Laboratory compression of scalped coal mine spoil materials tested under dry and wet conditions

The conventional laboratory compression testing of geo-materials is carried out in a water bath to create near-saturated materials. The aim of this is reduce the materials to two phases: solids and water, which are both essentially incompressible. This makes the results of the testing easier to interpret, since if highly compressible air were present the pore volume, degree of saturation and hence matric suction would change continuously during compression. Testing under saturated conditions will also generally represent a worst case situation, inducing greater compression than testing under unsaturated conditions. In addition, the limited scale of conventional laboratory test apparatus restricts the maximum particle size that can be tested. This necessitates that coarse-grained materials, such as coal mine spoil, be scalped to enable laboratory compression testing. Australian coal mine spoil materials selected to cover a range from essentially uncemented rocks to cemented sandstones, were prepared loose in a 150 mm diameter by 150 mm high, 10 MPa oedometer, and subjected to incremental compression under dry (assampled moisture content) and wet (in a water bath) conditions. The dry state represents the as-dumped condition, while the wet state represents the ultimate wetting-up of the material in the spoil pile due to rainfall infiltration and groundwater inflow. In the dry state, the air in the voids of the specimens is free to compress, leading to an increase in the degree of saturation and a decrease in the matric suction. The initial and final moisture contents and densities of the specimens were recorded. In the paper, the results for uncemented and cemented coal mine spoil materials tested under dry and wet conditions are presented and compared with data from the literature