Visualization of the microscopic mechanism of slurry infiltration and filter cake formation for slurry shield tunneling in saturated sand: A microfluidic chip experiment

This study investigated the effects of slurry concentration and pressure on the micro-mechanisms of the slurry infiltration and filter cake formation through microfluidic chip experiments. The formation process of the clogging skeletons and filter cake was revealed. The formation time of clogging skeletons, particle deposition area, average clogging depth and clogging frequency were measured to evaluate the effects of slurry concentration and pressure. The results show that the bentonite particles can clog the pores through the effects of sieving and bridging, thereby forming weakly-permeable clogging skeletons. Subsequent particles deposit on the clogging skeletons and the filter cake begins to form. Higher slurry concentration can accelerate the clogging skeleton formation and enhances its stability. Increasing the concentration from 30 g/l to 60 g/l can increase the frequency of bridging-induced clogging by 2.66 %, leading to a 26 % decrease in the average clogging depth. The shallower clogging depth facilitates a larger external deposition area, significantly reducing the filter cake permeability. Higher slurry pressure can also accelerate the clogging skeleton formation and compacts the external deposited particles. Increasing the initial pressure from 30 kPa to 50 kPa can decrease the frequency of bridging-induced clogging by 2.41 % and increase the average clogging depth by 13 %, which induces the larger internal deposition area and lower filter cake permeability. Considering the effects of the cutter tool penetration depth and rotation frequency on the filter cake, it is recommended the concentration and excess pressure of the slurry used in the engineering applications should not be less than 60 g/l and 50 kPa, respectively.