Design and validation of a large scale permeameter for mine waste rocks, and preliminary characterisation of hydraulic conductivity

Laboratory-scale hydraulic conductivity measurement for coarse granular materials is challenging. Notwithstanding the obvious difficulty in testing large samples at large scale, packed specimens of coarse particles tend to exhibit disrupted packing at a rigid boundary, which can lead to boundary zones of enhanced conductivity. A large-scale rigid wall permeameter is described and experimental results are presented showing the influence of test boundary conditions on the direct measurement of hydraulic conductivity. The equipment was constructed at the University of Newcastle for testing granular materials with particles up to 50 mm, with capability to accommodate vertical preloading of the samples up to 3 MPa. The equipment was designed with a customized double catchment end cap, introduced to isolate flows from the wall (outer) region of the sample, and the bulk (inner) region, where particle packing is less affected by the presence of the boundary. Results show that the phenomena responsible for disrupted packing differ for well graded and poorly graded materials, and, that preferential boundary flow can cause up to 10% difference between the inner and outer flow rates within the sample. For mono-sized materials, the size of the boundary effect depends on the ratio between the size of the permeameter chamber and the particle diameter, but satisfactory measurements can be obtained from the flow selectively measured through the core of the sample. For materials with poly-disperse gradings, careful sample preparation can reduce the boundary effect, giving meaningful results of satisfactory repeatability. In addition, tests with Waste Rock from a Hunter Valley (NSW, Australia) coal mine were performed. The specimens' permeability significantly varied with compaction and direction of flow.