Strength and Permeability of a Deep Soil Bentonite Slurry Wall: Fid-Bau Portal

Strength and Permeability of a Deep Soil Bentonite Slurry Wall

Ryan, Christopher R. / Spaulding, Charles A.

In 2006, a Soil Bentonite (SB) slurry wall was constructed at a brownfield redevelopment of a former steel mill site in Mayfield, NSW Australia. At this site, the slurry wall is designed to block groundwater flow that might contribute to the contamination of an adjacent waterway, the Hunter River. The wall was approximately 1500 m long and up to 49 m deep, constituting an apparent depth record for walls of this type. As a part of the construction QC, there was an extensive amount of testing done, including an unusual amount of in situ strength testing using both a static cone penetrometer and field vane shear measurements. These latter measurements offer a unique opportunity to determine the strength gain of SB backfill material. Results show a moderate stiffening of the SB material after it has been in the trench. This is consistent with field observations which show that, while SB backfill is placed in a semi-fluid condition, after some weeks it can be excavated with a vertical face. Results also show that the wall does not achieve a full static state of stress over its full depth. Rather, as the material "sets", arching occurs, in effect holding some of the weight of the backfill on the sides of the trench. Extensive permeability testing of field-mixed SB backfill samples also provides a basis for design of future walls. In an earlier design mix program, a good correlation between percentage of fines and reduced permeability was established. It is clear that the fines content must be at a certain minimum to achieve stability in the backfill and to permit the blending of a low permeability backfill mix. Data are also presented showing the effects of permeation over an unusually long test period with contaminated groundwater.