Environmental Engineering Reference
In-Depth Information
The foundation rock was curtain-grouted with three rows of grout holes beneath the entire
length of the dam and well into the abutments, and in places, blanket grouting was done.
The embankment was composed of several classes of compacted materials including: (1)
a core of clay, silt, sand, gravel and cobbles; and (2) a cover zone of selected sand, gravel,
and cobbles covered in turn at the lower elevations by rockfill.
During reservoir filling, when the water level was about 10 ft below sill level, minor
seepage was observed on the right bank downstream from the spillway on June 3. The
seepage was clear and flowing at about 20 gal/min, continuing at this rate until the morn-
ing of June 5 when it increased markedly to 50 to 60 cf/sec, although still remaining clear.
Within 1.5 h the seepage increased to about 1000 cf/sec, a whirlpool formed in the reser-
voir, and within a hour the embankment ruptured, releasing a wall of water downstream.
Two panels of experts decided that the cause of failure lay in the erodible, pipable silt
(loess) placed in the core trench in direct contact with the untreated, fractured rock. Water
flowing through the core trench, but over the grout curtain, opened a passage. One panel con-
cluded that the water flowed through cracks in the core material which was caused either by
differential settlement or by hydraulic fracturing from water pressure. The water flowing
through the fractures enlarged the openings by piping until failure occurred. Subsequently, it
has been thought that sealing the core trench walls with blanket grouting and designing the
filter layer between the core material and the trench walls might have prevented the failure.
Open Excavations
Most open excavations made below the water table require control of groundwater to per-
mit construction to proceed in the dry and to reduce lateral pressures on the retaining sys-
tem. The problems of bottom heave, boiling, and piping should also be considered.
Backslope subsidence and differential settlement of adjacent structures may be caused by
groundwater lowering, or by the piping and raveling of soils through the retaining struc-
ture if it contains holes.
Structures built below the water table must be protected against uplift when the water
table is permitted to rise to its original level and also against water infiltration and damp-
ness, as in basements.
Tunnels in Rock
High flows
under high pressures, occurring suddenly, are the most serious problem
encountered during tunnel construction in rock. Water pressures equal to full hydrostatic
head can cause bursting of the roof, floor, or heading, even in hard but jointed rock. High
flows, but not necessarily high pressures, can be encountered in porous rocks such as
vesiculated lavas or cavernous limestones.
Squeezing ground
refers to the relatively weak plastic material that moves into a tunnel
opening under pressure from surrounding rocks immediately upon exposure and can be
aggravated by seepage forces.
Running ground
, the sudden inrush of slurry and debris under pressure, occurs in
crushed rock zones, shear zones, and fault zones. The materials in fault zones can be highly
fragmented and pervious, saturated, and lacking in cohesive binder. Such materials are
usually associated with the foot wall or hanging wall of a fault, as illustrated in
Figure 8.34.
When encountered in excavation, the saturated debris flows as a slurry into the tunnel,
often under high initial pressures and quantities. Sharp et al. (1973) cite a case where
tunneling for the San Jacinto Tunnel in California encountered maximum flows of 16,000
gal/min (60,800 L/min) with water pressures as high as 600 psi (40 tsf). As is often the case,
the flows were associated with a fault zone filled with clay gouge. On the hanging wall
