Clay Minerals (1996) 31,243-252 PREDICTING THE PROPERTIES OF BENTONITE-SAND MIXTURES L.H. MOLLINS, D.I. STEWART AND T.W. COUSENS Department of Civil Engineering, University of Leeds, Leeds, LS2 9JT, UK (Received 5 June 1995; rev&ed 2 October 1995) ABSTRACT: One-dimensional swelling tests and hydraulic conductivity tests have been performed at vertical effective stresses up to 450 kPa on Na-bentonite powder and compacted sand/Na-bentonite mixtures (5, 10 and 20% bentonite by weight) to investigate the use of bentonite- improved soils for waste containment. It was found that bentonite powder swells to reach a final state described by a single straight line on a plot of void ratio against the logarithm of vertical effective stress, regardless of preparation technique. Swelling of sand/bentonite mixtures expressed in terms of the clay void ratio show a deviation from bentonite behaviour above a stress which depends on the bentonite content. Hydraulic conductivity data for bentonite and sand/bentonite mixtures indicate an approximately linear relationship between logarithm of hydraulic conductivity and logarithm of void ratio. A design model based on the clay void ratio, and the sand porosity and tortuosity is presented enabling the hydraulic conductivity of a mixture to be estimated. A modern landfill must be designed to prevent fluids in the waste migrating from the site. Currently, engineered liner systems are usually a composite of a compacted clay soil and a synthetic (usually high density polyethylene, HDPE) membrane, together with various drainage and fluid collection layers. The HDPE liner alone is sufficiently impermeable to prevent groundwater contamination by leachate from the waste but such liners are susceptible to puncturing, and an additional low permeability clay liner is essential to ensure containment. The engineering specifications for a compacted clay liner usually consist of a hydraulic conduc- tivity of <1 x 10 -9 m]s, and the need for stability during construction and operation of the landfill. Material selection is usually based on local availability, and many different soil types (both natural soils and processed clay minerals) have been used as liners. However, leakage can result from shrinkage cracking if the clay content of the soil is too high. One material that can meet the hydraulic conductivity criteria without suffering from shrinkage cracking is a sand/bentonite mixture. The sand component decreases the shrinkage on drying (Dixon et al., 1985) and below a limiting clay content the sand particles are in contact, providing mechanical stability and preventing shrinkage. When wet, the clay fills the sand voids producing a very low hydraulic conductivity for the mixture. Bentonite is a particularly effective clay for producing low permeability barriers because it has a high swelling capacity, which reduces the amount required. Depending on the type of bentonite, it can have a free swell (the volume increase when unconfined bentonite powder is inundated, divided by the volume of the original dry loose powder) between 200 and 1200% (Cowland & Leung, 1991; Wu & Khera, 1990). Processed (ground and dried) bentonite is normally used as the hydraulic conductivity of unprocessed bentonite can be higher (Oscarson et al., 1990). To minimize cost, and to avoid shrinkage cracking, it is important that the amount of clay added to a mixture is kept close to the minimum required to meet the hydraulic conductivity specification. Permeameter tests using distilled water indicate that mixtures containing more than 5% bentonite by dry weight achieve the required value, although the hydraulic conductivity may be much higher when a landfill leachate is used as the permeant (Alther et al., 1985; Hoeks et al., 1987; Wu & Khera, 1990). In addition, there is uncertainty about how hydraulic conductivity varies with bentonite content; Cowland & Leung (1991) found no noticeable relationship, Garlanger et al. (1987) found that hydraulic conductivity 9 1996 The Mineralogical Society