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2 Problems at great depth due to high rock pressureFrom geo-mechanics stand-point, great depth refers to distances below surface where creation of an opening results in an overstressed condition in the surrounding rock, resulting in failure or yield. Thus the depth at which a mine may be considered deep depends on the strength of the rock mass. In very strong, brittle rocks such as metaquartzites or granites; mines are not generally considered deep until their workings extend 1800m or more below surface. By contrast, coal and salt rock mines can be considered deep when they extend more than 450m below surface. Geomechanics behaviour in deep mines: openings in “deep” mines in weak, ductile (or pseudo-ductile) rocks, such as salt rock, shale, or bituminous coal, are characterized by viscoplastic (or pseudo-viscoplastic) deformation of the surrounding rock. This behaviour serves to mitigate the effects of the high stresses. In strong, brittle rocks, however, strain energy resulting from excavation is not dissipated through viscous flow, but is stored in the rock until a limit is reached at which failure occurs in an explosive manner. Such explosive failures, which are called rockbursts, are small-scale seismic events – micro-earthquakes accompanied by emission of acoustic energy.

3 Control measures to avert high rock pressureHigh rock pressure situation may be averted by proper planning and practicing right extraction methods that are as follows: Stop Planning: Planning of the stopping methods should be such that it can withstand the high rock pressure without endangering the stability and workmen. Speed of Stopping: Maintaining the extraction rate as fast as possible is a strategic measure for controlling rockbursts. Ground Control: Limited number of working faces, adoption of flat back stoping method with short strike lengths, orientation of mining faces at an angle of to the major trend of rock fracture and avoidance of mining the faces towards each other are some of the strategic measures for ground control. Geological Conditions: Geological discontinuities are hindrance to the transfer of stress concentrations. Whenever the orebody is intersected by dykes and faults or other geological structures, the stoping should be commenced from the dykes/fault position and moved away from them instead of advancing towards them. Shaft and level linings: In high pressure environment movement between the hanging wall and the foot wall and rock displacement in general affect both the shafts and levels. Shafts may loose their verticality hindering faster hoisting operation.

4 Problem of excessive rock temperatureEnvironmental control in deep mines can constitute as much as 25% - 30% of the total mine working cost. High environmental temperatures coupled with heat and humidity can produce on workers many adverse physiological effects such as heat cramps, heat stroke, heat exhaustion and collapse. In order to create acceptable working conditions for men, heat must be removed from workings. It has been found that in shallow mines about 1.6 m3/s ventilation air per 1000t of rock broken per month and for deep mines, about 4 times this quantity is desirable. However, since the power required to circulate air increases with the cube of the quantity of ventilation air, there is an economic limit to ventilation, in the region of 4.2 m3/s per 1000t rock broken per month. The economical depth limit is found to be around 2 km depth and the normal ventilation by circulation is found to be sufficient for rock temperatures up to 300C. When the rock temperature is about 400C, about 60% of the cooling requirements can still be provided by the normal ventilation air, the balance being provided by refrigeration (cooling service water). As rock temperature increases to about 450C, the ventilation air can provide only 30% of the cooling requirements, while 25% is provided by cooled service water and the balanced 45% by cooled ventilation air. At greater depths where rock temperatures are of the order of 550C, all cooling requirements must be provided by refrigeration. Of this, only 20% can be distributed by cooling the service water with a further 40% by cooling the ventilation air. The remaining 40% of cooling is provided by the successive re-cooling of the ventilation air as it passes through the mine. Also it is essential to ensure that maximum production is achieved from minimum number of working areas to keep ventilation costs to a minimum. More recently, use of ice for cooling the underground workings, rather than the conventional chilled water circulation has been considered in South African deep gold mines.

5 Measures to reduce temperatureChilled Water Cooling System: Chilled water at any temperature between 00C to 100C is produced by refrigeration plants situated on the surface and/or underground, and then distributed via an extensive pipe network to various parts of the mine for cooling purposes. A 300 – 400 mm dia insulated shaft line delivers chilled water down the shaft. Underground spot coolers and cold water chillers: Spot coolers are self-contained small spot refrigeration units and are generally used for localized or secondary cooling on jobs where temperatures are exceptionally high. Spot coolers reduced the temperature of air locally by allowing hot air to evaporate refrigerant directly. Use of ice for cooling the underground workings: Small ice crystals (60-70 mm square and 8-10mm thick) in slurry are conveyed pneumatically for long distances in underground. The air used in pneumatic conveying is cooled to 10C to minimize melting of the ice during transport to shaft bottom. It is estimated that for cooling a very deep mine, about 2 t of ice/t of rock hoisted, will be necessary. Backfill and recirculation: Installation of backfill gives rise to a substantial reduction in the rock surface area which emits heat. The environment is further improved by better ventilation flow, since the presence of backfill prevents the unwanted passage of air through back areas where it picks up heat. Recirculation of a portion of the ventilating air in the underground workings can provide an alternative to increased flow rates of downcast intake air. Advantages include smaller mine fans, saving in airways (including reduction in the number of additional ventilation shafts), reduced air leakage and more effective refrigeration system.

6 Other associated problems due to great depth Hoisting problems: Hoisting from great depths is often complex, time consuming, difficult and therefore expensive on the whole. Maintenance of deep shafts with expensive hoisting equipments itself an important, parameter for mine safety. Great depths require a system of primary, secondary and tertiary drops. Surveying: Great depths makes surveying operations very difficult, time consuming and complex. High precision instruments are necessary to do the normal traverses. Correlation survey is tedious and problematic. Dust control: This is another problem at great depths. One cannot simply spray water to control dust as it causes enormous humidity due to high rock temperature thereby making the workers life difficult. Ice blocks are at times used to cool, the water at the face which at times is used for drilling. It may be mentioned that combating all these problems of excessive rock temperature makes the over all mining operation expensive. Hence such precautions can only be practiced when the mineral to be extracted is either a priority item or can pay for its high cost of production. However, the national policies of any country dictate the terms for exploiting such deposits that involves high investment in capital equipment to overcome the problems of high rock pressure, rock temperature and hoisting from great depths.