160 PermeabilivFigure 9.10 Effective stresses for intact rock and discontinuities.
water pressure of u is introduced, all normal components of the stress
tensor are effectively reduced by u. In the Mohr's circle presentation, this
results in the circle being moved to the left by an amount u, which could
result in the circle then reaching the failure locus, which is Case 2.
In Fig. 9.10(c), we show the more complex problem of dealing with a
water pressure u in a rock discontinuity, i.e. within the context of secondary
permeability. There are two problems as compared to the primary
permeability: first, the water pressure does not, depending on the location
of the element in question, act on all normal components of the stress
tensor; second, the water pressure is a local phenomenon, i.e. it is only
acting in the discontinuities (within the timescales of engineering
changes). Thus, the presence of the water could well have a profound effect
on the mechanical behaviour of the discontinuities, but a much lesser effect
on the behaviour of the intact rock. In fact, we have two effective stress
concepts: one for the intact rock and one for the discontinuities. It is difficult
to integrate these into a global effective stress law, as is illustrated in Fig.
9.10 (d), showing elements in proximity to a discontinuity: the stress tensors
are different for each of these elements.
9.7 Some practical aspects: grouting and
blasting
One of the main engineering solutions for reducing the permeability of a
fractured rock mass is to inject a grout, which may be a suspension (e.g.
cement grout), an emulsion (e.g. bitumens) or a solution (e.g. a silicate),