The Failure Characteristics Of Overburden Strata Induced By Mining And Their Applications

In this dissertation, various methods, such as in situ measurement, physical and numerical simulation, are applied for the analysis of the failure characteristics of the overburden strata by coal mining under the conditions of slight dipping or dipping seams and mining the whole seam one time. In this analysis, various affecting factors, such as the properties of the overburden strata, the structures of strata, the parameters of working faces, faults and time, are considered in detail. In order to study the failure characteristics of overburden strata, the three-dimensional finite element program is written in Fortran. The program can be used for the analysis of the viscous-elastic and viscous-plastic deformation and failure. This program can also be used for considering faults and weak structures of rocks and obtaining the point safety factors, which are used to determine the failure state. The four models of considering overburden strata structures, such as hard-hard strata, hard-weak strata, weak-hard strata and weak-weak strata, are analyzed in detail. The different failure characteristics of overburden strata are obtained. The following conclusions are drawn.The relationship of the water conducting height of overburden strata and various affecting factors, such as the mining method, the structures of rock strata, the parameters of the working face and faults, are established. The failure height of overburden strata changes nonlinearly with the mining thickness of coal seam. The failure heights are related to the strength of overburden strata. The higher the strength of the overburden strata is, the less the failure height of the overburden strata is. And the failure height of overburden strata is also related to the structures of overburden strata. For non-fully mining, the failure height is determined by the short side of the working face. For fully mining, this effect is less. In addition, with the time elapsing, the failure height reaches the maximum value, then decreases and becomes a stable value. For the faulted zone, the variations of the failure height are different.The failure state of overburden strata is related to rock properties, the structures of strata, the parameters of working faces and the supporting conditions. If the strength of rocks is high, it is possible that the saddle-shaped failure boundary does not exist. The failure boundary of overburden strata is determined by the strata structures. The arch or saddle-shaped failure boundary is determined by the length of short sides of the working face. The failure boundary also changes with different supporting conditions. The models of calculating failure heights and the classical shapes are developed.The sub-zones of overburden strata failure and the strong, medium and weak sub-zones are researched by the water injected volume in the borehole.The relationship of the lateral boundary of overburden strata and the state of coal bed is established. The lateral boundary of overburden strata is determined by the failure state of the coal bed. The parameters and method of determining the failure boundary of overburden strata are presented.The strength index of comprehensive compressive strength and the classification method of overburden strata are presented by considering strata thickness and strength.The method of determining the influence of different factors on the failure height is also presented. These factors include the strata properties and structures, the parameters of the working face. The formulae of predicting the failure height for fully machined mining are proposed.For ascending mining of thick coal seam having medium-hard overburden strata, the standards of determining the distances of different fundamental layers are presented. Meanwhile, the methods are also proposed for determining the width of the coal pillar preventing water inrush and extracting methane from coal seam.The above-mentioned results of failure characteristics of overburden strata are much useful for safe mining of coal.