Estimation of Rock Participation Factor through Hydro Tests of Pressure Shafts: A Case Study

Abstract

Pressure shafts for conveying water from fore-bay reservoir to power generating units are designed by taking advantage of surrounding rock mass in sharing internal design pressure. The ratio of internal pressure shared by surrounding rock mass to design pressure is known as rock participation factor (RPF). It is a very important parameter in economizing the cost of pressure shafts through saving of high strength steel. However, to ascertain share of pressure to be transferred to surrounding rock mass is very complicated due to involvement of physical parameters of surrounding rock mass filled concrete and steel liner. Hydro test provides an effective means of ascertaining the structural validation and performance evaluation of pressure shafts along with branching having complex geometry. This paper presents a case study of Varahi Hydroelectric Project, Karnataka, India. The pressure shafts of Varahi projects have been designed with 50% RPF. For verification of structural design and estimation of RPF, hydro tests of pressure shafts have been conducted in two stages by CWPRS, Pune, India. In the first stage, after fabrication of prototype, hydrostatic test have been conducted up to 0.8 time of yield strength of steel without any rock participation. After completion of construction of the project, second stage hydro test of pressure shafts has been conducted during running condition of the generating units. Based on the results of measurement, the RPF has been found to be more than the design value, thus complementing the structural safety of the water conductor system.