Bentonite slurry infiltration into sand: filter cake formation under various conditions

Experiments on bentonite slurry infiltration into saturated sand have been carried out in a modified laboratory set-up that provides a hydraulic gradient comparable to real tunnels. Test series 1 investigated the characteristics of mud spurt and filter cake formation during water-bentonite slurry infiltration, before and after removing the external filter cake formed on the original boundary between the slurry and the sand. Test series 2 examined the conditions of filter cake formation during water-bentonite-sand slurry infiltration. The experimental results of series 1 indicate that, in the second infiltration, a new external filter cake will be formed for low concentrations of slurries (40 and 50 g/l), whereas hardly any new external filter cake formed for high concentration of slurry (60 g/l) owing to the formation of an internal filter cake in the sand during the first infiltration. The formation of this internal filter cake meant that the infiltration distance was much smaller for the second infiltration than that in the first for 60 g/l slurry. The experimental results of series 2 with a water-bentonite-sand slurry showed no external filter cake formation, but depth filtration was observed for high-density slurry (1300 and 1500 kg/m(3)). For low-density slurries (1050 and 1100 kg/m(3)), a thicker external filter cake, but with a higher permeability, than that in series 1 was observed on the sand surface, because the cake consisted to a large extent of sand particles. Mechanisms of mud spurt and filter cake formation are discussed, with a new solution for mud spurt and the theory of depth filtration.