Investigations of the stressed state of a rock mass by the stress-relieving method under conditions of the tashtagol iron-ore deposit

Abstract

The solution of the problem concerning the character of the distribution of stresses in a rock mass is extremely important for the proper interpretation of the results of observations of the development of rock pressure, calculation of underground structures, etc.Observations by the stress-relieving method can give some idea of the distribution of stresses outside the zone and in the zone of the effect of mine workings. A more reliable method of experimental investigations of the stressed state of rocks does not exist at present, although, as will be shown below, in this respect it has quite substantial shortcomings.The basis of the stress-relieving method is the use of the characteristics of the elastic recovery of the rock specimen upon artificial disruption of its bond with the mass. The method is widely used for those rocks whosespecimens retain their shape after removal from the mass.The investigations by the stress-relieving method were carried out in horizontal boreholes. The BA-100 machine was the drilling tool. A diamond bit 76 mm in diameter was used for drilling out the core. To record the elastic deformations of the core end released from loading we used strain gauges with a resistance of 100 ohms and a measurement base of 15 ram, a highly accurate galvanometer, and a special attachment. The boreholes were sunk from a horizontal working.The vertical o z and horizontal o x stresses were calculated in conformity with the manual [1] by the formulas of the elasticity theory applicable to the solution of the plane problem:where E is the modulus of elasticity of the rock specimen being relieved; e x, Sz are the relative deformations of the end of the core being relieved.The results of determining the stresses are shown in the table.It is apparent from the table that in all cases the horizontal stresses were on the average two times greater than the vertical stresses. If the vertical stresses were approximately equal to ~H, then the horizontal stresses were (1.5to 2.0)?,H. It was assumed that this character of the distribution of stresses could be explained by tectonic disturbances; by the effect of second and development workings; by the effect on the stressed state of the rocks in the ore zone of two hills at a distance of 500-1000 m away, whose absolute elevation was more than + 800 m, at an absolute elevation of the earth's surface above the mine workings of + 500 m and depth of the mine workings of 350 m. The presence of these hills can apparently have a definite effect on the distribution of stresses in the rock mass. However, a check by the formulas of the elasticity theory showed that the load from the hills cannot exceed 1.2 for the vertical stresses and 11.3 kg/cm ~ for the horizontal. 100

 On Classification of the Earth's Crust Areas by the Level of Geodynamic Threat

“…The source of earthquakes is the stress concentration in this area [30], associated with the interaction of the crust and mantle [31]. Much earlier, the very rst results of measuring rock stress showed that horizontal stresses exceed vertical values [38,39]. This nding was explained by the presence of tectonic processes, and was subsequently con rmed by numerous tests and general conclusions [40].…”

The state of stress in rock around a mine working, with consideration of the tectonic component of the stress

“…Thus at the Tashtagol' iron-ore deposite [4] and the Khaidarkansk [5] deposit, the Rasvmnchorr mine [6], and in various foreign deposits, the horizontal stresses are greater than the vertical ones by factors of 2=6. Movements of the Earth'~ crust and terrestrial seismicity are explained by the influence of tectonic stresses.…”

Relation between filling of reservoirs and earthquakes

“…The analyses indicate that for an earthquake with a magnitude of 6 the total width of the subvertical cracks parallel to the break per kilometer of rock mass in the region of the epicenter, in a direction normal to the break Measurements of the horizontal pressure in rock masses at great depths have permitted establishing not only small values of this pressure but also large values which exceed by a considerable margin the vertical pressures at those depths [13][14]. It foUows that the rock masses in the crust behave as a rigid elastic body, that the creep phenomenon in them is not so intense as to lead to the opening of a11 cracks and to equalization of the stresses according to hydrostatic law, and that in different portions of the crust there are downward forces which lead to compression in the horizontal direction.…”

“…The scheme of an active volcano is usually represented in the form of a system of cavities filled with meRed magma and a system of conduits connecting these cavities to each other and to the ground surface [11][12][13][14][15]. The sections of the conduits are measured in tens and hundreds of square meters.…”