Studies on hydraulic conductivity and compressibility of backfills for soil-bentonite cutoff walls

Fujian standard sand (Sand-F) was used to simulate a sandy soil layer. Hebei bentonite (Bent-H) and Jiangning clay (Clay-J) were served as additives for studying the hydraulic conductivity and compressibility of sand-bentonite/clay mixture backfills. The results indicate that there is an optimum mixing content (Copt) when Bent-H or Clay-J is mixed with the Sand-F. If the content of bentonite/clay is less than Copt, hydraulic conductivity k > 1.0 × 10− 7 cm/s and porosity and coefficient of compressibility decrease with the increase of the content of bentonite/clay. While the content of bentonite/clay are greater than Copt, hydraulic conductivity k ≤ 1.0 × 10− 7 cm/s and porosity and coefficient of compressibility increase with the increase of the content of bentonite/clay. As the content of bentonite/clay is less than Copt, clay minerals only fill the sand pore space without influencing the sand skeleton and porosity decreases with the increase of the content of bentonite/clay. While the content of bentonite/clay becomes greater than Copt, sand particles become disconnected and porosity increases with the increase of the content of bentonite/clay. A porosity model of sand-bentonite/clay mixtures was derived based on a micro-geometrical principle. Another equation was also developed to calculate hydraulic conductivity values with the changes of the content of bentonite/clay.