Environmental Engineering Reference
In-Depth Information
PROGRESSION TO
FORM A PIPE
INITIATION
OF EROSION
CONTINUATION
OF EROSION
FORMATION OF A
BREACH MECHANISM
Backward
erosion initiates
Pipe
forms
Pipe remains
open
Unfiltered exit
point of seepage
or
Filtered exit that
allows excessive
or continuing
erosion
Voids form in
the foundation
Settlement of
crest/sinkhole
Crest
overtopping
Internal
suffusion
(erosion of
fines)
Breach
Crest subsides
leading to
overtopping
Slope
instability
Increase in
pore pressures
Blow out
Figure 10.35.
Failure path diagram for failure by piping through the foundation - suffusion (Foster,
1999).
Backward
erosion
initiates
Pipe
enlarges
Pipe remains
open
Dam storage
empties
Effective stress
condition,
blow out occurs
Breach
Slope
instability
Crest
overtopping
Figure 10.36.
Failure path diagram for failure by piping through the foundation - blowout followed by
backward erosion (Foster, 1999).
Foundation filter
Unfiltered exit
Filtered exit
Filtered exit
Unfiltered exit
Open-work gravel layer
Filtered exit
Bedrock, open-jointed
Figure 10.37.
Examples of filtered and free exit points for piping through the foundation (Foster,
1999).
Figures 10.35 and 10.36 show the failure path diagrams for failure by suffusion and
blowout, followed by backward erosion.
Terzaghi and Peck (1967) and Von Thun (1996) believe if blowout were to occur it
would be expected the first time the reservoir reached a critical elevation. However, if
pore pressures increase with time due to internal erosion processes, or for example grad-
ual deterioration of pressure relief wells, the risk of blowout may in fact increase with
time. This has been observed at Mardi Dam (PWD of NSW, 1967).
Figure 10.37 gives an example of filtered and unfiltered exit points for piping through
the foundation.
 
Search WWH ::




Custom Search