New methods for the determination of mechanical rock properties in laboratory and in situ are described. The methods make possible speeding up and simplifying the investigations as well as increasing the reliability of the results obtained. Reported is the evaluation method of rock mass state of stresses based on the property of rocks "to remember" the strains which acted upon them.
When mining mineral deposits, mechanical rock properties considerably affect the processes of rock deformation and failure. The choice of shaft location, methods of shaft sinking and mining, pillar sizes, modes of supporting for development and main workings, to a great extent, depend on coal and rock properties. Hence, in planning and exploiting mines, especially in complicated geological conditions and at a great depth, it is necessary to take account of rock failure mechanism, stress strain characteristics and rheological properties (including post-failure domain). Carrying out simulation and analysis also needs for a deep insight into the physical and mechanical properties of rocks. Therewith, true evaluation of strain state of rock mass in situ is of great significance. VNIMI Institute has developed new methods and equipment for determining rock properties and strain state of rock mass in laboratory and in situ.
When mining deposits lying at a great depth, the stresses of rocks around the workings exceed the strength limits, therefore the rocks adjacent to the working contours fail. The failed rocks continue to be part of the rock mass, and their properties (residual strength, loosening, etc.) considerably affect the pressure on the support of the mine workings. Experimental investigation of the post-failure rock characteristics calls for special equipment which makes it possible to test rock samples controlling their deformation. In designing the equipment two tendencies are outlined: designing rigid presses with automatic control of testing conditions and designing rigid attachments to conventional hydraulic and mechanic presses. Nowadays rigid presses are of limited utility because of lack of their industrial output, high costs and complexity. To define strength and strain properties of rocks (down to complete failure) the BB 21 rigid stabilometer (Fig. 1) is designed by VNIMI. In the stabilometer the sample tests are carried out in the pre-set axial strain state under different values of lateral hydrostatic pressure. The press load P is passed to the rigid stabilometer and distributed among the rock sample, stabilometer chamber walls and practically incompressible fluid. When reaching the strength limit, the sample gradually fails and its carrying capacity smoothly decreases to the level of the residual strength. The stabilometer set comprises self-recorders, a pump unit for creating lateral pressure (б2=б3) on the sample and attached devices for measuring longitudinal and lateral deformations. To determine mechanical properties of disturbed rock mass a special procedure for testing rock samples in laboratory is worked out. The procedure consists in producing artificial faces of loosening in the samples followed by comparative tests of the above samples and monolithic samples.
