Stability of Long Trenches in Sand Supported by Bentonite-Water Slurry

Abstract

The most important mechanism by which bentonite-water slurry supports long trenches in sand is lateral pressure from the slurry. The effectiveness of this support can be compromised if filter cakes do not form on the trench walls. Criteria for filter cake formation are presented, along with the depth of slurry penetration and effects of the slurry on the strength of coarse granular soil when filter cakes do not form. Closed-form expressions are provided for global stability when filter cakes do form and for local stability when they do not form. A method for analyzing global stability when filter cakes do not form is also discussed. Increasing the bentonite concentration of the slurry has three beneficial impacts on stability: (1) the stagnation gradient increases, which improves local and global stability in cases where filter cakes do not form, (2) larger particles can become suspended in the slurry, which promotes filter cake formation on the walls of trenches excavated in coarse soils, and (3) more particles and larger particles can become suspended in the slurry, which increases the unit weight of the slurry and improves stability whether or not filter cakes form.