Baumgartl, T, Shao, Q & Filipovic, V 2026, 'Simulation of cover stability on a slope containing a sub-layer of low hydraulic permeability', in AB Fourie, G Boggs, J Heyes & M Tibbett (eds), Mine Closure 2026: Proceedings of the 19th International Conference on Mine Closure, Australian Centre for Geomechanics, Perth, pp. 1-12, https://doi.org/10.36487/ACG_repo/2615_44
(https://papers.acg.uwa.edu.au/p/2615_44_Baumgartl/)
Abstract: The intention of covers is to control water flow by reducing erosion risk through increasing infiltration, increasing water storage, but also minimising deep drainage. In conditions where an underlying sub-layer has very low permeability (e.g. brown coal) and where the landform is primarily sloped, soil stability may be reduced during events of high rainfall and exceedance of the storage capacity of the cover. When water accumulates in a cover on a low-permeability interface, hydrostatic stresses will increase, leading to a decrease in the Factor of Safety and potentially causing slope failure.
The objective of the study was to investigate the hydrologic performance of a sloped cover including a lowpermeability sub-layer and quantify the Factor of Safety under a typical rainfall regime of the Latrobe Valley, Australia. Hydrologic parameters were measured and calibrated from glasshouse-controlled column experiments using HYDRUS-1D. The model’s HYDRUS-2D and add-on module SLOPE CUBE were then used to simulate the hydrologic performance and slope stability.
The results showed that under current climatic conditions relatively high amounts of rainfall affected the stress distribution, mainly at the interface between cover material and coal. The calculated Factor of Safety was reduced but remained above the generally acceptable minimum design values (1.5~2.0). The column experiments showed that despite the very low permeability of coal, fractures through the coal matrix allow bimodal water flow and hence reduce the risk of water ponding on coal, reducing the risk of slope failure. However, predicted change in the regional rainfall regime may lead to situations of reduced stability during extreme events.
The concept of this study and pathway to quantify water flow and slope stability is transferable to similar cover designs, where a sub-layer of very low permeability, like geotextile, underlies the functional cover.