ABSTRACT
Pressurised waterways such as pressure tunnels and shafts are used as inlet structures in underground hydroelectric power stations. Particularly when a pressure shaft is driven along steep valley sides, the internal pressure in the tunnel may approach the surrounding rock stress. To prevent rock instability and excessive water leakage, impermeable tunnel linings are commonly installed in the deepest part of the pressure shaft. Up to date, the determination of the critical section of the shaft, beyond which lining is required, has been made with simple “rules of thumb” for lifting of the rock, and, alternatively, with a method which takes jacking into consideration. The latter method assumes that rock masses always contain critically oriented joints (parallel to the maximum principal stress). This paper demonstrates a complementary design method where both shearing and jacking of joints are considered. The proposed method requires that the geological preinvestigations include joint mapping and rock stress measurements. Based on these data and on an estimate of the friction angle of the most critical joint set, a rock mechanical design is proposed, based on a Mohr-Coulomb failure criterion with effective stresses acting on the rock joints. Out of this, the critical pore pressure inducing shearing and jacking along the pressure shaft is estimated. Some examples arc given to illustrate the method. The examples make use of a static water head; in a real design, however, the maximum dynamic water head should be compared with the critical pore pressure.
