THE COLLIERY GUARDIAN AND JOURNAL OF THE COAL AND IRON TRADES. ___________________________________ __________________________ Vol. CXVI. FRIDAY, NOVEMBER 1, 1918. ' No. 3018. ____________________________________________ ____________________________________ __________ Removal of a Vertical Shaft Pillar.* By J. CHILTON. The Village Main Reef Gold Mine is situated in the Witwatersrand central area, and, being surrounded on all sides by the workings of other mines, exceptional measures had to be taken for roof support, 6 per cent, of the total mining area being left in situ in the form of stope and drive pillars. These supports, with the aid of large unpayable blocks, were at the time thought sufficient to meet any pressure that was likely to be met with in the course of mining operations. The main vertical shaft is sunk to a depth of 1,375 ft. At 1,200 ft. the South Reef crosses the shaft at an angle of 30 degs., and the Main Reef leader is met with 00 ft. lower. To guard against any movement in the shaft, and to support the mill, engines, and other valuable machinery, a large shaft pillar, 60,000 ft. on the South Reef and 160,000 sq. ft. on the Main Reef leader, was left. Mining authorities differ widely in their estimates of the size of pillar that is necessary for shaft protection. The following table has been made, showing the divergent views of mining authori- ties on this question : — Shaft Pillar : Shaft Pillar: Authority Depth of Shaft Depth of Shaft 600 ft. 1,500 ft. Andre..... 135 ft. square ... 315 ft. square Foster..... 360 ft. diameter ... 570 ft. diameter Hughes ... 600 ft. „ ... 1,500 ft. Lupton ... 399 ft. ,, ... 999 ft. Merivale... 93 ft. square ... 150 ft. square Pamely ___ 195 ft. „ ... 420 ft. „ Wardle ___ 180 ft. „ ... 405 ft. „ The difference shown in this table is certainly re- markable : Hughes requiring in the latter case a shaft pillar that is almost 80 times as large as that desired by Merivale. If the sizes of shaft pillars here outlined were sufficient with the soft thill of a coal seam, it might reasonably be supposed that a much larger pillar standing on hard quartzites would be sufficient to resist either roof pressure or “ draw ”—i.e., that is the lateral disturbance beyond the point actually worked; yet in the year 1908 a vertical fissure in the south-east corner of the shaft appeared, indicating that some movement was taking place in the absolute roof of the mine. This crack extended up the shaft for about 300 ft., and gradually opened till it was 3 in. wide. It was then decided, on the advice of Mr. Stuart Martin, the consulting engineer of the Rand Mines, to carry out extensive sand-filling opera- tions to strengthen and support the area round the shaft, and over half a million tons of sand were flushed into these workings. Some time previous to this slight earth tremors had been recorded on the surface, and shattering and scaling of pillars had been noticed in the drives and stopes north of the shaft, but nothing to cause alarm or uneasiness, and the main arteries of the mine were still regarded as absolutely secure. Round the shaft pillar there were no signs of creep or roof weighting, and timbers seemed to be under little strain. On the morning of November 10, 1914, the shaft pillar collapsed, and on examination it was found that the roof, sides and floor of all the excavations near the shaft had been ruptured. The timbers in the vertical shaft had been broken and the shaft thrown out of line. The pumps, pump chambers and sumps had been destroyed, and the stations and ore-bins completely wrecked. A fall of hanging wall had occurred in a neighbouring incline shaft reaching over 50. ft. above the shaft timbers. The floor, sides and roof in the dis- turbed area had been shattered, and drives and stations were completely filled with debris, the rails being forced up against the hanging. It appeared as if the whole weight of the overlying strata had been carried on outer edges of the shaft pillar and these had suddenly failed. Strange to say, some timbers standing in the drives showed no signs of roof pressure, and the sand-filled areas round the shaft were undisturbed. This collapse was partly due to the fact that the sand had been flushed into the stopes round the shaft some years after these had been worked. By that time the roof had sagged between its supports, and was, owing to the compressibility of the sand, only partially supported. That the sand- filled area did receive pressure is evident from the complete flattening of a 12 in. air pipe that had been left in the sand for ventilating purposes. Although the sand-filling did not entirely prevent the accident, there is no doubt that it minimised its effects. To get the fullest benefit from sand-filling it ought, how- ever, to follow the stope faces as closely as practicable. In a few months the mine was repaired and restored to its normal condition and work resumed, particular attention being directed to strengthening and support- ing the sides of drives, and stations with concrete pillars reinforced with wire ropes and cushions of sand were placed between the concrete and the country rock to distribute the pressure evenly. * Journal of the Chemical, Metallurgical and Mining Society of South Africa. On September 27, 1915, the shaft pillar again col- lapsed. Bad as the first accident was, it was com- pletely dwarfed by this one. The vertical shaft was completely closed for over 200 ft., and stations and ore bins were wrecked beyond repair. The concrete pillars were cracked and shattered, and fissures were visible in the shaft 300 ft. above the disturbed area. Unfortunately, some native miners lost their lives in the mishap, and many were injured. For some time the situation was almost hopeless, and grave fears were expressed that the shaft would have to be abandoned. An attempt to open up the 10th level station had to be given up, as timbers 48 in. square were broken in 24 hours. As the lower part of the vertical shaft could not be re-opened, it was decided to abandon this and fill it up with sand, and thoroughly reorganise the new shaft bottom. The incline shaft was lengthened; new or bins were cut and new cross-cuts driven; the vertical shaft was straightened and re-timbered, and after a stoppage of about five months hoisting of rock was possible, and normal conditions again prevailed. It may be contended that the shaft pillar in this instance was too small for the weight it was called upon to bear, but in mining it is always difficult to correctly ascertain the forces to be dealt with, and it matters not how large a pillar may be, if pressure accumulates beyond the elastic limit of the rock of which it is composed it will certainly fail, often with a single burst of great severity. As reclaiming opera- tions were taking place, necessitating the removal of I IM Fig. 1. pillars and small blocks of ground, a continual re- adjustment of pressure was taking place, and more and more weight was being transferred from pillars unto the solid blocks around the shaft. Shaft pillars that may be adequate in the early years of a mine may become overloaded and fail as the excavations extend and roof pressure increases. As it seemed likely that further rock bursts might take place in this area, Mr. Stuart Martin, the con- sulting engineer of the property, decided that the somewhat heroic measure of removing the shaft pillar entirely should be attempted. As this shaft was the only one through which the ore could be hoisted, and its destruction meant the closing down of the property, it was realised that it was an operation demanding great judgment and skill, and that it was not lightly to be embarked upon. Probably only once before has a similar mining operation been attempted, and this was at the South Kirby Colliery, Nottingham,t where, owing to creep round the shaft, it was decided to remove the shaft pillar. It was necessary to do this t “ Removal of Shaft Pillar at South Kirby Colliery.” C. Snow. Trans. Inst.M.E., Vol. 46. work in such a manner as to prevent any stoppage in the winding operations and interfere as little as possible with the ordinary activities of the mine. The shaft consists of four compartments—two for hoisting rock and two for men and materials—and a start was made from the shaft timbers of the south com- partment or man cage way. A winze 6 ft. wide was driven from the shaft to connect with a raise coming up from a lower level, thus dividing the pillar and facilitating the handling of the broken ground, and forming a starting place for hand-stoping operations. From this winze an excavation about 20 ft. wide was made, completely encircling the shaft. As soon as a space of 6 ft. by 20 ft. had been excavated, a pigsty was placed in position. These supports were formed of blue gum beams 12 in. by 12 in. in thickness and 16 ft. in length. These were framed together, so that when the pigsty was completed it measured 4 ft. in breadth and 16 ft. in length. Shorter lengths would have been easier to handle, but it was feared,that the smaller pigsty might be crushed into the shaft if roof pressure became severe. A space of 12 in. was left between the shaft timbers and the supports. The pig- sties were carefully wedged and hand packed, the entire space being completely filled so that shrinkage could be reduced to the smallest possible amount. With the shaft as centre, the pigsties radiated in all directions, and were put in as rapidly as the rock was excavated; at no time was a space greater than necessary left unsupported. The broken ore was passed down the winze to the level below, and as blasting only took place when winding was stopped no interruption occurred, the holes being so placed as to blow the rock away from the shaft timbers. After the shaft had been encircled in this manner, ordinary stoping operations were started, working outwards to Fig. 2. Fig. 3. V the boundaries of the shaft pillar. The large pigsties were only employed round the shaft, the remainder of the ground being supported by a new form of pigsty designed by Mr. H. C. Hilton. This new type has an open construction that permits of the building in of the filling material, thus increasing the resistance to pressure and forming an almost solid support. Spaces between the pigsties were filled with waste rock until the whole area round the shaft was completely filled, with only spaces left being over an engine chamber and the ore bins. Filter mats were placed among the pigsties so that sand-filling operations could be undertaken, but, owing to the flooding of the mine, this had to be put off till the dry season again comes round. Some months have elapsed since the shaft pillar was totally removed, and little movement has been noticed. That some subsidence, probably not more than 2 in., has taken place is evident by the fracture of the main air column on two occasions, but an expansion joint will remedy this. The shaft timbers show no sign of movement, and no surface subsidence has occurred. The Main Reef leader shaft pillar, which lies imme-