Zhang et al. / J Zhejiang Univ-Sci A (Appl Phys & Eng) 2019 20(3):218-228 218 Compressibility and hydraulic conductivity of sand-attapulgite cut-off wall backfills * Wen-bing ZHANG 1 , Wen-bo RAO †‡1 , Lei LI 1 , Ye LIU 2 , Shuai WANG 1 , Ke JIN 1 , Fang-wen ZHENG 1 1 Institute of Isotope Hydrology, College of Earth Sciences and Engineering, Hohai University, Nanjing 210098, China 2 College of Resources and Environment, Henan Agricultural University, Zhengzhou 450002, China † E-mail: raowenbo@163.com Received Sept. 27, 2018; Revision accepted Jan. 17, 2019; Crosschecked Jan. 18, 2019 Abstract: Soil-bentonite cut-off walls have been used widely to control pollution in landfills but their antifouling property (their ability to prevent contaminants in landfills from polluting the surrounding environment) decreases significantly over time due to a variety of factors (e.g. contaminant concentrations). In recent years, attapulgite has been considered as a backfill material for cut-off walls, but relevant studies are lacking. In this study, the compressibility and hydraulic conductivity of sand-attapulgite backfills were investigated using consolidation and hydraulic conductivity tests. In these tests, attapulgite comprised 10%, 20%, 30%, 40%, 60%, 80%, or 100% (dry weight) of the backfills. The results showed that (1) the compression (C c ) and swell (C s ) indexes of the backfills increased linearly with increasing attapulgite content (A p ); (2) both the consolidation coefficient (C v ) calculated by the Casagrande and Taylor methods and the hydraulic conductivity (k theory ) calculated according to Terzaghi con- solidation theory decreased with increasing attapulgite content. In the case of an effective consolidation stress σ′<100 kPa, k theory <10 −9 m/s when A p ≥30%, which was supported by the hydraulic conductivity tests. Two methods were developed based on la- boratory data, for predicting the hydraulic conductivity of sand-attapulgite backfills. We conclude that the use of sand-attapulgite backfills applied to cut-off walls as substitutes for soil-bentonite backfills is technically feasible. Key words: Attapulgite; Sand-attapulgite backfill; Cut-off wall; Compressibility; Hydraulic conductivity https://doi.org/10.1631/jzus.A1800548 CLC number: X5; TU41 1 Introduction According to the statistics of Professional Committee of Urban Domestic Refuse Treatment of China Association of Environmental Protection In- dustry (2017), the production of municipal solid wastes (MSWs) approached 200 million tons/year in 2015, and in China more than 60% of MSWs are disposed of in landfills. However, most landfills constructed early in China are relatively simple and have no seepage control systems (Xu et al., 2016). Leachates in the landfills contain a wide range of harmful substances released from MSWs, contami- nating the surrounding soils and groundwater systems through lateral leakage (Xie et al., 2009; Laner et al., 2012). Therefore, it is necessary to construct vertical cut-off walls to prevent the leakage of leachates from the landfills (Evans et al., 1995; Sharma and Reddy, 2004; Zhu et al., 2014). A variety of cut-off walls have been applied to existing landfill pollution control systems (Garvin and Hayles, 1999; Inazumi et al., 2006; Joshi et al., 2010; Takai et al., 2014; Xu et al., 2016). Among them, soil-bentonite cut-off walls are a popular choice Journal of Zhejiang University-SCIENCE A (Applied Physics & Engineering) ISSN 1673-565X (Print); ISSN 1862-1775 (Online) www.jzus.zju.edu.cn; www.springerlink.com E-mail: jzus@zju.edu.cn ‡ Corresponding author * Project supported by the National Natural Science Foundation of China (No. 41273015) and the Fundamental Research Funds for the Central Universities (No. 2017B19614), China ORCID: Wen-bing ZHANG, https://orcid.org/0000-0002-8225-8042; Wen-bo RAO, https://orcid.org/0000-0002-1924-6305 © Zhejiang University and Springer-Verlag GmbH Germany, part of Springer Nature 2019