Environmental Engineering Reference
In-Depth Information
Table 20.6.
Influence of the material in the downstream zone of the embankment, or in the founda-
tion, on the likely time for development of a breach (Fell et al., 2001, 2003).
Material description
Likely breach time
Coarse grained rockfill
Slow - medium
Soil of high plasticity, including clayey gravels
Medium - rapid
Soil of low plasticity, all poorly compacted soils, silty sandy gravels
Rapid - very rapid
Sand, silty sand, silt
Very rapid
(d) Initiation of erosion by suffusion (internal instability) is likely to be a more slowly
developing process, accompanied by more gradual increases in seepage and changes
in pore pressure with time. Some cases where sinkholes develop in the reservoir
upstream of a dam founded on alluvial or fluvioglacial soils may be due to suffusion.
(e) “Blow-out” or “heave” in dam foundations where seepage forces create a zero effec-
tive stress condition is a situation which should be readily detected by carefully posi-
tioned and well monitored piezometers. Often these low effective stresses occur below
lower permeability layers which act to confine the seepage flow and it is important
that piezometers are installed to measure pore pressures in these areas. These pres-
sures are usually directly related to reservoir levels and it is important to monitor the
relationship between the pore pressures and reservoir levels. Most often these condi-
tions will occur on first filling or at historic high reservoir water levels, but the
authors are aware of cases where pressures have increased with time possibly due to
suffusion in the foundation soils or blockage of drains and pressure relief walls.
(f ) Failures from piping in the foundation and from the embankment to the foundation
are mostly from backward erosion, or backward erosion following hydraulic fracture,
“blow-out” or “heave”. These would not necessarily be expected to be preceded by
large increases in seepage during the time the erosion is gradually working back from
the downstream exit point. When the erosion has progressed to within a short dis-
tance of the reservoir/foundation interface, it breaks through rapidly or very rapidly.
(g) Accidents are also usually detected in the progression phase, rather than in the initia-
tion and continuation phase. Progression ceases due for example to sealing of eroded
materials on filters/transitions which satisfy excessive erosion criteria, flow limitation
by an upstream dirty rockfill zone, or concrete face (on concrete faced dams), or due
to collapse of the pipe. The latter is possibly a more common mechanism in founda-
tion piping accidents than in embankments, because most embankment cores will
have sufficient fines to support a roof to the pipe.
(h) Timely intervention can change a potential failure into an accident. As discussed in
(b), changes in pore pressure, seepage, and settlement may be detected before internal
erosion progresses too far.
(i) The ease of detection of internal erosion and piping in the foundation by visual and
seepage means is more readily achieved if the area downstream of the dam is not veg-
etated, or is at least cleared of larger vegetation. Detection is difficult if the area down-
stream is densely vegetated or if the dam toe area is covered by water.
(j) Detection by pore pressure measurement is more likely if there is extensive instru-
mentation read regularly and if the erosion is widespread.
(k) The frequency at which seepage is measured can be related to the likelihood that a
dam may fail or experience an accident by internal erosion and piping, the conse-
quences of failure and the likely time for internal erosion to progress to form a pipe
and the dam breach. It might also be related to the reservoir level, with enhanced
measurements at times of high reservoir level.
