Influence of mineral or polymeric modification on bentonite-based tunnel face support

Abstract

The time-dependent penetration of bentonite-based support fluids into non-cohesive soil is a current subject of investigation, especially in the field of shield tunnelling with fluid face support due to the cyclic excavation process. Research has so far focused on linking the time-dependent penetration of clean bentonite suspensions with the penetration rate of the cutting wheel. However, experience shows that bentonite slurries in-situ often contain suspended fines, which can change the penetration behaviour of the slurry significantly. Moreover, in recent years, it has become common practice to improve stability or rheological behaviour of bentonite suspensions by adding water-soluble polymers. Understanding the effect of such slurry modifications on the time-dependent penetration behaviour is an essential basis for realistic face support models. The present study analyses the influence of a mineral or polymeric modification of a typical bentonite slurry by means of penetration tests with sand columns at over-pressure. The sand type was chosen so that a pure unmodified bentonite slurry does not create an external filter cake, but penetrates into the pore space of the soil to be supported. Pore water pressure sensors along the height and measurements of inflow and outflow using electronic balances enable a time-dependent determination of penetration depth and pressure transfer. For selected tests, a special focus lies on the evaluation of the effect of filter cake formed and penetration on the soil permeability. From the results, it can be concluded that a bentonite modification with fines results in a significant reduction in the penetration depth with the formation of an external filter cake. The addition of specific polymers reduces the amount of bentonite needed to create a stable suspension with defined rheology. Special care must be taken regarding the choice of fines and polymers.