Water: Surface and Subsurface - Geotechnical Engineering Investigation - page 653

Water quality is of concern in water supply from wells and with respect to the deteriora-
tion of concrete and the corrosion of other materials. Groundwater requires protection
against pollution not only from the aspects of water supply but also from the aspects of
contamination of adjacent water bodies, and the effect on aquatic life.
Groundwater and seepage control are discussed in Section 8.4 , and pollution control in
Section 8.5.2.
Water Supply
Soil formations normally providing suitable quantities for water supply are clean sands or
sand and gravel strata. Overwithdrawal on a long-term basis can result in surface subsi-
dence (see Section 10.2).
Rock masses constituting the most significant aquifers are some sandstones, intensely
fractured or vesicular rocks, or cavernous limestone. In igneous and metamorphic rocks,
groundwater sources are most likely to be found in major fault zones or concentrations of
joints. Groundwater extraction from soluble rocks can cause rapid cavern growth and
ground collapse (see Section 10.4).
Slopes
As discussed in Chapter 9, the increase in pore pressures in soil slopes and the increase in
cleft water pressures in rock slopes are the major causes of slope failures.
Flows from rock faces are common but can become dangerous when increased by rain-
water infiltration or blocked by freezing. A dry slope face does not necessarily indicate a
lack of seepage forces in the mass. In slow-draining masses, such as tightly jointed rock,
the face may be dry because the evaporation rate often exceeds the seepage rate, but seep-
age forces remain in the mass at some depth and their increase can result in failure.
Reservoirs
Reservoirs have a regional effect on groundwater conditions as rising reservoir levels
cause a regional rise in piezometric levels.
In rock masses, more or less horizontally bedded alternating pervious and impervious
strata can be the cause of high pore pressures in the foundation rock. Within the reservoir
area, the effect of rising piezometric levels is partially offset by the weight of the reservoir act-
ing on the valley floor and sides, but downstream piezometric changes can have substantial
effects, causing heaving of the downstream valley floor, boils (Figure 8.33), and slope failures.
Slope instability in the reservoir area can present a possible hazard when the slopes are
resting in a barely stable condition. Steep slopes with colluvium, or sedimentary rocks dip-
ping in the direction of the slope with the beds daylighting along the slope, are particu-
larly susceptible to slides when rising pool elevations change the hydrostatic conditions in
the slope. The slope failure at Vaiont Dam (see Section 9.2.3) appears to have been at least
partially triggered by rising water levels.
Dams
Foundation Stability
Stability analysis of dam foundations is performed as a matter of standard procedure, but
the case of the failure of the Malpasset Dam, near Frejus, France, bears some discussion
because the conditions were unusual and the results catastrophic. The buildup of seepage
pressures in the rock mass beneath the thin-arch concrete dam is considered to be the
cause of the failure on December 2, 1959, which took 400 lives (Jaeger, 1972).