Dams and Reservoirs - Hydrogeology: Objectives, Methods, Applications - page 290

Figure 135 Malpasset dam (Var). Relationship between the dam and the geologic structure
in map view and cross-section (from Bellier, 1967).
dips upstream and passed underneath the dam at a depth between 15 and
30 m depending on the location.
In addition, tests on the gneissic rock revealed a spectacular decrease
in its permeability under pressure, with an average difference of 100
between the permeabilities measured at pressures of 1 bar and 50 bars
(Bellier, 1967).
The results of the studies indicate that the upstream-dipping fault
played an important role as a hydraulic barrier, behind which the foliations
of the rock, compressed by the pressure P of the dam, closed and created a
veritable “impermeable coating” between the base of the support and the
fault. The infi ltrations possible because of the foliation and induced by the
fi lling of the reservoir enabled the creation, under the double hydraulic
barrier, of an intolerable hydrostatic pressure H. The upward pressure R
developed in this zone then led to a true cleavage of the supporting rock and
freed a dihedral mass of rock limited by the two “impermeable” surfaces
(Figure 136).
The structural conditions, however, enabled the dissipation of pressure
below the right bank support.