence of time
Another major factor of importance is the influence of time. On the one
hand, our engineering material is millions of years old and, on the other,
our engineering construction and subsequent activities are generally only
designed for a century or less. Thus, we have two types of behaviour: the
geological processes in which equilibrium will have been established, with
current geological activity superimposed; and the relatively rapid engi-
neering process. Over the millions of years, in some areas, the in situ rock
stresses will have been continually in a stable state yet, in other areas, the
stresses will have been continually altered by tectonic activity. Similarly,
the pore pressures even in the most impermeable of rocks will have
stabilized, but geological activity could be causing overall hydrogeological
changes. In contrast, the reponse of the rock to engineering occurs over a
very short time.