51. The ratio of weight per unit volume of solid rock mass in the bench and weight per unit volume of loose rock mass after blasting or loosening is called
52. It is the mining method in which masses, panels or block of ore are undercut to induce caving, permitting the broken ore to be drawn off below
53. Risk of premature collapse and over-ridding of pillars are maximum with
54. Danger of air blast is maximum with
55. In a coal mine the depth of working is 200 m from surface, width of galleries 3 m, height of galleries 3 m and distance of square pillars between centres of bords is 22.5 m. What is strength of the pillar
56. A surface mine blast design has 9 holes in a row, each of 8 m length and 200 mm diameter. The spacing and burden are 6 m and 5 m respectively. The length of subgrade drilling is 1 m and the density of in-situ rock is 2.43 t/m3. Assuming no back break, the output per blast in t is
57. Wagon drill working at minimum compressed air pressure of
58. It is a solid block of coal surrounded on all its side by galleries. Identify it
59. Crown Pillar is
60. Apart from mining of coal, the longwall mining method has been practiced for mining the deposits of
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