UNDERGROUND WORKINGS OF A WESTERN COLLIERY'. XLV. 



and even if they are not altogether crushed, a large proportion is 

 ground into small coal; if the floor is weak the downward pressure 

 of the roof upon these small pillars causes the floor to rise between 

 them as illustrated in diagram D., now however an improved 

 system has been adopted in which 12 yard bords and pillars 

 predominate. 



The 'creep' is to be much dreaded, and having once set in it 

 spreads slowly but surely over the whole mine, no timbering can 

 arrest its progress, roadways and airways require constant and 

 costly repairs, and it is often so severe in its effects as to necessitate 

 the abandonment of the district or mine. Much damage has 

 already occurred owing to the deficiency of pillars, and over a 

 large area pillars have been crushed and tops have fallen. 



In addition to the danger caused by narrow pillars there is the 

 great loss of coal when the pillars are being removed; if the pillar 

 is too small it cannot carry the pressure of the roof without 

 having its sides and ends split and fractured, so that the working 

 of the pillar results in an abnormal quantity of slack. In small 

 pillars it is generally estimated that only 50% of the coal can be 

 extracted, but in large pillars 80% can generally be relied upon ; 

 in narrow bords the proportion of waste is considerably more than 

 in wider bords, so that the wider the bord and pillar the more 

 economical is the working, this is seen in diagram E. 



This view as to the size of the pillars is corroborated by the 

 Chief Inspector of Coal Mines in his report for the year 1899, 

 where the following remarks occur : — "The pillars left in the bord 

 and pillar system still continue to be made too small at some of 

 the collieries in the first instance, having regard to a successful 

 "broken" or second working and also to the extraction of the 

 greatest possible percentage of coal from a given area. It is a 

 difficult matter to persuade some colliery managers of the unsuit- 

 ability of the time honoured 8 yard bord and 8 yard pillar even 

 under very much increased depth. The following condition bear- 

 ing on this point is now included in the leases which are issued 

 for working coal under Crown lands. The percentage of coal to 



