Figure 7 – Subdivision of Project into Components and Construction Activities in Risk Identification,
Assessment and Rating of Risk Factors
nature or discipline such as hydrology, hydraulics,
geology, geotechnical, structure, mechanical,
electrical etc. The process performed in the risk
analysis involved the following:
● ●
Separating the project into its component
parts and functions.
● ●
Identifying the construction activity or
influence that has potential to impact
negatively the design, construction and
operation of the project, a third party or the
environment (illustrated in Figure 7).
● ●
Analysing risks of each factor, its probability
of occurrence and consequences.
A project risk register describing the risks and
source of these risks and the parties responsible
for them was drawn up. An assessment of these
risks was undertaken by evaluating their potential
impact or consequences and their likelihood of
occurrence, from which a risk rating was derived.
A project risk management plan detailed the
initial risk control action to reduce the rating of
each risk such that the residual probability and
impact was reduced to an acceptable level. The
register was under continual review and revision
throughout the construction stage. An initial risk
control plan with mitigation measures was applied
for risks with a medium to high rating, but for
those with a low rating, monitoring was sufficient.
In cases where the consequences or impact
were high but the probability of occurrence was
uncertain, a Level 2 risk analysis was undertaken.
Level 2 Risk Assessment
Altogether 13 risk factors were identified for
Level 2 risk assessment as shown in Table 1;
their locations are shown in Figure 8. A semi-
quantitative risk analysis was conducted. Some
risks constituted a threat throughout more than
one stage of the project from design, construction,
commissioning and operation. Some risks
had an impact only at the construction and
commissioning stage, while others had an impact
on the future operational safety of the dam. The
risk reduction measures (that would impact the
future performance of the dam) to be adopted
during the construction stage were identified.
The selection of mitigating options can be
more difficult for cases where the probability of
failure is low, but the consequences are very high
involving potential loss of life, extensive damage
to the environment and economic losses. In cases
where the decision making process involved
several parties a long lead time was required to
resolve the issues concerned.
Three risk factors identified were determined
to require Level 2 detailed analysis, as follows:
- Risk to the old dam due to construction of
new dam.
- Risk to the old dam due to construction of
dam raising works.
- Risk to pipes at left abutment of new dam
adjacent to the Weng Lee Quarry.
Level 3 Risk Assessment on Failure of the
Existing Dam
The existing dam was filled to its full supply level
EL 43m at the time of construction of the project.
During excavation for the foundation preparation
works at the left abutment of the old dam, a
concentrated leak was observed in an excavated
trench about 100m away from the existing dam.
The leak was coming from a thin porous layer of
highly weathered granite as shown in Figure 9. The
leakage through the existing dam abutment was
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