A consolidation model for lumpy composite soils in open-pit mining

In open-pit coal mining, the excavated clayey cover is dumped without any compaction to form a landfill with a relatively open structure. In this type of material, water flowing through the landfills prefers the inter-lump voids, which have a permeability significantly higher than the clay lumps. With increasing time, the lumps in the upper layer may be partially transformed into a reconstituted soil, occupying the inter-lump voids with consequent decrease in permeability of the landfill. In the study presented here, a consolidation model is proposed based on the double porosity concept and the homogenisation theory. The analysis follows these steps: step 1, a representative volume of lumpy composite soils is divided into four parts and the governing differential equations are formulated based on conservation of mass; step 2, the inter-lump porosity for the lumpy composite structure is formulated as a function of the overall porosity and the porosity of its constituents; step 3, to account for stress (strain) concentrations within the lumpy composite material, a homogenisation relation is used based on analysis of the soil structure; step 4, the hydraulic conductivities of the inter-lump material (lumps) and intra-lump material (reconstituted soil) are approximated by the same set of parameters; step 5, for the lumpy soil, a new relationship between the strains and the absolute velocities of the solid skeleton is proposed, which eliminates the influence of the rigid displacement of the lumps. The model simulations are compared with experimental data, indicating that the proposed model can well represent the consolidation curves of the lumpy composite soil observed in the laboratory.