4 - Design of Large Powerhouse Caverns in Weak Rock

Abstract

This chapter presents design of large powerhouse caverns in weak rock. Weak rock is rock that fails when subjected to the stress levels induced by the excavation of large underground caverns at depths of 100–300 m below surface. Sedimentary rocks such as bedded sandstones, shales, siltstones, and mudstones are among the rocks that fall into this category. Tunnels and caverns associated with underground hydroelectric projects are sometimes excavated in rock masses of this type. Estimating the strength and deformation characteristics of weak rock masses is an uncertain process, and large variations in properties can be anticipated, particularly in bedded sedimentary rocks. Precise analysis of the stresses and deformations, induced by the excavation of the caverns, is not possible and the designer has to rely on parametric studies, in which the in situ stresses and material properties are varied over their maximum credible range to establish general behavioral trends.