INGENIEUR
failure model related to internal erosion at the
abutment soils was considered likely to occur
during foundation excavations at the left abutment
of the existing dam.
Assessing Whether Erosion Will Initiate
The hydraulic shear stresses (HST) in the leakage
conduit were computed based on the Wan and
Fell method (2004). The reservoir level assumed
leakage conduit dimensions and location
relative to the reservoir surface were taken into
consideration so the hydraulic gradient could be
determined.
The HST computed was compared to the
critical shear stress (CST), at which erosion of
the soil in the abutment will initiate. CST was
estimated based on results by Wan and Fell (2002)
who developed a method to estimate critical shear
stress using laboratory test results of the Hole
Erosion Test. Piping analysis using an analytical
method was faced with uncertainty on factors
such as the hydraulic gradient, permeability,
variability of materials and their properties
and erodibility of materials. A probabilistic risk
assessment was considered more appropriate to
assist in the decision making process to address
the issue of the safety of the existing dam due to
construction works.
Postulated Breach Mechanism and Event
Tree Construction
An internal erosion failure mechanism might result
from progressive sloughing and unravelling of the
downstream abutment slope as a result of flows
undercutting and eroding of the abutment, until
the reservoir is breached at which point rapid
erosion of the abutment soils is likely to cause
instability in the embankment and eventually
creating a breach to the dam.
If internal erosion and piping failure modes
were to occur, it would be difficult to detect during
rainy days and at night. The failure modes could
be well developed and in progress by the time
they were detected. Once the abutment to the
reservoir was breached, rapid enlargement and
complete loss of the reservoir filled with 22 MCM
of water could occur. Also, even if the breach
was detected, there are uncertainties on the
evacuation efficiency. Figure 11 shows the Event
Tree constructed based on the postulated failure
mechanism.
Probabilistic Risk Analysis
The assigned conditional probabilities for each
event tree were based on a method proposed
by Fell, R et al, in 2008. In the event tree, the
Figure 11 – Event Tree for Postulated Failure Modes
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VOL
VOL 73
55 JANUARY-MARCH
JUNE 2013 2018