Destruction Mechanism of Filter Cake under Air Pressure during Chamber Opening

Abstract

Abstract During chamber openings, the filter cake that has formed on the excavation face is thin and cannot be monitored directly, resulting in an unclear understanding of its destruction mechanism. To investigate the destruction mechanism of filter cake under air pressure, a series of air breakthrough tests (ABTs) were conducted in this study, during which the water discharge and pore pressure were monitored. The experimental results indicate that the filter cake undergoes significant compression during ABTs, but it tends to be stabilized after a testing time of 40,000 s. Cracks and pores are observed on the surface when the filter cake is destructed, and the cracks are mainly distributed between 80 and 100 % of the radius. The effective working time (EWT) of Filter Cake B with a compression time beyond 40,000 s is much longer than that of Filter Cake A with a compression time below 40,000 s. Because the EWT of Filter Cake B is much longer than that of Filter Cake A, the slurry composition that promotes the formation of Filter Cake B should be used as much as possible during the slurry shield. Additionally, it is proven that the EWT increases with higher slurry density and filter cake thickness, whereas it decreases with increasing slurry viscosity. The analysis of the filter cake destruction mechanism shows that the compression of filter cake, capillary effect, mechanical effect, and interfacial effect all play important roles in filter cake destruction. Except for the compression of the filter cake that can extend the EWT, the other effects will accelerate the filter cake destruction.