Impact of highly weathered geology on pipe-jacking forces

For the Kuching Wastewater Management System Phase 1 project in Kuching, Malaysia, 7·7-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 35 m 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 and φ′ t,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 μ avg and σ 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 and φ ′ t,p and the back-analysed parameters μ avg and σ EV can be reliably used to predict jacking forces in highly fractured, highly weathered geology.