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In regions and countries where sodium bentonite is scarce, the resourceful calcium bentonite can be considered as an alternative material for soil-bentonite slurry trench walls. However, the lower swelling capacity of calcium bentonite as compared to Na-bentonite may result in the unacceptable high hydraulic conductivity of the sand/calcium bentonite backfills, and, therefore, amendment of calcium bentonite is necessary. However, studies on the feasibility of using amended calcium bentonite for soil bentonite backfills are limited. This study investigates the feasibility of using sodium hexametaphosphate amended calcium bentonite for the backfill material through a comprehensive laboratory investigation. First, a series of viscosity, density and filtrate loss tests are conducted using bentonite-tap water slurries with different amounts of phosphate to determine the optimal amendment dosage. Subsequently, the Atterberg limits and free swell index are evaluated for the amended/un-amended calcium bentonite and backfills, which consist of sand and amended/un-amended calcium bentonite. Finally, slump, oedometer tests and flexible-wall hydraulic conductivity tests are conducted on the backfills. The results indicate that the slurry with 20% calcium bentonite amended by 2% phosphate results in the target slurry, and the optimum phosphate dosage for amended calcium bentonite is found to be 2%. The phosphate amendment causes an increase of 1.21 times in the liquid limit, and an increase of 1.40 times in the free swell index of the amended bentonite relative to those of the un-amended bentonite. The sand/calcium bentonite backfill changes from non-plasticity to plasticity after the amendment. The amended backfill displays two times higher swell index value and one order of magnitude lower consolidation coefficient value than the un-amended backfill. The hydraulic conductivity of un-amended backfills containing 15% to 25% un-amended bentonite is greater than the commonly targeted value of 10 − 9 m/s for slurry trench walls. In contrast, the hydraulic conductivity of the amended backfill with 15% to 25% amended bentonite varies from 8.3 × 10 − 10 m/s to 2.7 × 10 − 10 m/s, which is lower than 10 − 9 m/s. In addition, the amended backfills containing 20% and 25% amended calcium bentonite have nearly the same magnitude of hydraulic conductivity as the conventional sand/sodium bentonite backfill. This study demonstrates that phosphate is a promising amendment for achieving low hydraulic conductivity of sand/calcium bentonite backfills for soil-bentonite slurry trench walls.

Bentonite cake and filter cake on the surface of porous media result from the filtration of stabilizing slurry into the adjacent porous formation. This paper provides measurements of hydraulic conductivities of bentonite cake formed on Korean standard sand under various pressure levels. Two types of bentonite (Tixoton and Bentonil GTC4) were used to create the slurries with three different concentrations (4, 6, and 8% by weight). An estimation of the hydraulic conductivity of the bentonite cake based on the filtration theory was carried out. The range of hydraulic conductivities of the bentonite cake was from 2.1 × 10− 11 m/s to 5.7 × 10− 10 m/s. The hydraulic conductivity of the bentonite cake deposited on Korean standard sand were compared with that of the bentonite cake deposited on filter paper to show the effect of the filter medium. The slurry concentration of 6% seems to be a value that most stimulates the effect of the filter medium on the permeability of the Bentonil GTC4 bentonite cake. In addition, a simple approach was developed to characterize the hydraulic properties of the filter cake, which has usually been confused with the bentonite cake in recently published literature.

The ageing and stability behaviour of Bentonite slurries were evaluated under the influence of Mg2 +, Ca2 + and Ba2 + ions at concentration ranging from 0.005 to 1 M. Stable gels were formed at low metal ion concentration of

article Soil-bentonite vertical cutoff walls, consisting of a sandy soil mixed with Na-bentonite as backfill, are used extensively as engineered barriers for contaminant containment. However, suitable sandy soil and Na-bentonite may not be available at some sites. Consequently, locally available clayey soil and Ca-bentonite may be considered as an alternative backfill. The use of clayey soil/Ca-bentonite backfill may be advantageous to achieve relatively low hydraulic conductivity, which has equivalent performance as that of conventional sandy soil/Na-bentonite backfills. However, studies on the compressibility and hydraulic conductivity of these clayey soil-bentonite backfills are very limited. This paper presents a comprehensive laboratory investigation on the compressibility and hydraulic conductivity of clayey soil/Ca-bentonite backfill through a series of oedometer tests. Kaolin is used as the control clayey soil and it is amended with different contents of Ca-bentonite, 0 (kaolin alone), 5, 10, and 15% (by dry weight basis), to prepare the clayey soil/Ca-bentonite backfills. The initial water contents for the backfills are selected to be 0.75, 1.0, 1.25, and 1.50 times their corre- sponding liquid limits. The results reveal that the backfills exhibit a noticeable inverse 'S' shaped e-log(σ') compression curves attributed to the existence of the remolded yield stress (σ'yr). The compressibility, in terms of σ'yr and the compression index (Cc), is significantly affected by the initial water content and bentonite content.The voidratioataneffectiveverticalcompressionstress of 1 kPa (denotedase1) isa useful characteristic parameter touniquely correlate withCcfor theclayeysoil/Ca-bentonitebackfills inthis study as well asfor sandy soil/Na-bentonite and sandy soil-clay backfills that are reported in previous published studies. Unique relation- ships are also foundbetween the σ'yr,initial voidratio(e0), e1, andthevoidratioatliquidlimit(eL).The hydraulic conductivity of the clayey soil/Ca-bentonite backfills is significantly reduced by the bentonite content; generally to less than 10 �9 m/s. An empirical method based on the framework of Kozeny-Carman equation is proposed to predict the hydraulic conductivity of the clayey soil/Ca-bentonite backfills, and the predicted hydraulic conduc- tivity values using these methods are found to fall in the range of 1/3 to 3 times those obtained from the oedometer tests. The proposed method is shown to estimate the hydraulic conductivity for both the clayey soil/Ca-bentonite backfills inthisstudy and the sandy soil-bentonite backfills from published study with reason- able accuracy. Additional research is warranted to prepare the backfills to simulate typical field practice (e.g., use