ABSTRACT:
Discontinuities and displacements at structural block boundaries arising from loading a hard rock result in stepped, discrete distribution of deformations. Level of concentration of deformations at the block boundaries depends on stress parameters, the structural block shape and dimensions, and conditions at the block boundaries. The concentration of deformations should be taken into account during support design, estimating stability of underground structures affected by mining. Especially, influence of the discreteness becomes stronger at increased depth where stresses vary in a wide range. In this paper are generalized the results of investigations on distribution of deformations at a iron mine in the Urals(USSR).
1 INTRODUCTION
The discrete nature of rock mass deformation was investigated in working conditions by means of comparison of actual vertical and horizontal displacements of monitoring station datum marks (measured in situ geodesically), which give the distribution of deformations in actual blocky mass, with theoretical ones for a solid elasto-isotropic medium. A profile line design allowed displacements at sites of 24 to 325m long to be compared. The investigations have been performed between 1968 and 1982. The discreteness in distribution of deformations in case of actual rock mass was characterized by a coefficient of correlation between calculated and measured values of vertical and horizontal displacements.
1.1 Experimental test conditions
The monitoring station comprising about 150 datum marks along profile lines at surface and underground was used as a test site. Enclosing rocks were porphyre and its tuffs, diorites, and scarns with uniaxial compression strength of 90–120MPa. Mining was performed at a depth of 530m. In 1971, a surface subsidence has occured, its diameter now being about 800m. The values of tectonic compression stresses measured in situ in the mine area were 1,5 and 9,6MPa, with the greater value along meridian. For calculations, the tectonic stresses were used as initial boundary conditions.
1.2 Displacements of the datum marks of surface monitoring station
By the moment of the surface subsidence in 1971 the surface monitoring station had 4 profile lines with 33 datum marks. The diameter of subsidence zone was 210m. In case of the profile lines up to 300m in length the coefficient of correlation between theoretical and measured displacements was equal to 0,9. For intervals of 20–25m in length, however, its value did not exceed 0,55. Therefore, with shortening a measurement base the correlation between theoretical and measured displacements significantly decreases that seems to be account for discrete nature of displacements at the structural block boundaries. Analogous results were obtained during tests between 1974 and 1982 in the process of surface subsidence expansion. The displacements were measured along 6 profile lines with 77 datum marks. On comparing vertical and horizontal displacements along the profile lines exceeding 300m in length, the correlation coefficient of 0,85 was calculated while one for intervals decreased to 0,3.
1.3 Displacements of the datum marks of underground monitoring stations
At the same time, analogous tests were performed at monitoring stations underground in cross-cuts at depth of 160 and 300m.
