Variations of Undrained Shear Strength of EPB Mud and a Calculation Model Based on Consistency Index

Abstract

If earth pressure balance (EPB) shields are used for tunneling in clayey soils, dispersants are often applied to condition the mud. For the conditioned mud, the undrained shear strength is a key indicator determining the shield tunneling operational settings and assessing soil conditioning effectiveness. The consistency index, a parameter for evaluating clay softness, is directly associated with the undrained shear strength. To explore changes in undrained shear strength, measurements were conducted on clay using a direct shear instrument under varying consistency indices and normal pressures. Moreover, a computational model for predicting undrained shear strength was established. Based on the findings, internal friction angle and soil cohesion decline progressively with a reduction in the consistency index. The internal friction angle is close to zero as the soil’s saturation nears one. Under identical normal pressure and consistency index conditions, the undrained shear strengths of different soil samples exhibit similar values. Additionally, the undrained shear strength of clay was found to increase exponentially with a rise in the consistency index. When dispersant was added, the shear strength of the soil samples gradually decreased by about 10%–50% at the same water content. The average shear strength of the soil samples decreases as the dispersant addition ratio increases from 0% to 4%. Given that the dispersant affects the clay Atterberg limits, soil samples with an identical consistency index demonstrate nearly identical shear strengths when subjected to an identical normal pressure.