2012 – Angat Multipurpose Dam Remedial Works Project

John Grimston, David Leong, Robin Dawson

The Angat Multipurpose Project, originally constructed in the 1960’s, is located 60 km north-east of Manila, and provides power, irrigation and domestic water supply and flood mitigation. The major water-retaining structures of the scheme are a 131 m high main rockfill dam and a 55 m high rockfill saddle dam.
Previous seismology studies have identified the presence of a possible branch of the West Valley Fault crossing under the saddle dam. If the fault dislocated, the branch under the saddle dam could produce horizontal and vertical shear displacements. Further, earthquake shaking poses a risk outside the fault zone. If the main dam/saddle dam were to fail in such an event, there would be major consequences in respect to both the water supply (serves a population of approximately 10 million) and the large population living below the dams. The dams are thus in the highest hazard category under any internationally accepted standard.
A study to investigate the dam safety aspects and identify remediation works which would bring the seismic performance of the main dam/saddle dam system up to an acceptable level was undertaken and included:

• Investigations and topographic survey of main dam/saddle dam
• seismic dynamic response studies
• review of current Probable Maximum Flood (PMF) to assess spillway capacity
• preparation of remedial actions plan for dam remedial works
• dam break analysis
• preparation of Emergency Action Plan
• site specific seismic hazard assessment
• preparation of concept design for remedial works including Design-Build contract documentation.

The main conclusions were:

• the peak PMF inflow into Angat reservoir is now estimated to be 12,000 m3/s compared with the previous PMF estimate of 8400 m3/s
• the ultimate discharge capacity of the spillway before the dam is overtopped at the abutments (assuming zero freeboard) is 7,180 m3/s
• the spillway capacity is just short of the PMF standard and, the ultimate capacity of the spillway corresponds with about a 70,000 year return period flood event based on consideration of flood and volume frequency analyses of historical floods
• an auxiliary spillway would be needed to safely pass the PMF
• the main dam/saddle dam require remediation due to the potentially high degree of seismic shaking and the potential for fault dislocation under the saddle dam.

Keywords: Dam, Remedial, Seismic, Fault, Spillway.