Influence factors and calculation model of the adhesion strength of clayey soil for EPB shield tunnelling

When earth pressure balance (EPB) shield tunnels are constructed through clayey ground, the soil adheres to the cutter, cutterhead and chamber bulkhead due to the high adhesion strength between the steel and the clay, thus clogging the shield. To investigate the influence of different factors on the adhesion strength, this study used montmorillonite, kaolin and mixtures of the two as test soils. The adhesion strength between the steel and clay is determined with a customized rotary shear apparatus. The results show that when the consistency index of the soil specimen is less than 1, the adhesion strength between the steel and clay increases with the consistency index. As the consistency index decreases, the effect of the normal pressure on the adhesion strength gradually weakened. As the contact angle of the shear plate increases, thus reducing the hydrophilicity, the adhesion strength decreases. When the soil specimens with different plasticity index values have the same consistency index value, the adhesion strengths are similar. The adhesion strength increases gradually with increasing surface roughness. Based on grey incidence analysis, the order of the factors affecting the adhesion strength is as follows: normal pressure > consistency index > contact angle > plasticity index ≈ surface roughness. The normal pressure, consistency index and contact angle all have important effects on the adhesion strength between steel and clay. However, because there are no large differences in the contact angle among the metal materials, the approximate adhesion strength can be calculated by considering the effects of only the normal pressure and consistency index. The measures that are effective for preventing EPB shield clogging are increasing the soil softness and decreasing the hydrophilicity of the cutterhead by applying new materials for the cutters and cutterhead.