Compressibility and hydraulic conductivity of clayey soil mixed with calcium bentonite for slurry wall backfill: Initial assessment

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 corresponding 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 void ratio at an effective vertical compression stress of 1 kPa (denoted as e1) is a useful characteristic parameter to uniquely correlate with Cc for the clayey soil/Ca-bentonite backfills in this study as well as for sandy soil/Na-bentonite and sandy soil-clay backfills that are reported in previous published studies. Unique relationships are also found between the σ′yr, initial void ratio (e0), e1, and the void ratio at liquid limit (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 conductivity 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 in this study and the sandy soil-bentonite backfills from published study with reasonable accuracy. Additional research is warranted to prepare the backfills to simulate typical field practice (e.g., use of tap water) and at workable initial water contents (based on the slump testing).