The Synergy Mechanism Between Rock Mass And Soil In Mining Subsidence And Its Prediction

The Coal Mining Areas covered with Thick Alluvium Soil (CMATASs) are widelydistributed in China, and the ground subsidence in these mining areas shows uniquefeatures, for example, subsidence coefficient is greater than1.0(that is, the maximumsubsidence is greater than the extracted coal seam thickness). Thence, there has been alot of interest in studies of mining subsidence regularity and mechanism in theCMATAS. Huainan Coal Mining Area shows a mining subsidence characteristics ofboth general conditions (thin alluvium) and thick alluvial soil, and the laws of miningsubsidence differences reflect the particularity of mining subsidence in thick alluviumareas, which provides favorable conditions for the study of the mining subsidence inthick alluvium areas mechanism. Taking advantage of the characteristics of HuainanCoal Mining Area, Based on a large number of measured data and a combination ofphysical modeling, numerical simulation and theoretical analysis method, thedissertation has done some research on dynamic movement and fracture in strataoverlying of coal mining areas with thin alluvium soil, has analyzed the synergymechanisms between rock mass and alluvium soil, has proposed a new approach forstudying soil response to subsidence in CMATASs, has investigated systematically themechanism of mining subsidence in CMATASs, Studied Preliminary the subsidencecoefficient during multi-seam mining in coal mines with thick alluvial soil andestablished a combination prediction model for mining subsidence which can reflectthe effect of alluvium soil on the mining subsidence.Using the similar material modeling experiment, we studied dynamic movementand Fracture in strata overlying of coal mining areas with thin Alluvium soil. The resultsshow that firstly the height of Fractured Water-Conducting Zone (FWCZ) increasesgradually during the mining moving forward, and then, when the mining area reachenough to collapse of overlying strata, the height of FWCZ increases rapidly, lastlytends gradually to stability; during the transmit process of the damage of rock mass dueto mining from low to top, the subsidence is gradually decreasing and the horizontalmovement is gradually increasing; the three zones, which are (1) a zone with themovement vector oriented vertically downward,(2) a zone with the movement vectororiented towards the goaf center and (3) a zone with the movement vector orientedtowards the coal pillar, are formed in the overlying strata; the law of dynamic movementwas gained through the research on dynamic movement tracking lied on different zones. The overall analysis reveals that the movement boundary in the strata overlying therock mass is not a straight line, but an “S-shaped” curve, in contrast to the traditionalfindings with respect to the movement boundary. These findings may prove significantin guiding better prediction of movement and deformation inside a given rock mass,and related improvements in protecting engineered infrastructure.When a coal mining area is covered with a thick layer of alluvium soil, the height,width, and distribution pattern of the FWCZ in the area shows unique features. Theinternal mechanism by which thick alluvium soil affects the FWCZ and the Synergymechanisms between rock mass and alluvium soil are still unknown. We investigatedthe impact of thick alluvium soil on the height, width, and distribution pattern of theFWCZ through numerical simulation using the Distinct Element Method, theoreticalderivation, and data validation. The results indicate that a thick layer of alluvium soilinhibits the FWCZ height. For soil rock ratio>5/6, the FWCZ height decreases byabout10%and the FWCZ width increases; the shape of the FWCZ changes from talland thin to short and wide.The internal mechanism of thick alluvium’s impact on the FWCZ is as follows:the load from the thick alluvium spreads to the underlying rock mass, affecting thefractured rock mass above the goaf and the bed separation in the goaf. This affects theheight, width, and distribution pattern of the FWCZ. When the thickness of alluviumincreases, the fractured rock mass in the caving zone in the goaf is more compressedunder the force of the alluvium load and the expansion of the broken rock decreases;the fracturing and bed separation inside fractured zone tends to become more closelypacked, thus inhibiting and reducing the height of the FWCZ. However, the space inthe broken rock mass, fractured areas, and bed separation is limited; therefore, thereduction in FWCZ height reaches about10%, then gradually stabilizes. The displacedgaps in the rock are transferred to the surface in the form of subsidence, and increasesthe surface subsidence, it is also one of the reasons why surface subsidence in thickalluvium areas is relatively larger. Because the process of transferring down thealluvium load is affected by the hard rock strata, the vertical and lateral changes in theFWCZ dimensions are similar, both showing a step pattern.This dissertation puts forward a new approach namely the research ideas of"separated–comprehensive pattern" for studying soil response to mining subsidence inthe CMATAS, which can make full use of the field measured data. The prominentfeature of the proposed approach in the study is that the overlying rock mass and alluvium soil can be separated and the measured data of different boundaries can beobtained, which makes up the shortage of too few measured data in the current studymethods. The proposed method has been applied in Huainan Coal Mining Area, Chinasuccessfully. The method proposed in this study, having a characteristic of universalapplication, can provide a new idea and technical approach for the researchers in thisfield, provide measure data for research of mining subsidence in the areas covered thickalluvium soil and promote the more comprehensive development of the subject.Based on a wealth of measured data and the special occurrence condition of coalseam in the Huainan Coal Mining Area of China, using the proposed research ideas of"separated–comprehensive pattern", the internal mechanism behavior and thecharacteristics of the surface subsidence in coal mining areas with thick alluvium areinvestigated. The results show that the subsidence in thick alluvium coal mining areasconsists of four parts:(1) the subsidence of alluvium soil following the bedrocksubsidence caused by coal mining;(2) the synergy subsidence between alluvium soiland bedrock;(3) the subsidence due to groundwater loss and consolidation of thealluvium soil in coal mining area and (4) the compacting subsidence of shallow soilunder the disturbance of coal mining. Analysis of the changes in each type ofsubsidence during multi-seam mining has allowed determination of the subsidencecoefficient during multi-seam mining in the CMATASs. The subsidence coefficientduring multi-seam mining in coal mining area with mega-thick alluvial soil, instead ofincreasing as in areas with thin or no alluvium, will decrease with an increase infrequency of multi-seam mining. Field measurements in the Huainan Mining Area ofChina have confirmed this conclusion, and the results of this study are expected to beuseful in prediction of subsidence during multi-seam mining in CMATASs.Aiming to solve the common problems existed in the current prediction modelsfor mining subsidence, we deduced a calculation model for the compacting subsidencein the shallow alluvium soil and a calculation model for the consolidation subsidencein the deep alluvium soil based on the mechanism of mining subsidence in CMATASsand the soil mechanics and the mining subsidence theory, and then build a combinationprediction model for mining subsidence through the principle of “repeat-addition”,which is meaningful for impoving the predition accuracy in CMATASs.