Application of bentonite suspensions as supporting media in foundation engineering an other underground workings

Abstract

Investigations of the physicochemical properties of suspensions of active (sodium-) bentonite in water and of the interactions of such suspensions with loose soil show that the following phenomena are responsible for the applicability of bentonite suspensions as supporting media for the walls of underground excavations (slurry wall technique):

i) The long-known formation of a watertight film or penetration zone at or in the loose soil which makes possible the transmission of the full hydrostatic pressure of the suspension to the walls.

Besides that, there are further important phenomena that have not been considered in technical literature concerning the slurry wall technique up till now:

• ii) The development of an osmotic pressure between groundwater and bentonite suspension, which is caused by the formation of a semipermeable structure of montmorillonite particles at the phase boundary.

• iii) The development of an underpressure in the penetration zone in the soil, which is caused by a very slow volume contraction of the solidified suspension in the soil pores.

• iv) A considerable increase of the soil cohesion in the penetration zone of the suspension, caused by the formation of a supporting structure of montmorillonite platelets linking the soil grains together.

The additional effects of ii) and iii) cause a formal increase of the hydrostatic pressure of the suspension in the excavation and prevent or restrict, together with the cohesion increase iv), the detachment of single grains from the walls, which is the initiating step of wall collapse in cohesionless soils.