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. ... Read more