Volume changes of biochar-amended landfill cover soil under a thermal cycle

Biochar has been identified as a favourable amendment for landfill cover soils. So far, the effects of biochar on soil response to temperature variations are still unclear. Waste heat generates when municipal solid waste decomposes, and the temperature of landfill can increase up to 90°C. The present study aims to investigate the thermally induced volumetric behaviour of biochar-amended sand with different biochar application ratios of 0%, 5% and 10%. Experimental results show that, during heating from 23–83°C, pure sand expands. However, during the heating process biochar-amended sand samples contract first and then expand. This is because biochar treatment not only decreases soil density, but also promotes the formation of big size macro-aggregations with more macro-pores, which may be destroyed when particle rearrangements induced by thermal expansion occur. During the cooling process from 83–23°C, all the specimens with three biochar application ratios show contraction. Moreover, with an increasing biochar application ratio, the thermal expansion coefficient of the soil specimen decreases. This is because biochar is insensitive to temperature variation as it is produced in very high temperature. Therefore, biochar-amended soil is a promising alternative landfill cover material for municipal solid waste landfill, and the effect of temperature effect on its volume change in landfill cover design is also essential to be considered.