Vol 61 Iss. 1

Mining engineer, Professor Dmitry F. Borisov for fourteen years, from 1954 to 1968 headed one of the leading departments of the Leningrad Mining Institute - the Department of Stratiform Deposit Development.

Room-and-pillar systems of development with cemented backfill are becoming more and more widespread. This causes the need for further development of calculation methods for determining the basic parameters of the systems and strength properties of the backfill. The purpose of such calculations is a reasonable choice of the width of inter-chamber pillars, chamber spans, width of a panel or block, dimensions of inter-block (panel) pillars, determination of the required strength of artificial pillars. The results of calculations, of course, will be different depending on the order of development of sections (blocks), chambers within one block, the structure and properties of the filling massif and other conditions.

The problem of mining thick steep seams with backfilling has become especially acute in Kuzbass, where due to a number of technical circumstances it is required to significantly expand its use, while coal mining with backfilling is developing here very slowly. The unpopularity of backfilling in Kuzbass is explained, first of all, by the low efficiency of the mining systems used there. Thus, extraction of thick steep seams with backfilling is still carried out by the old systems, with slicing.

Mining of potash seams is complicated by sudden salt and gas outbursts. Such outbursts are observed in the USSR during the mining of the Verkhnekamsk and Starobinsk potash deposits.

At domestic brine fields deposits of rock salt of various thickness (from 40 to 700 m) and depth (from 300 to 1400 m) are developed by solution mining, through boreholes. As a rule, cylindrical caverns of 100 m diameter are used. The caverns are arranged on a 200x200 m grid, and the width of inter-cavern pillars is 100 m. Such sizes of pillars are insufficiently justified due to limited experience of solution mining and lack of tested solutions. As a result, there is a danger of pillars destruction and surface subsidence.

At the Yar-Bishkadak field the rock salt deposit with the thickness of 200-700 m and the thickness of overburden of 500-600 m is developed by solution mining through boreholes from the surface. The development is carried out by individual vertical caverns, the diameter of which, according to the project, is equal to 100 meters. Due to untimely control of erosion results, there are deviations from the project, sometimes the diameters of the caverns reach 160-190 m, so it is not clear how much the actual outcrops of excavations meet the requirements of stability. The current absence of surface subsidence and large cavings in the caverns associated with the cessation of operation of boreholes is not yet a sufficient guarantee of the safety of further development, as the duration of operation of the caverns (3-15 years) is short compared to their total service life (50-80 years). As a result, there is a need to predict the stability of rocks and determine the maximum allowable spans of solution-mine workings.

Employees of the Leningrad Mining Institute together with workers of Mine No. 1/2 of the trust "Uglegorskugol" in 1967-1968 conducted at the mine observations of the manifestations of rock pressure at different methods of extraction and roof management. The purpose of the observations was to improve the technology of extraction of gently dipping Seam VI and to choose the most rational method of roof management. Seam VI is relatively uniform, the workable thickness is 0.95-1.10 m, the dip angle is 8-15° The roof of the seam is represented by layered claystones (0.4-0.6 m) and dense fine-grained sandstones (15-20 m). The floor of the seam is composed of claystones (0.1 m) and siltstones of medium strength, up to 8-10 m thick.

Through joint efforts of the staff of the Leningrad Mining Institute and the Sakhalinugol Combine, in 1967-1968 numerous mine and laboratory investigations were carried out, aimed at improving mining systems, face operation mechanization, and roof control for thick gently dipping seams.

In the development of thin and medium-thickness steeply dipping seams using systems with long stopes along the dip, the most labor-intensive and arduous is supporting the face area, which takes up to 55-60% of working time.

Significant spread in both the coal and ore industries is gained by shield and flexible canopies, for the selection of rational and reliable designs of which it is necessary to know the loads acting on them.

The methodology and results of laboratory studies of the process of caving of overlying rocks are outlined. The research was carried out in the Leningrad Mining Institute for conditions of mining the apatite deposits of the Khibiny, hanging wall of which is represented by hard fractured rocks.

The magnitude of rock falls, the load on mine support, and the displacement of their contours are determined mainly by the extent of the inelastic deformation zone formed in the rock mass surrounding the workings. In the practice of maintenance, the majority of development workings are affected by abutment pressure, and there are no calculation methods for assessing this effect on the size of the inelastic deformation zone.

In the practice of maintaining mine workings there are often cases of their mutual influence. Especially significant is the impact of extraction operations on the stress state in the vicinity of development openings. The solutions of these problems are complicated and cumbersome, and therefore they are mainly available for cases of mutual influence of openings of the same shape. Only some solutions for different shapes are known. However, these solutions due to the complexity of the final formulas have not become widely used to determine the stresses in the vicinity of openings. It is necessary to create simpler calculation methods.

When studying the manifestations of rock pressure on flat and three-dimensional models, various types of dynamometers are now widely used to create elastic measuring bases of stands.

Violation of the continuity of the rock mass, associated with the mining of deposits, is accompanied by a number of physical processes. Rocks in the zone of influence of mine workings are in a stress-strain state, which is a complex function of the structure of the mass and mining conditions of deposit development.

The steady increase in coal production and the transition to mining new deposits with increasingly complex conditions of occurrence makes the problems of monitoring rock pressure manifestations and predicting the behavior of the roof in mine workings a paramount task, which has not yet found its solution. This is due to a number of reasons, in particular, insufficient knowledge of rock pressure processes, and the lack of a methodology for systematic prediction based on monitoring measurements of the state of the rock mass.

At the Department of Construction of Mining Enterprises of LGI a method of stress measurement, instruments and equipment was developed, which is characterized by simplicity, reliability and sufficient accuracy. The method is based on the assumption that if a sensor is placed in a borehole in a rock mass, providing it with sufficiently good contact with the borehole walls, then as a result of displacement of the borehole walls under load, the sensor readings will change simultaneously. The forces acting on the sensor will decrease or increase depending on whether the rock mass is subjected to compressive or tensile stresses.

Physical and mechanical properties of salt rocks were investigated in laboratory and underground conditions with respect to the problems of rock mechanics – calculation of rational parameters of room-and-pillar mining and identification of patterns of rock pressure manifestations. These tasks are of great importance in mining the Starobinsk potash deposit, represented by several potash seams, occurring at a depth of 420–640 m from the surface. At the deposit, two potash seams are mined with thicknesses of 1.8–4.0 m and 4.5–7.0 m, occurring respectively at depths of 420 and 580 m.

Along with the development and improvement of traditional methods of pressure determination, based on the study of elastic strain values of rocks, various physical methods are increasingly used, including gamma methods of determining rock density.

In general, geologically complex areas of stratiform deposits represent zones of faults that differ slightly in electrical properties from the well-conducting surrounding rocks. To study faults located in a highly conductive medium and at considerable (up to 200 m) distances from the source of electromagnetic waves, relatively low frequencies are required, and it is often impossible to use conventional methods of interpreting the results of radio-wave transmission.