Thought of full-stratigraphy and its practice in ground control

Ground control is a science of mitigating or eliminating mining damage by controlling the breaking motion of mining rock strata, with the aim of ensuring safe, efficient and green mining. There are two limitations in the traditional thoughts and methods of ground control research: only studying the local rock movement instead of the overall movement of the overlying rocks in the whole stratum, and homogenizing the overlying rocks instead of grasping the main contradictions. In view of these problems, in the process of key stratum theory research and engineering application, the ‘full-stratigraphy’ thought of ground control has been formed, whose basic connotation is as follows: firstly, ‘grasping the main contradiction’, to find the key stratum (i.e. the main contradiction) that plays a major role in controlling the overburden movement without homogenizing the overlying rocks. Secondly, it is ‘overall view’, which takes the overburden as a global object, considers all the factors that may have a significant impact on the breakage and instability of the key stratum from a global view, and analyses the manifestation of mining pressure, water and gas transport, and surface subsidence from the point of view of the whole activity of the overburden movement. According to the basic connotation of the thought of full-stratigraphy, when carrying out ground control research, it is necessary to collect full-stratigraphy data and key stratum discrimination studies within different mining scales (mine, mining area, workings and blocks), the complete coring columnar that contains all the information of the rock strata from the vicinity under the mined coal seam all the way to the surface, and then form an overall portrait of the rock stratum endowment characteristics in the research area through the comparison of the columnar of multiple drill holes. The comparison of multiple drill hole columns will form an overall picture of the characteristics of the rock strata in the study area, rather than using only partial columns or comprehensive columns, or homogenizing the whole column. It is necessary to consider the possible effects of changes in the location of the columns and key stratum in different regions on the movement of the rock layer. Secondly, it is also necessary to fully consider the original rock stress, geological structure, topography, water load, mining deployment and so on, which may lead to abnormal loads on the key stratum and its breakage. On this basis, the relation between key stratum movement and various mining engineering phenomena or disasters is established, based on which the mechanism of the problem is analyzed and the targeted rock control methods are put forward. The full-stratigraphy thought has promoted the theoretical development and practical innovation of ground control, solved a series of engineering problems of disaster prevention and control such as supports crushing, water-inrush, subsidence, etc., and achieved remarkable results. The main innovations in the theory of rock stratum control achieved by using the full-stratigraphy thought include revealing the accumulative effect of overburden strata expansion induced by stress relief, establishing a structural model of ‘key stratum - loose stratum arch’ of the mining overburden, proposing a new method to predict the height of fractured water-conducting zone by location of key stratum, and initially establishing a method for estimating the surface subsidence based on the structure of key stratum, and so on. It lays a foundation for engineering application. Typical examples of the full-stratigraphy thought in engineering application include: finding out the root cause of pressurized racking in the uphill section of shallow buried coal seam gully and mining under unconsolidated confined aquifer, completely solving the engineering problems of pressurized racking prevention and control; quantifying the influence of the location of the key stratum on the ‘water flowing fracture height’, solving the problems of preventing and controlling the abnormal water breakout of the roof; proposing the ‘three-zone’ distribution of the unloaded gas in coal seam, which has guided the practice of unloaded gas extraction; it has developed isolated overburden grout injection and filling technology, and created a new mode of ground drilling and one-injection high-efficiency filling and green mining, which has solved the problems of surface subsidence control, and has been comprehensively popularized and applied.