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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. DOI: 10.1061/(ASCE)1090-0241(2004)130:9(915) CE Database subject headings: Slurry trenches; Core walls; Sand; Slurries; Filters.

Statistical analyses of data sets from five case histories indicate that soil–bentonite hydraulic conductivity is distributed log normally. The advection–diffusion equation was used to investigate the impact of log-normal variation in hydraulic conductivity on both steady-state and transient contaminant flux through a cutoff wall with idealized initial and boundary conditions. The results demonstrate that contaminant flux through cutoff walls increases as the variability in hydraulic conductivity increases while all other variables are held constant, including the area-weighted average conductivity. The effect of variability is most pronounced when advective transport and diffusive transport act in opposite directions, as occurs for circumferential cutoff walls that are operated with inward-directed hydraulic gradients to contain contaminated ground water. In this case, the increase in total outward flux due to variability of hydraulic conductivity occurs because the increase in inward advective flux in areas where the seepage velocity is higher than average is more than offset by the increase in outward diffusive flux in areas where the seepage velocity is lower than average. [ABSTRACT FROM AUTHOR]

The hydraulic conductivity of soil-bentonite backfill in three pilot-scale cutoff walls was measured using laboratory tests on disturbed samples, laboratory tests on undisturbed samples, piezocone dissipation tests, and piezometer tests (also known as slug tests or single-well tests). In addition, a global measurement of the average hydraulic conductivity of the soil-bentonite backfill in one of the cutoff walls was made using the pilot-scale test facility. Two main factors distinguish these five different methods of measuring hydraulic conductivity: remolding and sample size. Remolding of samples tested in American Petroleum Institute filter press equipment significantly reduced their hydraulic conductivity compared to the hydraulic conductivity of undisturbed samples, which were of similar size. For the other tests, where the degree and extent of remolding were less significant, hydraulic conductivity was found to increase as sample size increased, with the global measurement producing the highest value. The existence of bentonite filter cakes on trench walls reduces the influence of sample size on the equivalent hydraulic conductivity of the barrier. Findings regarding locating defects with a piezocone and hydraulic fracture in piezometer tests are also presented. [ABSTRACT FROM AUTHOR]

The article examines the factors that tend to reduce the influence of sample volume on the hydraulic conductivity measurements. It was thought that an increased hydraulic conductivity with increasing sample volume remains valid for pilot-scale cutoff wall W3. A soil bentonite wall plays a significant role on the water balance in the barrier pit. The hydraulic conductivity of the treated concrete is believed to be low. However, the magnitude of seepage of the concrete walls was important since the soil-bentonite wall of the hydraulic conductivity was also low.

The article focuses on the comments made to the study entitled "Stability of Long Trenches in Sand Supported by Bentonite-Water Slurry." It agrees with the comment that for this type of construction, three-dimensional safety factors are better than two-dimensional safety factors. It states the importance of three-dimensional effects. It also agrees to the suggestion on the development of analytical solutions for factors of safety applicable to instances where there is no formation of filter cakes.