Back-analysis of Frictional Jacking Forces Based on Shear Box Testing of Excavated Spoils

Abstract

Trenchless technology has become the preferred technique for installation of buried infrastructure due to advantages such as minimised ground surface disturbances as well as reduced social disruptions. These benefits have led to an increased demand for trenchless technologies such as the pipe-jacking method, with its application well-suited for urban environments. The construction of trunk sewers in Kuching, Sarawak has been carried out using the pipe-jacking method. Critical to this micro-tunnelling technique were the associated frictional forces anticipated during the jacking process. Excessive jacking forces could damage installed pipes. The Authors have studied the use of a few well-established empirical equations to validate frictional resistance during jacking supplemented by laboratory direct shear box tests carried out on spoils obtained from pipe-jacking drives. The tested spoils were by-products of the slurry transport system for spoil removal at the tunnel face. Shear box test results have been attempted for use to predict the pipe-jacking forces based on the well-established empirical equations. The predicted jacking forces for this study show fair agreement when benchmarked against actual, measured forces. However, it is the intention of the Authors to carry out more tests to further verify the reliability of this method for a wider variety of local rock types, especially in geologically ‘young’ weathered rock where competent rock cores are usually difficult to obtain.