NEW WINNING OR, SEA PIT. 291 



l)ord is then dropped, and preparations are made for drawing 

 the next pillar as in the previous case. The posts that sup- 

 port the roof temporarily are drawn as far as possible for 

 future use before the workings in which they stand are aban- 

 doned. By employing plenty of posts, less timber is lost 

 through crushing than if one is more sparing in its use. The 

 posts have lids or slabs wedged tightly between them and the 

 roof; the wedges may be driven in from any direction. If 

 the post is not quite long enough, one may use two lids, and 

 drive wedges in between them. Where the ground is "made," 

 i.e., the roof has fallen in, sole pieces are used for the posts 

 to rest on. 



In many cases it is immaterial whether the surface of the 

 ground above the workings of a colliery is allowed to subside 

 or not, but in other cases, where buildings, railroads, reser- 

 voirs, etc., are overhead, the workings must be properly sup- 

 ported. Much of the ground mined by the A. A. Co. underlies 

 the city of Newcastle, and 011 more than one occasion a sub- 

 sidence has taken place, much to the alarm of those who own 

 house property in the affected area. A Iloyal Commission 

 was appointed to enquire into the last subsidence, which took 

 place on 17th January, 1908, when damage was done to vari- 

 ous surface buildings, the obelisk reservoir, footpaths, gas 

 main, service pipes, sewer, etc. The creep area was about 100 

 acres, and under this about 65 acres of the yard seam has been 

 worked, partly by the Government in the early days, and 

 partly by the A. A. Co. This yard seam is 2ft. lOin. thick, 

 and lies 170ft. above the borehole seam. The borehole seam 

 under this area is practically worked out, except the pillars 

 left, which amount to 60 per cent, of the total seam. The 

 greatest depth of cover above the borehole seam in the dis- 

 turbed area is 450ft. The Commissioners considered that the 

 third creep was a continuance of the first and second creeps, 

 and that the movement in the workings of the borehole seam 

 would not in itself have been sufficient to cause the damage, 

 but was intensified on the surface by the old excavation in the 

 yard seam. The size of pillars it is advisable to leave in those 

 cases where the surface must be left intact is one that must 

 always be left to individual judgment, for one can never hope 

 to obtain sufficient reliable data to enable one to make exact 

 calculations. The strength of the coal varies, even in the 

 same seam, some bands of which it is composed being softer 

 and more pliable than others, and such weak bands really de- 

 termine the strength of that particular coal. Then a pillar 

 which is strong enough in limited workings, may not be suf- 

 ficient to support all the weight thrown upon it when the 

 workings are extended, and lateral support is nil. The pre- 

 sence of dykes and faults, by interfering with the continuity 



