Thermal effects of the hydraulic conductivity and threshold gradient of sand–clay liners in municipal solid waste landfills

Abstract

High leachate heads and dry cracking of clay barriers are common problems in landfills. The use of sand-clay barriers with a non-darcy percolation patterns can not only prevent leakage of large amounts of leachate but can also potentially reduce the development of dry fractures due to loss of water. Thus, it has been suggested that sand-clay materials could be used as barriers at the bottoms of landfills or as a vertical barriers. However, biochemical degradation of waste in a landfill generates much heat, and the temperature can reach 60 °C. A high-temperature field can inevitably affect the permeability of sand-clay barriers. The change in sand-clay barrier permeability under the influence of temperature has received increasing attention. Therefore, in this study, a temperature-controlled permeameter with flexible walls was used to conduct experiments on permeability of sand-clay mixtures at four temperatures. The results indicated that the hydraulic conductivity gradually increased and the threshold gradient gradually decreased with increasing temperature. The hydraulic conductivity and threshold gradient exhibited exponential relationship with the temperature, respectively. Experiments were designed to measure changes in the permeate viscosity at different temperatures. The previously established hydraulic conductivity equation (that considered only the influence of temperature on the permeate viscosity) greatly differed from the measured values. In contrast, the newly established hydraulic conductivity equation (that considered the influence of temperature on both the permeate viscosity and intrinsic permeability) agreed well with the measured values. Statistics based on a large amount of data (collected previously) showed that the intrinsic clay permeability exhibited a power law relationship, with increasing temperature to a threshold gradient at 25 °C (Eq. (9)). Above the threshold, the intrinsic permeability experimental data increasingly deviated from Eq. (9). To consider the effect of the temperature on both the intrinsic permeability and permeate viscosity, an equation for calculation of the threshold gradient was proposed, and it could be found that the calculated values suitably agreed with the measured values. The results of this study provide an important theoretical basis and data to support the development of high-performance barriers with long service life for landfills.