Determination of jacking forces based on highly weathered non-linear 'soft rock' strength parameters considering arching

Abstract

As microtunnelling by pipe-jacking continues to remain an efficient method of constructing buried infrastructure in densely populated urban areas, it is paramount to understand the effects of geology on pipe-jacking forces. There continues to be a knowledge gap when understanding the accrual of frictional jacking forces for drives negotiating geological rock formations, particularly highly erratic and highly weathered geology. This paper presents a methodology for understanding the effects of highly weathered geology on pipe-jacking forces. This method was developed during the construction of a 7.7 km long trunk sewer network at depths of up to 30 m below the central business district of Kuching city, Sarawak, Malaysia. At such depths, the encountered lithologies from the Tuang Formation predominantly presented RQD values of 0%, which created difficulties when extracting rock samples for strength characterization. Therefore, excavated tunnelling rock spoils were instead collected and subjected to direct shear testing. The reconstituted tunnelling rock spoils demonstrated behaviour, characteristic of the nonlinear power law. For application to assessment of jacking forces, the developed peak tangential strength parameters, c't,p and ϕ't,p were found to have been closely related to the mineralogy of the respective lithological units, which subsequently affected the lubrication and accrual of jacking forces. This was explained through back-analysed values of pipe-rock frictional coefficient, μavg and vertical stresses at the pipe crown, σEV. These back-analyzed parameters provided insight into the effectiveness of lubrication efforts and the development of arching, which were closely related to the traversed lithologies. It was found that the highly weathered geology was able to arch, suggesting that the highly weathered and discontinuous 'soft rock' masses demonstrated soil-like behaviour.