Abstract: The authors investigate the mechanical behavior of internal soil cohesion and the adhesion between clay soils and metal cutterheads during mud cake formation. The shear failure modes of soil samples under two shear methods are analyzed. A self-developed rotational torque test device is used to conduct vane and plate shear tests on four types of soil: pure illite, pure kaolinite, pure montmorillonite, and sand-illite mixtures. The findings are as follows: (1) With increasing normal pressure, both peak and residual torque increase for vane and plate shear, with peak torque showing a strong linear correlation with pressure. (2) Compared to pure illite, sand-illite mixtures exhibit higher peak torque and lower residual torque. (3) For illite, kaolinite, and montmorillonite, peak and residual torque under both shear methods exhibit significant linear correlations; montmorillonite yields the highest torque values, followed by illite and then kaolinite, indicating montmorillonite poses the greatest clogging risk. (4) Vane and plate shear torque values are linearly correlated. For the same soil, the slopes of fitted lines for both peak and residual torque are nearly identical under both test methods. When clogging risk is highest, i.e., when the consistency index reaches 0.625, shear failure occurs within the soil, and cohesion is lower than adhesion to the cutterhead.

Key words: vane shear, plate shear, adhesion, cohesion, rotational torque, mud cake