On the critical particle size of soil with clogging potential in shield tunneling

Shield tunneling is easily obstructed by clogging in clayey strata with small soil particles.However,soil clogging rarely occurs in strata with coarse-grained soils.Theoretically,a critical particle size of soils should exist,below which there is a high risk of soil clogging in shield tunneling.To determine the critical particle size,a series of laboratory tests was carried out with a large-scale rotary shear apparatus to measure the tangential adhesion strength of soils with different particle sizes and water contents.It was found that the tangential adhesion strength at the soilesteel interface gradually increased linearly with applied normal pressure.When the particle size of the soil specimen was less than 0.15 mm,the interfacial adhesion force first increased and then decreased as the water content gradually increased;otherwise,the soil specimens did not manifest any interfacial adhesion force.The amount of soil mass adhering to the steel disc was positively correlated with the interfacial adhesion force,thus the interfacial adhesion force was adopted to characterize the soil clogging risk in shield tunneling.The critical particle size of soils causing clogging was determined to be 0.15 mm.Finally,the generation mechanism of interfacial adhesion force was explored for soils with different particle sizes to explain the critical particle size of soil with clogging risk in shield tunneling.

DEM investigation on the effect of sample preparation on the shear behavior of granular soil

The effect of initial fabric anisotropy produced by sample preparation on the shear behavior of granular soil is investigated by performing discrete element method （DEM） simulations of fourteen biaxial tests in drained conditions. Numerical test specimens are prepared by three means： gravitational deposition, multi-layer compression, and isotropic compression, such that different initial inherent soil fabrics are created. The DEM simulation results show that initial fabric anisotropy exerts a considerable effect on the shear behavior of granular soil, and that the peak stress ratio and peak dilatancy increase with an increase in the fabric index an that is estimated from the contact orientations. The stress-dilatancy relationship is found to be independent of the initial fabric anisotropy. The anisotropy related to the contact orientation and contact normal force accounts for the main contribution to the mobilized friction angle. Also, the occurrence of contractive shear response in an initial shearing stage is accompanied by the most intense particle rearrangement and microstructural reorganization, regardless of the sample preparation method. Furthermore, the uniqueness of the critical state line in e-logp＇ and q-p＇ plots is observed, suggesting that the influence of initial fabric anisotropy is erased at large shear strains.