Environmental Engineering Reference
In-Depth Information
responsible for the investigation and design. The grouting as shown was highly successful,
closing to 1-3 lugeons. What was overlooked was that the dolerite sill daylighted in the
storage, was highly permeable due to jointing and water simply went around the ends of
the grout curtain. The water level in the storage dropped by
25 mm/day, and the water
could be heard flowing in the dolerite sill in a borehole about 100 metres downstream!
Similar problems can occur with cutoffs into foundations of alluvial or lateritic soils
(
Figure 10.3
)
and it is often necessary to take the cutoff beyond where the crest of the dam
meets the natural surface.
Another example of a 3 dimensional problem is seepage from tailings dams is given in
Figure 19.33 which shows the plan of the tailings storage for the Boddington Mine, Western
Australia. At the time of the first author's initial involvement, the emphasis on estimating the
seepage from the storage had been on the seepage under the dam structures. The foundation
rock was deeply weathered and lateritised and likely to have a high mass permeability. Hence
seepage would occur through the hills to the north and east, not just under the dams. The
groundwater table to the south was towards the dam. Being a tailings dam, the tailings them-
selves can control the seepage, except where the water on the tailings is in contact with the
foundation.
10.6
THE MECHANICS OF INTERNAL EROSION AND PIPING
10.6.1
Some general issues
For internal erosion and piping to occur four conditions must exist (adapted from von
Thun 1996):
1. There must be a seepage flow path and a source of water;
2. There must be erodible material within the flow path and this material must be carried
by the seepage flow;
3. There must be an unprotected exit (open, unfiltered), from which the eroded material
may escape;
4. For a pipe to form, the material being piped, or the material directly above, must be
able to form and support “roof” for the pipe.
Piping may occur in the embankment, foundation and embankment to foundation.
10.6.2
Piping in the embankment
Piping in dam embankments initiates by one of three processes: backward erosion, con-
centrated leak and suffusion.
Back ward erosion
piping refers to the process in which erosion initiates at the exit
point of seepage and progressive backward erosion results in the formation of a continu-
ous passage or pipe.
Concentrated leak
piping involves the formation of a crack or concentrated leak
directly from the source of water to an exit point and erosion initiates along the walls of
the concentrated leak.
Figure 10.31
shows conceptual models or the development of failure for backward ero-
sion piping and concentrated leak piping. The sequence of events leading to failure by the two
models is essentially the same, however the mechanisms involved in the initiation and pro-
gression stages are different.
Suffusion
involves the washing out of fines from internally unstable soils. Soils which
are gap-graded, or which have only a small quantity of fine soil in a mainly coarse sand
or gravel are susceptible to suffusion.
