Environmental Engineering Reference
In-Depth Information
INITIATION
CONTINUATION
PROGRESSION
BREACH/FAILURE
Leakage exits the core
into the foundation and
backward erosion
initiates as core erodes
into the foundation
Continuation
of erosion
Backward erosion
progresses to form a
pipe. Eroded soil is
transported in the
foundation
Breach mechanism
forms
Figure 10.38.
Model for development of failure by piping from the embankment into the foundation
(Foster, 1999).
INITIATION
OF EROSION
CONTINUATION OF
EROSION
PROGRESSION
TO FORM A PIPE
FORMATION OF A
BREACH MECHANISM
Crest settlement or
sinkhole on dam or
abutment
or
Gross enlargement
of pipe
or
Unravelling of toe
or
Saturation/high
pore pressures in
d/s slope
Crest
overtopping
Dam storage
empties
Crest subsides
leading to
overtopping
Pipe remains
open
Unfiltered exit
point of seepage
or
Filtered exit that
allows excessive
or continuing
erosion and open joints or
gravel which allow excess
or continuing erosion
Erosion into
open joints or
coarse gravels
Breach
Piping hole
enlarges
Downstream
slide
(for low permeability d/s zone)
Pipe collapses
Crest
overtopping
Crest settlement
or sinkhole
Figure 10.39.
Failure path diagram for failure by piping from the embankment into the foundation -
backward erosion piping (Foster, 1999).
10.6.4
Piping from the embankment into the foundation
Piping from the embankment to the foundation involves backward erosion, or suffusion
initiated by erosion of the embankment soil into open joints or open gravels in the foun-
dation. Figure 10.38 shows conceptually the model for development of failure. Figure 10.39
gives a failure path diagram.
10.7
FACTORS INFLUENCING THE LIKELIHOOD OF INTERNAL
EROSION AND PIPING
Foster (1999), Foster and Fell (1999b, 2000) present a detailed discussion of the factors
which influence the likelihood of initiation and continuation of erosion, progression to
form a pipe and formation of a breach of the dam. The following is a summary of the
information, presented mainly to assist those who are assessing existing dams, but the
points are also a useful guide to good practice to reduce the likelihood of piping problems
in new dams.
