No abstract

The Cone Penetration Test (CPT) is widely used for classifying soils and assigning soil properties to the subsurface because it is robust and can be used to quickly collect continuous data with depth. During and subsequent to the construction of a very deep soil-bentonite (SB) slurry trench cut-off wall in Mayfield NSW, Australia, the CPT was used to check the continuity and quality of the SB backfill material. The numerous CPT profiles conducted in the Mayfield wall provided a unique research opportunity for examining how shear strength and effective stress vary with depth. A method is proposed to estimate the undrained shear strength of the Mayfield Wall from the CPT combined with some vane shear data. The results support previous studies that show that the shear strength and the effective stress distributions in SB slurry trench cutoff walls are less than would be expected from a geostatic stress distribution. While there is some increase in shear strength with depth, these data show the increase is modest and consistent with the less than geostatic stresses. In addition, the authors recommend a general procedure for shear strength and horizontal stress estimation using CPT data in any SB slurry trench application.

Vertical cutoff walls have been used to control the movement of contaminants and contaminated groundwater since the remediation of contaminated sites began. There are, however, significant hydraulic conductivity differences between soil-bentonite, cement-bentonite, plastic concrete, and in situ mixed cutoff walls. The results of laboratory and field studies were assessed to show the influence of material properties, confining stress, permeameter type, water table position, and state of stress, on the hydraulic conductivity of vertical cutoffs. The results of these studies show the range of hydraulic conductivity expected for each of the cutoff wall types. Increasing confining stress markedly decreases the hydraulic conductivity of soil-bentonite and has a measurable but reduced impact on stronger backfill materials. Studies on soil-bentonite cutoff walls show that the stress at depth is less than predicted using the effective weight of the overlying materials. This reduction in stress is a result of soil-bentonite materials "hanging-up" on the side walls of the trench. Thus, applying the effective stress calculated from the effective weight of the overlying backfill overestimates the stress to be used in the laboratory tests and results in unconservative measures of hydraulic conductivity. Field data also reveals that, with time, the hydraulic conductivity of soil-bentonite above the water table may increase substantially. Further, the hydraulic conductivity does not significantly decrease upon re-saturation.

A total of 16 inorganic aqueous solutions were utilized as permeants to determine their effects upon the permeability of both contaminant resistant (polymerized) bentonite and untreated bentonite. It was found that, of the aqueous solutions tested, those with potassium (K+) cations or chloride (Cl−) anions or both induced the largest permeability increases with increasing electrolyte concentration. Conversely, solutions with sodium (Na+) cations or carbonate (CO−3) anions had the least impact upon the permeability of bentonite. It was also observed that doubly charged cations (+2) have a greater initial effect on the permeability than do singly charged cations (+1). Furthermore, a “saturation limit” was in evidence for +2 cations, indicating that beyond certain concentrations the further addition of the soluble salts had only limited additional impact upon the permeability. For selected bentonite-contaminant combinations, slurry cracking pattern tests were conducted. A correlation between the permeability changes and the results of the cracking pattern tests was demonstrated. Based upon the findings of this study, it is concluded that the character of the solute anions, as well as the primary cations, affect the permeability of bentonite clays. Further, the Gouy-Chapman model of diffuse double layer was found to be generally consistent with the test data.

This paper presents the results of laboratory tests on a compacted clayey liner material using landfill leachate. Laboratory permeability tests were conducted with continuous permeation for a period from three to to six months in duration in specially designed traxial cell permeameters. The soil samples were obtained from a borrow pit in eastern Pennsylvania and proposed for use as a compacted liner material. Routine soil tests, including Atterberg Limits, gradation, compressive strength, and compaction tests were conducted on samples exposed to both tap water and leachate. Limited tests were conducted on bentonite-clay mixtures to illustrate the behavioral difference between high-swelling and non-high-swelling clays. It was found that permeation of the clayey liner material with landfill leachate did not significantly alter permeability of the material. Further, the physical properties remained relatively unchanged. In contrast, test results conducted on bentonite-clay mixtures resulted in more significant changes. It is concluded that leachate permeation of natural silty clays of low activity, such as the material investigated herein, can result in inconsequential change in engineering properties. In contrast, leachate permeation of a high-swelling sodium montmorillonitic soil could result in significant changes in physical and engineering properties of the soil.