1.INTRODUCTION
During the mining course, when the roof above coal seams is constituted by thick and hard sandstone(conglomerate),sudden roof movements often take place with great force and destruction which is the biggest danger to the safety and production. Such sever movements of the roof in mining directly relate to the rock mechanical properties which are the main factors that influences the regularity of hard roof movement and its destruction form. A significient work and important technical way for applying rock mechanics into mining engineering is studying the rock mechanics of the hard roof and altering some of the rock mechanic parameters which induce the improvement of the rock properties in order to efficiently control the movement of the hard roof.
2.THE MECHANICAL PROPERTIES OF HARD SANDSTONE(CONGLOMERATE)
The coal seams being mined at DATONG coal area belong to Jurassic period,Mesozoic Era with the roof of clasolite of continental deposit in various sizes of grains, main part of which is of sandstone and conglomerate whose colloidal matter is mainly of argillaceous and carbonate matrix. Sandstone consists mainly of quartz and feldspar while the matrix of conglomerate is of coarse and medium size quartzous feldspathic sandstone with smooth rolling circle and diameter of 20 to 300mm and 70% of the maximum gravel content, most of which are quartz, quartzous gravel, silex and gritstone,etc. The roof of strata formed by conglomerate and coarse and medium size sandstone is of under-developed stratification, joint and fracture. So after caving in the goal, the stratifications of the strata are very thick, above 3m averagely and up to 10m occasionally. The data gained from underground mining shows that without effect of faulted structure the spacing between microcracks and fractures including joints is above lm and in the spacing of several decameters, weak plane can be seen occasionally. The caving roof in the goaf is in the large-size blocks, normally above 10 cubic meters.
2.1 The mechanical properties of rock gained from lab.
Table below is the physical mechanic properties of the rock sample from the roofs above the present operating seams at DATONG coal area,
TABLE 1. physiced Mechanic properties of Sandstone(conglomerate)(available in full paper)
Hydrophysics is the physical and mechanical properties of rock after immersed in water. The mineral contents in the rock can be either hydrophilic or hydrophobic, so with the action of water, the more hydrophilic minerals are in the rock, the more unstable its mechanical properties are. Such rock samples, when the water content is lower, have higher strength and lower compressibility with the sign of brittle fracture and elastic medium; and have lower strength with the sign of plastic failure when the water content is higher. The ratio between the compressive strength of wet rock test piece and dry rock testpiece is named rock softening coefficient which is the main parameter to the hydrophysics of rock. The colloidal matter of sandstone in DATONG coal field is mainly or clay mineral matters and carbonate matters and the test results showed that the softening coefficient for the sandstone of clay mineral colloidal matters is 57%, and 81% for the sandstone of carbonate colloidal matters, after immersed in water.
