Properties of various types of roof

The immediate roof in a roadway or workplace in a coal seam may consist of coal, sandstone, fireclay or shale.

Coal roof is common in all seams more than 3 m thick if coal near the floor is worked. It is reliable and with occasional dressing down of the hanging portions will not prove dangerous when it stands for long periods, even a year or more. Compressive strength of coal varies but maybe considered as 2.25 kg/mm² (1.5 te. per square inch).

Sandstone roof bends slightly before breaking and will give enough warning before fracture. It is, therefore, reliable and is considered "good" in mining terminology. Coal parts readily from sandstone roof. Crushing strength of sand stone is nearly 13.5 kg/ mm².

Shale roof is treacherous and most unreliable. It rarely gives any warning before collapsing. It weathers quickly when exposed to atmospheric action and spalls off, that is one layer after another comes down at intervals. It is a good practice to keep 0.6 m of coal intact near a shaly roof during development of a mine if the seam thickness so permits. The coal left intact in the roof may be completely extracted when depillaring the area.

Two terms, laminated roof and massive roof, are generally used in mining in case of roof consisting chiefly of sandstone. Laminated roof is common in most of the coalfields. During depillaring operations, if the roof is laminated, local" roof fall occurs 4 to 10 hours after the props are withdrawn. This relieves the coal pillars of local weight -and is, therefore, an advantage when extracting the pillars. Massive roof consists mostly of sandstone which has no laminations, e.g. roof in Giridih coalfield. Such roof will not come down easily and will not give enough warning before its break. In the depillaring area such roof will stand for a long period even after withdrawal of supports. When the roof fall takes place, it is generally over a wide area followed by a heavy air blast and crushing of pillars in the vicinity of goaf.

TABLE 1

A is surface with multi-storeyed pucca buildings. B is the surface with one-storey building, main river like Damodar and main railway line. C is the surface occupied by kutcha buildings, jores or tanks D is the surface on which sidings and roads are laid. T is the thickness of the seam being extracted in (in meters).
Method of extraction	Safe cover for surface
	A	B	C	D
1.  with caving	60 x T	52.5 x T	45 x T	37.5 x T
2.  With hydraulic sand-stowing (Assuming 15% shrinkage)	9 x T	7.5 x T	6.8 x T	5.6 x T
3. With dry packing (as suming 30% shrinkage).	18 x T	15.75 x T	13.5 x T	11.25 x T
4. Splitting of pillars to the extent that pillars left contain at least 40% of pre-mining coal content upto 60 m depth, and hydraulic stowing of de-coaled voids. For every 60 m extra cover, the percentage of coal should be increased by 1%	5.5T	5.5T	4T	3.5T