Workability/cuttability of hard coal seams with igneous intrusions
To evolve methods which would help in selection of suitable coaling machines for hard coal seams, a number of field and laboratory tests were carried out on more than 15 coal seams of compressive strengths ranging from 19 MPa to 44 MPa. The effect of physico-mechanical properties on cuttability were studied in the laboratory for all these coal seams to identify the relevant parameters affecting the specific energy of coal cuttability. These data were subjected to regression analysis to find the best fit for estimation of laboratory specific energy of coal samples on the basis of simple laboratory and field tests for the strength parameters. Field studies were also conducted over a large number of active mechanised coal faces to study in situ cuttability along with geo-mining conditions of the site. The field and the laboratory data so generated were correlated and an attempt is made to establish a relationship for the estimation of capacity of coaling machine by considering the geo-mining domain of the field in totality.
On the basis of physical inspection of exposed area in a gallery, the coal mass near an igneous intrusion band is divided into four zones; called as (1) normal, (2) pulverized, (3) Jhama and (4) mixed zone excluding the dyke/sill. Some simple field and laboratory studies were carried out to visualise the nature and extent of variation of strength and cuttability across these zones of igneous intrusion in a coal seam. Influence of intruded igneous materials over the in situ strength of the coal mass was studied by Schmidt hammer while the influence over cuttability of the seam was studied through monitoring of current drawn by roadheaders during mechanised gallery drivage in coal seams consisting of igneous intrusions. Both, strength and cuttability across the band of igneous intrusion were found to be highly dependent upon the proximity and extent of the intrusion. In fact, requirement of current for gallery drivage across the coal mass with igneous intrusion was observed to be quite high in comparison to that required for the normal coal mass. To understand this phenomenon in detail, a number of samples were collected from the coal-intrusion interface and were subjected to laboratory tests for physico-mechanical properties along with cuttability testing with a drag bit type coal plough rig. The laboratory study showed wide variation in physico-mechanical and cuttability properties of different samples collected from different zones of the affected coal mass surrounding the intrusion band.
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[1] Sheorey, P. R., Das, M. N., Barat, D., Prasad, R. K. and Singh, B., 1987. Coal pillar strength estimation from failed and stable cases. International Journal of Rock mechanics and Mining Sciences, 24: 347-355.