When conditions in the gate roads of a longwall panel at Sigma Colliery suddenly started deteriorating, the matter was investigated in detail. Observations indicated that there were distinct zones of tensile and compressive rock failure. The observations corresponded to the predicted state of stress in the roof of an area where pillars had failed.
It was then concluded that the pillars in a previously mined bord and pillar panel in a lower seam had failed. Further analyses indicated that progessive pillar failure could take place, which would seriously jeopardize the stability of the main gate. When underground observations indicated that pillar failure was in fact progressing, the face equipment was extracted before the gate roads collapsed.
In a multi seam situation, such as at Sasol Coal's Sigma Colliery, pillar instability on a particular seam may have serious consequences on overlying seams. The case history presented in this paper describes such an event, where pillar failure on the No.2 Seam ultimately resulted in the collapse of longwall entries on the overlying No.3 Seam.
Current understanding of rock mechanics led to the recognition of the failing state of pillars on the inaccessible lower seam by observing rock conditions on the upper seam. These remote observations, coupled with analyses, enabled certain predictions to be made. Consequently, a sound decision regarding the workings on the upper seam could be made and more serious losses were prevented by taking timely action. From the purely technical point of view, the events described here afforded the unique opportunity to visually observe the behaviour of a rock mass overlying an area subjected to pillar failure. The upper seam development, situated over the failing area, was a window which allowed a look into the rock areas subjected to various magnitudes and classifications of stress. Specifically, it allowed one to visually observe the behaviour of a rock mass subjected to overall tension.
Four mineable coal seams are present at Sigma Colliery, as shown in the simplified stratigraphic section in Figure 1. The No.1 Seam has only been mined on rare occasions, due its varying width and uneven floor. The No.2 seam has been split into the 2A and 2B Seams, and has been extensively mined by bord and pillar, pillar extraction and longwall methods. The No.3 Seam has also been extensively mined by a variety of mining methods. It is characterised by a weak, rapidly weathering mudstone roof of as thick as 30m, and is the most difficult to mine of the three seams which are usually exploited.
The mine has been in operation for some 35 years, and vast areas have been mined. It pioneered mechanised longwalling in South Africa and played the leading role in the development of the Rib Pillar Extraction mining method for South African conditions.
Multi seam operations have been practised for several years. Many combinations of mining methods have been implemented. These include longwalling/longwalling, longwalling/pillar extraction, longwalling/bord and pillar, pillar extraction/pillar extraction, etc.
