Assessment of non-linear rock strength parameters for the estimation of pipe jacking forces. Part 1. Direct shear testing and backanalysis

A method of backanalysis was used for the assessment of pipe-jacking forces from three pipe-jacking drives in laminated shale and interbedded lithological units of metagraywacke, siltstone, greywacke and phyllite found in the highly weathered and fractured lithologies of the Tuang Formation (Malaysia). Direct shear tests were conducted on reconstituted tunneling rock spoils sampled from three pipe-jacking sites. The direct shear test results were initially assessed using the Mohr-Coulomb failure criterion, which demonstrated severe shortcomings in capturing the nonlinear shear strength behavior of the reconstituted tunneling rock spoils. Subsequently, a simple power law function was proposed together with the generalized tangential method to obtain useful equivalent tangential cohesion, c'(t,p) and friction angle, phi'(t,p) for the assessment of pipe-jacking forces in fractured lithologies. The outcomes from this study show that the tangential Mohr-Coulomb strength parameters can be used to backanalyze the frictional coefficient, mu(avg) and the vertical stress at the tunnel crown, sigma(EV). These backanalyzed values were successfully related back to the measured jacking forces and jacking speeds for pipe-jacking drives in various geological conditions and lubrication efforts. The use of the backanalyzed parameters c'(t,p), phi'(t,p), mu(avg) and sigma(EV) in the assessment of pipe-jacking forces from highly weathered rocks comprising of laminated and interbedded lithological units could lead to the prediction of jacking forces using numerical models. A companion paper to this study has also been produced to evaluate the reliability and suitability of these backanalyzed parameters for assessing pipe-jacking forces using the finite element method.