In favourable mining conditions, in particular at the Starobinskoye potash deposit (Belarus), longwall mining systems are used. They cause a high human-induced load on the subsoil, including intense deformation of the ground surface. The presented investigations are aimed at studying the dynamics of the ground surface displacement during the longwall face advance. Mathematical modelling was carried out in an elastic-plastic formulation with numerical implementation by the finite element method. The condition for the roof rocks collapse was opening of the contact between the seams when its boundaries were reached by shear fractures or formation of the tensile stresses area at the outcrop. With the working front advance, an increase in subsidence is observed, followed by its stabilization to a value determined by the process parameters of mining operations and the physical and mechanical properties of collapsed rocks. In this case, each point of the ground surface experiences sign-alternating horizontal deformations: when the front approaches, it causes tension, and when it moves away, compression. The obtained results of mathematical modelling are in good agreement with the data of instrumental measurements of the ground surface displacements, which indicates the adequate description of the rock mass deformation during the slice excavation of sylvinite seams by longwall faces.
In the development of practically all potash salt deposits, the study of gas-dynamic phenomena (GDP) is one of the most difficult tasks to ensure mining safety. Sudden salt and gas outbursts, dynamic breakdown, which are accompanied by intense gas release and possible broken rock carry-over into the mine workings, are associated with GDP. Geological preconditions for the GDP development are often the layered structure of the salt rock mass, the presence of interlayers and layers of salt clays. For the conditions of the Usolsky potash plant mine, complex studies of factors that characterize the possibility of gas-dynamic roof fall of the stoping rooms were carried out. In mine studies, free gases pressure and the initial velocity of gas release in the rocks of the roof workings were determined. The obtained experimental estimations were used as a parametric basis for mathematical modeling of geomechanical processes under conditions of a near-contact accumulation of free gas. The deformation of a layered salt mass produced by a room development system was described by the model of an ideal elastic-plastic medium with internal friction. The parabolic envelope of Mohr circles was used as a plasticity criterion in the compression area. In the numerical implementation, the deformation of clay contacts was modeled by Goodman contact elements. Based on the results of multivariate numerical calculations, it is established that the main factors determining the possibility of implementing GDP are the additional gas pressure at the contact, the width of the workingspan, and the distance from the roof to the first gas-containing contact. With multi-level lamination of roof rocks, there is a danger of large sources of GDP formation and the mechanism of successive fall of layers in an instant mode is implemented.
Development of water-soluble ore deposits is associated with the necessity to preserve water blocking strata (WBS), which separate aquifers from the mine gob. One indicator of the rate of man-induced load on WBS layers is subsidence of the earth surface, which defines the character of shift trough formation of the earth surface. The greatest threat of WBS discontinuity is posed by the areas located at the edges of a shift trough. From the perspective of Upper Kama deposit of potassium and magnesium salts, by means of mathematical modelling methods authors demonstrated that in the capacity of threat indicators of WBS hole destruction it is possible to use the following parameters of a shift trough: edge length scaled to the depth of mining operations and maximum subsidence of the earth surface. Critical combination of these factors is responsible for the discontinuity at the edges of water blocking strata. These parameters of a shift trough can easily be controlled by instrumental procedures and can be included in the basics of a general monitoring system of WBS state at potash mines. In order to protect the mine from the inrush of fresh water, it is necessary to form softening zones at the edges of mined-out areas near permanent or temporary borders of mining operations. Authors review different options of softening zone formation. Numerical tests have demonstrated that the most efficient way to protect water blocking strata is the formation of softening zones by means of backfilling the stopes of the workable seam or its exclusion from mining operations.
The research of potash salt rock deforming and damaging features has made it possible to develop the method of load bearing element state examination by chamber-and-pillar system. This method consists in instrumental control of pillars by geophysical and in situ methods with following prognosis of their residual life by mathematical simulation methods.
The research of potash salt rock deforming and damaging features has made it possible to develop the method of load bearing element state examination by chamber-and-pillar system. This method consists in instrumental control of pillars by geophysical and in situ methods with following prognosis of their residual life by mathematical simulation methods.
