Abstract

FENN, D.; DU KANDA, A.  and  DUKHAN, D.. Determining settlement rates and surface stability using in situ density of backfill as a proxy for displacement. J. S. Afr. Inst. Min. Metall. [online]. 2015, vol.115, n.11, pp.1035-1043. ISSN 2411-9717.  http://dx.doi.org/10.17159/2411-9717/2015/v115n11a8.

Gibb Consulting Services was contracted by Eskom Line Engineering Services to conduct an investigation to determine the absolute displacement to date and infer the current rate of displacement of unsurveyed colliery backfill. At the test site, where a 400 kV power line is to be constructed, neither the surface nor the backfill material had been monitored and it was not possible to state whether the surface will be subject to future settlement that may compromise surface infrastructure. Therefore indirect methods of estimating current and future displacements needed to be found to act as a proxy for a comprehensive long-term settlement monitoring exercise and provide a reasonable estimate for the settlement that has occurred in the decades of mining. The innovative solution was to recover essentially undisturbed core from backfill using a sonic drill rig and use the measured dry density values from the samples as a proxy for the rate and absolute displacement in the backfill. The success in obtaining an essentially undisturbed backfill core and the clear relationship of the backfill density with age and depth allowed the conclusion to be issued that the power line infrastructure can be constructed over the backfill; with a warning that once the water table recovers after mining ceases some collapse settlement may occur. With this method, the amount of settlement can be predicted and, if proven conclusively, provides a way to confidently release huge areas of currently waste ground for future surface infrastructural development.

Keywords : settlement; subsidence; mine backfill; undisturbed backfill core; sonic drilling; bulk density; surface infrastructure construction over backfill.

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