Back-analysis and finite element modeling of jacking forces in weathered rocks

• A novel approach is proposed for pipe-jacking through weathered geology.
• Nonlinear rock strength parameters were developed to assess pipe-jacking forces.
• Higher rock strength leads to arching, which reduces pipe-rock friction and jacking forces.
• 3D FEM confirms reliability of direct shear test results in assessing jacking forces.

Prediction of jacking forces has been well-established for pipe-jacking drives traversing soils. However, the accrual of jacking forces for drives negotiating weathered rock formations has not been well understood. Three pipe-jacking drives in Kuching City, Malaysia spanning weathered lithological units of sandstone, phyllite and shale were studied. In the absence of in-situ pressuremeter testing during the investigation stage, tunneling rock spoils were collected and characterized through direct shear testing. The “generalized tangential” technique was applied to the nonlinear direct shear test results to obtain linear Mohr–Coulomb parameters, c′p and ϕ′p. This allowed for back-analysis of frictional coefficient, μavg through the use of a well-established predictive jacking force model. The reliability of using c′p, ϕ′p and μavg was assessed through 3D finite element modeling of the studied pipe-jacking drives. Based on these parameters, the results obtained from the numerical analyses of the studied pipe-jacking drives show good agreement with the jacking forces measured in-situ. The outcome of this research demonstrates that the derived strength parameters from direct shear testing of tunneling rock spoils has the potential to be used as reliable input parameters in finite element modeling to predict pipe-jacking forces in highly weathered geological formations.