Torque estimation for tunnel boring machine design in soft ground

In recent years, numerous models have been developed to predict the torque requirements of Tunnel Boring Machines operating in soft ground. Friction between the TBM cutterhead and surrounding ground is a main contribution to the required torque. As a result, friction coefficients are a major input factor in recent torque estimation models. Conditioned soil samples have been tested in a direct shear box, to assess whether a common lab setup can be used to improve the determination of TBM-soil friction coefficients. Three different soils were each mixed with water, bentonite or foam and sheared against a TBM steel plate. Test results show unconditioned friction coefficients matching with those reported from literature. Compared to the unconditioned friction coefficients, all bentonite-conditioned specimens show reduced friction coefficients, while foam-conditioned sand specimens do not show any reduction in friction. Data from literature and the laboratory tests resulted in two regressions, for sand and clay respectively, relating steel-soil friction coefficients to relative density, grain size, liquid limit and steel roughness.