SOIL-BENTONITE SLURRY TRENCH CUT-OFF WALLS

David J. D'appolonia
Christopher R. Ryan

Soil-bentonite cut-off walls constructed by slurry trench methods have been employed for about three decades and have gained widespread acceptance during the past five years. Nevertheless, there is virtually no information in the engineering literature concerning the engineering properties pertinent to the design of these facilities.

Soil-bentonite cut-off walls are constructed by excavating a continuous narrow trench under a bentonite slurry, which stabilizes the sides of the excavation. The trench is backfilled subsequently with a blended mixture of natural soil and bentonite, thereby displacing the slurry. The completed wall acts as a barrier to lateral flow of water and most fluid pollutants.

During the past several years, the writers have been involved in laboratory and field investigations to determine the design factors relating to the impermeability of cut-off walls. One significant question has been the relative contributions of the filter cake and of the backfill itself. This paper presents data which will help the designer in determining the long-term permeability of a cut-off wall and in writing specifications which will achieve the required performance at the lowest cost. Information is presented relating permeability, strength, and compressibility to various types of construction materials that can be employed.

A design factor that is increasingly important is the ability of slurry cut-off walls to maintain their integrity under the effect of long-term permeation by various pollutants. Laboratory procedures for modeling the degradation process are described, along with typical results and design methods for minimizing the deterioration of slurry trench cut-off walls due to pollutants.