Mining

Numerical modelling produced upper and lower bound strength formulae for rib pillars, the lower bound covering weak roof and floor contacts. The formulae apply between width to height ratios of 0.5 and 2.5, with linear interpolation below that, and give conservative designs above 3. Adjustments were suggested where regular joint sets occur.

Back analysis of 77 collapsed and 245 uncollapsed pillar cases showed that much of the scatter in pillar strength comes from differences between coalfields, so a single national formula is unrealistic. Seam specific formulae were derived for the major coalfields; Free State and KwaZulu Natal data remained too sparse. The lognormal safety factor distribution was retained.

A rating and ranking system was applied to South African coalfields and, separately, to the coal bearing countries elsewhere in the Southern African Development Community. The Waterberg ranked highest in South Africa and is the coalfield most worth detailed investigation as Witbank output declines. Botswana ranked highest among the other member states on contained resources and exploitation potential.

Tonnage estimates for the remaining low phosphorus resource could not be produced, because the phosphorus analyses supplied by mining companies did not state what had been measured. Distribution trends were still mapped. Coal at or below 0.010 per cent phosphorus is widespread in the Witbank Coalfield, restricted to the far northwestern Highveld, and follows a northwest to southeast band in eastern Ermelo.

Electrical and electromagnetic ground surveys were run over a Waterberg profile with borehole logs and three dimensional seismic data as control, to test whether cheaper methods can match seismic reflection for locating coal and faults. Resistivity tomography at two electrode spacings, frequency and time domain electromagnetics, controlled source audio magnetotellurics and magnetics were all acquired. Results are preliminary.

Geotechnical factors affecting secondary extraction were identified from interviews with collieries already re-mining pillars and with people experienced in stooping, then quantified and ranked against the different aspects of the work. No single factor governs the outcome. Factors interact, so choosing a method and working safely in old workings depends on managing them together across disciplines.

Shallow coal suitable for small scale opencast working was mapped in the northern Witbank and Highveld coalfields, using a limit of 30 m from surface to the top of the first seam. Most outcrop areas examined proved already mined out, and depth to the top seam increases southwards, though some ground remains available. Obtaining resource data was the main obstacle.

About 3.19 billion tons of coal now sits underground in pillars, growing by roughly 110 million tons a year under current bord and pillar practice. Pillars with a safety factor of about 1.8 or higher can be extracted successfully, and 1.5 is permissible in thin seams where the pillar width to height ratio exceeds 10.

Sub-seams within the No 2 Seam were characterised at a single test site using lithology, seam thickness, proximate analysis, petrography and geophysical logs, and a paleoenvironmental model was built from them. Wireline logs separated coal from interburden but could not resolve the individual zones, because the physical differences are too subtle. Better sedimentological models gave better reserve and resource estimates.