hydraulically connected to the existing reservoir . This observation prompted the need to critically assess the safety of the existing dam due to the construction of the project .
Purpose of Level 3 Risk Assessment
A quantitative risk assessment was conducted to determine whether the do-nothing option was acceptable and if not , whether lowering the reservoir water level would reduce the probability of breaching the dam to an acceptable level and how it would compare with the option of soil improvement works or the option of controlling the construction activities near the existing dam . The process involved in Level 3 risk assessment is shown in Figure 10 .
Risk Identification
The study was not aimed at assessing the overall probability of failure of the existing dam under all possible causative events . Thus , the following types of specific events were not included : events which cause a slip of the upstream shoulder of dam such as rapid draw down , floods which cause overtopping of dam ; and earthquakes which cause cracks , liquefaction or slip failure of the dam . The
Figure 9 - Concentrated Leak at left abutment of existing dam
risks associated with the above events would be similar for all the options assessed and the risks would not affect the overall risk comparisons . In so far as the reservoir water level was concerned , floods need not be considered as a separate causative event as the dam has adequate freeboard , has an ungated spillway and the flood risk is similar to all options .
Initiating Events
A plausible initiating causative event would be piping due to excavation at the abutment and the downstream toe of the existing dam . A potential
Figure 10 – Process in Level 3 Risk Assessment
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