Experimental Study on Filter Cake for Slurry Infiltration in Different Permeability Strata

To solve the problem of filter cake formation in highly permeable strata and provide a reference for slurry preparation in construction with a slurry shield, in view of the process of bentonite slurry infiltration in the sand stratum, a series of pressure filtration tests was conducted based on the shield construction practice of a certain high-speed railway tunnel. A self-designed apparatus with an inner diameter of 300 mm was used in the test, and the slurry pressure was transformed into the effective support pressure by means of staged pressure. The relationships among ground permeability, thickness of filter cake, and quantity of filtration in slurry infiltration was analyzed, as well as the method of judging filter cake formation. The results indicate that the thickness of the flter cake increases and then decreases as permeability increases, and the amount of discharged water increases accordingly. The thickness of the flter cake is positively correlated with the amount of discharged water in the increasing stage, but negatively correlated in the decreasing stage. The maximum point of thickness of the flter cake is the turning point of valid formation in slurry infiltration. The k-line, exceeding the maximum thickness of the flter cake, can provide a reference for the judgment of slurry adaptability. On the left side of the k-line is the stable area of filter cake formation, while on the right side, the consumption of slurry particles is not directly proportional to the retention, and the adaptability of the slurry is poor. The value of D15/d85 can roughly divide the formation of filter cake. A D15/d85 value of approximately 10 is the transition between valid and invalid formations of the flter cake. This transition area corresponds to the area adjacent to the right side of the k-line, where an unstable flter cake can be built up. Therefore, the presence of a filter cake cannot be taken as a sign of the valid formation of the filter cake. By increasing the particle size of the slurry, the ability of slurry cake formation can be improved, thus moving the k-line to the right, which can effectively solve the problem of slurry cake formation in high-permeability strata. © 2020, Editorial Department of China Journal of Highway and Transport. All right reserved.