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Dam Construction

Excavation for power house

Philippines: Located about 300 km north of Manila, the San Roque Multipurpose Project involves the construction of a very large embankment dam, a 345 megawatt hydropower plant, two diversion tunnels, and a number of other structures.

When completed, the dam will be 200 meters high and nearly 1100 meters long, making it Asia's largest embankment dam and the world's 12th highest rockfill dam. The two diversion tunnels, an intake tunnel and an outlet tunnel are both over 1200 meters long. The dam will create a reservoir of approximately 850 million cubic meters and is expected to supply irrigation water for about 87,000 hectares of farm land.

The powerhouse box (above) was cut to a depth of 77 meters by blasting and excavating. Rod extensometers, were used to monitor movements of the rock wall during the excavating process. If excessive movements occurred, additional rock anchors would be installed to stabilize the walls.

Extensometer dataThe rod extensometers proved their worth during the excavation process. While they detected only acceptable movement, including some movement during a low-magnitude earthquake), the extensometers provided assurance that the walls were stable. This allowed the excavation to proceed more rapidly, since no additional rock anchors were required.

Other instrumentation included twenty-seven vibrating wire piezometers, seven pneumatic piezometers, and four open standpipe piezometers. The piezometers are Installed within the dam and its foundation to monitor pore-water pressure, water levels, and seepage. The vibrating wire and pneumatic piezometers are used to check each other for accuracy. Additional vibrating wire piezometers were installed at an upstream coffer dam and at the intake tunnel along with a VW MiniLogger.

Wiring a rod extensometerSlope Indicator's representative in the Phillipines, Drilling Advisory & Logistics Corp, supplied the instrumentation. The photograph at left shows the final steps of installing a rod extensometer. All instruments are now in place and monitoring is ongoing.

The main contractor for the project, Raytheon Ebasco Overseas Limited, was assisted by geotechnical engineers from United Engineers International Golder associates Inc. These engineering firms made improvements to the 20-year old design, modifying the upstream cofferdam section, utilizing excavated material that would otherwise have been spoil, and widening the dam at its crest. The filter, transition, and drain zones of the dam were modified to comply with current filter criteria and reduce the required quantity of processed materials. The engineers also conducted liquefaction studies and then set foundation excavation depths at a minimum of 2 meters and as much as 6 meters in certain areas.

Thanks to Mr Noel Frigillana, Engineer, of Raytheon Ebasco Overseas Ltd and to Mr Tito Tuason of Drilling Advisory for providing this story.

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