Environmental Engineering Reference
In-Depth Information
INITIATION
Leakage exits on
downstream side of
core and backward
erosion initiates
→
CONTINUATION
Continuation of
erosion
→
→
BREACH/FAILURE
Breach
mechanism forms
PROGRESSION
Backward
erosion
progresses to
form a pipe
(a)
Backward erosion piping
INITIATION
→
CONTINUATION
→
PROGRESSION
Enlargement of the
erosion hole to form
a concentrated leak
→
BREACH/FAILURE
Breach
mechanism forms
Concentrated leak
forms and erosion
initiates
Continuation of
erosion
(b)
Concentrated leak piping
Figure 10.31.
Model for development of failure by piping in the embankment (a) backward erosion,
and (b) concentrated leak (Foster, 1999)
Potential breach mechanisms are:
-Gross enlargement of the pipe hole;
- Unravelling of the toe;
-Crest settlement, or sinkhole on the crest leading to overtopping;
- Instability of the downstream slope.
Figure 10.32
shows a failure path diagram illustrating the possible sequence of events
leading to dam breaching.
Only marginal increases in the permeability of the core may be required to increase
pore pressures in a low permeability downstream zone sufficiently to initiate downstream
sliding, so it is assumed that the progression of piping to form a hole is not necessarily
required for this failure mechanism. It is assumed that failure by gross enlargement of the
pipe or unravelling of the toe requires continuing flow and therefore this is only possible
if the pipe remains open.
10.6.3
Piping through the foundation
Piping in the foundation initiates by one of four processes: concentrated leak, backward
erosion, suffusion, or blowout/heave followed by backward erosion.
As for piping through the embankment, it is possible to develop a single failure
path diagram for the assessment of concentrated leak piping and backward erosion pip-
ing.
Figures 10.33
and
10.34
show the failure path diagram for piping through the
foundation.
