Impact of highly weathered geology on pipejacking forces

For the Kuching Wastewater Management System Phase 1 project in Kuching, Malaysia, 7 center dot 7 km of trunk sewer lines were constructed in the highly fractured, highly weathered Tuang Formation using a pipe-jacking method. The pipelines were founded at depths of up to 35m below Kuching City, where the majority of the pipe-jacking activities would traverse the Tuang Formation. Jacking forces in highly fractured geology are not well understood as most jacking force models were derived for drives traversing soils. Therefore, a novel method was developed, whereby equivalent rock strength characteristics were interpreted from direct shear testing on reconstituted tunnelling rock spoils. Tangential peak strength parameters, c'(t,p), p and phi'(t,p), p, were developed from the nonlinear behaviour of the reconstituted spoils and applied to a well-established jacking model to assess arching and development of jacking forces from four documented drives. The back-analysed parameters mu(avg) and sigma(EV) were used to demonstrate that geology had significantly affected the development of jacking forces. The back-analysis of the studied drives was successfully validated through finite-element modelling. This research shows that the developed parameters c'(t,p), p and phi'(t,p), p and the back-analysed parameters lavg and rEV can be reliably used to predict jacking forces in highly fractured, highly weathered geology.