Rock mass and Earth’s surface movement during underground working of cryolithozone placers

Abstract

The results of the researches on the processes of movement of rock mass and the Earth’s surface during underground working of alluvial mines in the cryolithozone are presented. Geologically, the area of the deposit is an alluvial placer. The productive seam is represented by pebble deposits. The cementitious composition of pebbles is clay, sand and ice. The thickness of the productive seam is on average 1.8 m. The average thickness of loose deposits is 30 m. The temperature of the rocks is minus 6-8°C. The moisture content of the rocks is 22%. Mine-field parameters: width - 50 m; length - 320 m; development depth - 30 m. Development system -pillar. Roof control - the collapse of the roof rocks onto the T-13K powered support. The first spontaneous roof caving occurred during a longwall face span 28 m from the contour of the mine-field. The beginning of the movement of Earth’s surface was noted at bottom hole deviation from the contour of the mine-field by 55 m 15 days after the first caving. During the development of the second half of the mine-field, the rates of maximum subsidence increased significantly, reaching in some cases 30 mm./day. An increase in the rate of subsidence of surface bench marks (12-18 mm/day) refers to secondary roof sediments. The following features of the rock mass movement have been established: the general deformation of the roof rocks begins at 2500 - 3000 m2, the angles of strike shift and across the strike of the mine-field are practically the same and fluctuate within 75-80° the maximum subsidence of the shift trough with full additional working is 0.75-1.0 m and is located in the middle of the mine-field.