Appraisal of Friction Coefficients Between TBM and Conditioned Soil

Abstract

During the advance of a tunnel boring machine (TBM), the torque applied to rotate the cutterhead must overcome the resisting moments acting on it. Hence, one of the major concerns of TBM design is to determine the torque and power requirements of the excavation machine. The first empirical approach for torque estimation was developed in the 80’s by JSCE. This estimation fits the recorded mean torque for 5-8m diameter projects, but overestimates torque as TBM diameter increases. Newer approaches estimate TBM torque and thrust requirements as sum of multiple components, focusing on the friction between ground and cutterhead. However, the recommended or selected friction coefficients range between a wide interval (typically 0.05-0.75), depending on soil and operational conditions. Consequently, the torque and thrust estimations relying on the above-mentioned friction coefficients can be considerably imprecise, as well. Previous research regarding soil-machine/steel friction coefficients has been collected and studied. Successively, interface shear tests have been performed at TU Delft, to study whether and how friction coefficients could be determined consistently in the laboratory. Three soil types have been examined, ranging from coarse sand to kaolin clay. A metal plate cut out from a worn TBM cutterhead has been added to a direct shear apparatus to test the abovementioned soils. The latter are either water saturated or treaded with bentonite or foam, to assess the influence of conditioners on soil interface friction. Bentonite slurries and foam suspensions have been prepared in the laboratory using common industry products. Overall, in excess of 60 interface shear tests have been performed, considering various permutations of soil, load and conditioning. Results show that lubricated friction coefficients (i.e. when soil samples are conditioned) are up to 25% and 50% lower than for water-saturated conditions, for sand and clay respectively. A direct shear apparatus proves to be useful to study and select friction coefficients for TBM preliminary design, as it provides satisfactory results for non-conditioned and bentonite-conditioned soils. The same apparatus, however, cannot fully capture the lubricating effect of foam on sand. Improved estimation of friction coefficients requires more advanced equipment, or substantial modifications to the direct shear apparatus.