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( sigma ') compression curves attributed to the existence of the remolded yield stress ( sigma ' sub(yr)). The compressibility, in terms of sigma ' sub(yr) and the compression index (C sub(c)), 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 e sub(1)) is a useful characteristic parameter to uniquely correlate with C sub(c) 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 sigma ' sub(yr), initial void ratio (e sub(0)), e sub(1), and the void ratio at liquid limit (e sub(L)). The hydraulic conductivity of the clayey soil/Ca-bentonite backfills is significantly reduced by the bentonite content; generally to less than 10 super(- 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