Engineering Geology

Chapter 9
Impervious linings consume large amounts of head near the source of water, thereby
reducing hydraulic gradients and saturation at the points of exit and increasing resistance to
seepage loss. Clay blankets or layers of silt have been used to seal exits from reservoirs.
Because of the occurrence of permeable contacts, close jointing, pipes, and the possible
presence of tunnels and cavities, recent accumulations of basaltic lava flows can prove highly
leaky rocks with respect to watertightness. Lava flows frequently are interbedded, often in an
irregular fashion, with pyroclastic deposits. Deposits of ash and cinders tend to be highly
permeable.
Reservoir sites in limestone terrains vary considerably in their suitability. Massive horizontally
bedded limestones, relatively free from solution features, form excellent sites. On the other
hand, well-jointed, cavernous and deformed limestones are likely to present problems in
terms of stability and watertightness. Serious leakage usually has taken place as a result of
cavernous conditions that are not fully revealed or appreciated at the site investigation stage.
Indeed, sites are best abandoned where large numerous solution cavities extend to considerable
depths. Where the problem is not so severe, solution cavities can be cleaned and
grouted (Kannan, 2003). In addition, reference has been made by Milanovic (2003) to the
application of a blanket of shotcrete to seal areas of karstic rock in reservoir basins. However,
wet rock surfaces are not suitable as far as the application of shotcrete is concerned and
neither is it wise to allow groundwater pressure to build up beneath shotcrete.
Sinkholes and caverns can develop in thick beds of gypsum more rapidly than they can
in limestone. Indeed, in the United States, they have been known to form within a few years
in areas where beds of gypsum are located below reservoirs. Extensive surface cracking and
subsidence has occurred in Oklahoma and New Mexico due to the collapse of cavernous
gypsum. The problem is accentuated by the fact that gypsum is weaker than limestone and,
therefore, collapses more readily. Uplift is a problem that has been associated with the hydration
of anhydrite beneath reservoirs.
Buried channels may be filled with coarse granular stream deposits or deposits of glacial
origin and, if they occur near the perimeter of a reservoir, they almost invariably pose leakage
problems. Indeed, leakage through buried channels, via the perimeter of a reservoir, is usually
more significant than through the main valley. Hence, the bedrock profile, the type of deposits
and groundwater conditions should be determined.
A thin layer of relatively impermeable superficial material does not necessarily provide an
adequate seal against seepage. A controlling factor in such a situation is the groundwater
pressure immediately below the blanket. Where artesian conditions exist, springs may break
the thinner parts of the superficial cover. If the water table below such a blanket is depressed,
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