Calculation model of the equivalent spiral shear stress of conditioned sand

Soil is the medium that balances the soil and water pressure on the excavation surface of an earth pressure balance (EPB) shield machine, and the soil flow plasticity directly affects the working performance of the shield machine. The spiral shear stress is the main determinant of the soil flow plasticity. During the progress of the EPB shield screw conveyor normal operation, the shear effects between the muck in the spiral tube and the screw shaft, as well as the flight, are different from the shear action, such as the direct shear or the vane shear. The direct or vane shear test is now widely used to evaluate soil flow plasticity. To better investigate the shear properties between the soil and the shield conveyor casing, a model screw conveyor was developed. Based on the analysis of the stress conditions and the movement of the conditioned soil plug, the theoretical calculation model of the equivalent helical shear stress of the EPB shield screw conveyor was deduced based on the steady-state equilibrium conditions of moment of momentum. The conditioned standard sand was used in the indoor tests. The results of the indoor test of the model machine verified the rationality and effectiveness of the theoretical model. The model provides an important theoretical basis and references for the design and optimization of soil conditioning during shield tunneling.