Centrifuge modelling of permeability in a heterogeneous coal mine backfill sequence

Abstract

Opencast mining has been and continues to be a favoured method for the extraction of the vast coal reserves in the Highveld of South Africa. Previously backfilled and restored open cast areas are generally zoned for agricultural uses, but with growing pressures on land use, such areas are increasingly being considered for the expansion of infrastructure and regional development. Understanding the backfill permeability and hydraulic behaviour is therefore an important component in defining the land use restrictions placed on a previously backfilled area. Centrifuge modelling provides a means of better understanding the hydraulic behaviour and measuring the permeability of opencast backfill under controlled laboratory conditions. Based on a preselected backfill prototype, an appropriate centrifuge model was developed. Using miniature pore pressure transducers, the pore pressures were measured at discrete locations in the model during falling head tests in a geotechnical centrifuge. Using the measured volumetric discharge, spacing between the transducers and the measured pore pressures, the permeability of the backfill was calculated. Due to polarized opinions on the scaling of permeability in a centrifugal field, a control model was tested at 1g and 23g to validate this scaling law. It was demonstrated that the respective permeabilities calculated at 1g and 23g were effectively the same and that it is in fact the hydraulic gradient that is scaled N times in the centrifuge. Knowing this allowed the calculated centrifuge permeabilities to be directly related to the prototype represented by the model. To determine the accuracy of the centrifuge model, the results of field percolation tests were compared to the results of an analogous centrifuge model. There was no correlation between the results and it was not considered meaningful to compare the results, as the model and percolation test site (prototype) conditions differed significantly. To simulate the preselected prototype backfill sequence, a model configuration that represented the geometry and material properties of the prototype was tested at 35g (half scale) and 70g (full scale) in the centrifuge. The results of the centrifuge model were used to make reasonable predictions on the long term permeability and hydraulic behaviour of the backfill prototype. It was found that the permeability of the backfill is likely to decrease over time due to consolidation settlement. The bottom of the backfill sequence is expected to have the lowest permeability and the top is likely to maintain a higher permeability. It was further demonstrated that the horizon interfaces acted as flow restrictors and resulted in poor vertical permeability between the horizons in the backfill sequence. Overall the centrifuge methodology provided a unique and efficient means of modelling the long term permeability and hydraulic behaviour of the backfill sequence.