Environmental Engineering Reference
In-Depth Information
Potential problems here in
downstream uplift and slope stability
Reservoir level
Piezometric level
post-reservoir
Water level
river
Piezometric level
prereservoir
Dam
River level
Low permeability rock
Regional aquifier
FIGURE 8.33
Possible stability problems caused by reservoir changing the regional piezometric system. (From Patton, F.D.
and Hendron, Jr., A.J.,
Proceedings of the 2nd International Congress,
1974.)
The rock foundation of Malpasset Dam was a tightly jointed, finely fissured gneiss con-
sidered as competent, with more than adequate strength to support the compressive
stresses and arch thrust. Failure studies hypothesize that as the water load behind the dam
rose to 58 m, the dam deflected slightly and placed the rock under the heel in tension,
causing the normally tight fissures in the gneiss to open slightly, increasing its permeabil-
ity. As the water level rose, uplift pressures increased and a progressive displacement of
the foot of the dam and rotation of the shell began. The shell transferred a tremendous
thrust to the left abutment that failed, resulting in the collapse of the dam.
Seepage beneath dams is normally controlled by grouting, constructing a cutoff trench,
or installing a pressure relief system (see
Section 8.4.7)
,
none of which was provided at
Malpasset because of the apparent tightness of the rock. Since Malpasset, it has become
standard practice to drain the foundation rock beneath concrete-arch dams
(Figure 8.50).
Seepage Losses and Piping Failures
Seepage can account for large losses of storage through abutment and foundation rock that
is porous, heavily fractured, or cavernous, and, in the case of earth dams, can cause the pip-
ing of embankment materials, which can lead to failure. During the inspection of earth
dams, all seepage points are noted, including flow quantities and water condition.
Normally, the inspector becomes concerned when flows are muddy, which indicates that
piping and erosion are occurring. In the case of the Teton Dam failure of June 5, 1970, cited
below, however, seepage was reported to have remained clear, almost to the time of failure.
The collapse of the Teton Dam in eastern Idaho, which resulted in 11 deaths and $400
million in damages, was caused by uncontrolled seepage (Penman, 1977; Civil
Engineering, 1977; Fecker, 1980).
Below the valley floor of the Teton River were alluvial materials. Underlying the allu-
vium and exposed along the valley sides was a badly fractured and porous rhyolite,
described as a welded tuff. A core trench was excavated through the alluvium and back-
filled with loessial soils compacted slightly on the dry side of optimum moisture. The core
trench was placed in direct contact with untreated, fractured rock. In both abutments the
rock was badly fractured near the surface, so key trenches 70 ft deep were excavated by
blasting and backfilled. The sides of the key trenches were steep (0.5H:1V). At the bottom
of the key trench, one or three rows of holes were drilled, and grout pumped into them.
