New clogging potential assessment method for conditioned soil based on modified pullout and direct shear tests

During EPB (earth pressure balance) shield tunnelling in cohesive soils, the excavated soil may stick to the cutters, causing clogging. Clogging could reduce the efficiency of tunnelling as the cutters may be covered with excavated soil, the opening ratio of the cutterhead may be reduced by the sticked soil, and increased torque may be required. This paper studies clogging by testing adhesion strength and tangential adhesion strength with modified pullout tests and direct shear tests, respectively. Strong weathered granite and 11 artificial soil samples are tested to check the feasibility of the modified pullout tests and direct shear tests in the clogging study. The repeatability and accuracy of the modified pullout test are improved by building a new soil container and using a new loading method. Soil conditioning by injecting chemical additives such as foam and dispersant into the excavated soil can be used to reduce the clogging potential during shield tunnelling. The influences of foam and dispersant on Atterberg limits, adhesion and tangential adhesion in strong weathered granite are also studied. Test results show that the Atterberg limits decreased in conditioned soil, and soil conditioning has a greater influence on tangential adhesion than on adhesion. A new clogging potential assessment method for conditioned soil is proposed based on the pullout and direct shear test results. Validations using strong weathered granite show that this method is more suitable for conditioned soils than the traditional methods.