Permeability and Seepage Flow - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 161

The investigation of the disturbed rock zone around underground openings in rock
masses with low permeability such as granite or rock salt with the aid of in-situ per-
meability tests (Section 15.8.2) allow us to derive an expression for the specifi c stor-
age coeffi cient for confi ned aquifers which account for all the above-mentioned effects
(e.g. Stormont et al. 1991, Wittke 1999). The specifi c storage coeffi cient of jointed rock
where seepage fl ow in the intact rock can be neglected can be calculated by a corre-
sponding expression:
(6.75)
The specifi c storage coeffi cient of unjointed porous rock can be calculated by a formula
which was originally derived for soil (e.g. Freeze & Cherry 1979):
(6.76)
In (6.76) the compressibility of the grains and of potential isolated voids is neglected.
Comparison of (6.75) and (6.76) with (6.63) and (6.64) indicates that the specifi c storage
coeffi cient of a confi ned aquifer is orders of magnitude smaller than that of an uncon-
fi ned aquifer.
Considering (6.75) and (6.76), the specifi c storage coeffi cients of a confi ned double
porous aquifer such as the jointed porous sandstone illustrated in Fig. 6.19 can be ex-
pressed as
(6.77)
(6.78)
It should be noted that the defi nition of a specifi c storage coeffi cient does not make
reference to the state of stress existing in the rock. Therefore, S 0 is only a useful quantity
if the infl uence of stress changes in the rock mass due to external forces on seepage fl ow
(Chapter 7) is ignored.
6.4.3 Seepage Force and Hydrostatic Uplift
Fig. 6.22 shows the lines representing equal piezometric heads, the so-called “equi-
potentials”, of a seepage fl ow through a slope in a horizontally and vertically jointed
rock mass. In addition, this fi gure shows the distribution of piezometric heads
which act on an individual rock block in the slope if the intact rock is assumed to
be impermeable. This can be subdivided into a pressure head distribution and a
distribution of geodetic heights as illustrated in Fig. 6.22. The corresponding water
pressure given by Equation (6.7) is obtained by multiplying the pressure head by the
unit weight of water (blue lines in Fig. 6.23). The resultant of the water pressure can
be separated into a hydrostatic component (represented as green areas in Fig. 6.23)
and a hydrodynamic component (represented as red areas in Fig. 6.23).