Summary

The Batang Padang hydro-electric scheme in West Malaysia is located in an area of tropically-weathered granite. Major factors in the siting, design and construction of the engineering structures, which include three dams, twelve miles of tunnel and a power station 900 ft underground, were the nature of the residual soil mantle and the structure of the underlying rock.

Site investigation included over 10 000 ft of cored borings carried out in two stages. The first stage confirmed that there were no major geological impediments to the scheme and enabled basic design concepts to be established. Stage two obtained data for detailed design of the dams and tunnels. Principally because of the presence of deep weathering, earth-fill dams were the most economic and suitable. As the pressure tunnels were located mainly in competent granite with tight joints, unlined designs were chosen.

Exploration of the power station site was deferred until excavation of the tailrace tunnel permitted an exploratory adit to be constructed. The investigation included measurements of elastic modulus, strain measurements in drillholes from which were computed the in situ stresses in the rock, and a discontinuity survey. Laboratory tests were made on cores. The results confirmed that the cavern would be excavated in intact rock suitable for permanent support by bolts, gunite and mesh.

During construction it was possible to evaluate the success of the site investigation. Overall rock tightness led to a major reduction in the scale of grouting. Intact rock in the power station proved ‘defective’ in that it resulted in exfoliation of the cavern walls necessitating more extensive guniting than expected. A major geological problem proved to be tunnelling through residual soil below the water table. This led to flow slides necessitating tunnel diversions.

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