A Study On The Law Of Rock Movement And Soil Variation In Mining Of Coal Seams Of Thick Loess And Thin Bedrock

Damages to soil caused by mining have long been a problem. The use of fully mechanized long wall caving mining in the coal seam of thin bedrocks and thick surface soil, due to its unique characteristics, causes more severe damages to the soil surface than traditional mining, and yet few reports have been made on the research on the influences of overburden movement destruction on soil quality.On the basis of summing up the existing research results and in combination with the actual situations of Sima Coal Mine of Lu'an Mining Group, a large-sized coal mining enterprise in China, this thesis, through on-the-spot investigation and observation, physical simulation in laboratories, computerized random simulation, surfece measurement , theoretical estimation, test and analysis of site samples, theoretical analysis, studies, in a systematic manner, the law of strata movement, features of surface subsidence, and the law of soil quality variation in Coal Seams # 1101 in Sima Coal Mine, which features thick loess and thin bedrock, so as to provide theoretic bases for .the prevention of mining damages and the afterward restoration.New points of the thesis are as follows:(1) A comprehensive study is carried on the interrelations between the law of soil destruction in mining subsidence area and the law of rock movement in the mining process, and a brief exploration is done between their correlations.(2) For the first time, the reliability of the interrelated calculated parameter is analyzed in the mining subsidence prediction process, and the choosing methods are improved. The specific choosing methods are given based on random simulation theory, which have been proved to be good through practice.(3) For the problem of deep rock movement being difficult to be monitored, a sensor, monitoring system and system installation and measuring method are creatively designed to monitor rock movement; meanwhile, a theoretical description of measuring large-scale strain via micro strain is deduced,so that foundation is laid for further laboratory and industrial tests.The following main results and conclusions have been achieved:(1) For the shallowly-buried thin bedrock with thick loess, if the surface sand is loose(without self loading capacity), the bedrock will not form self stable structure until it reaches certain thickness. If the clay layer of certain thickness exists above the upper shock ply of the bedrock, its loading capacity is increased because the clay layer is possessed with certain loading capacity. The combination of bedrock of fixed thickness and increased thickness of clay layer will form stable structure with increased loading capacity and reduced and smooth pressure to the surface.(2) A new displacement sensor is innovatively designed to effectively monitor the dynamic movement of the deep bedrock and surface soil. Theoretical calculating and measuring methods are deduced to measure large-scale strain via micro strain; a monitoring method is studied based o the sensor of movement in that layer, and its feasibility is expounded on. (3) Simulated tests have discovered a steeper surface subsidence basin, a larger subsidence coefficient, yet a smaller horizontal movement coefficient after the Coal Seam 1101 is mined under the condition of thin rock and thin loess. Some unique features are observed, which are different from the features under the normal geological conditions: subsidence curves changes rapidly around the拐点with a steep curve; meanwhile, the convergence takes place in a very slow manner at he boundaries, which is shown in extended boundaries and smaller horizontal movement coefficient. The rock layers cracks with certain angles and is spread upward. The upper rock layer subsidence value is bigger than the surface value with a relatively small rock layer subsidence value above the coal wall.(4) Through monitoring the surface subsidence of the primary mining phase of Sima Coal Mine, the laws and characteristics of the surface subsidence of coal seam of thick loess and thin bedrock is discovered:Surface is very sensitive to mining influence in that the influence of underground mining activities are soon spread to the surface, and, due to a short initial surface movement period, surface in a very short time enters the subsidence-active period with severe and centralized movement and deformation, long-lasting surface moving and deforming span of the mining area, and relatively developed surface cracks. When the seam is mined, the influencing area of the surface movement and deformation is expended, the surface subsidence basins become steep surrounding mined boundaries with obvious and centralzedly-distrubited movement and deformation, and the surface subsidence basins become smooth outside mined boundaries, which is reflected in the following: the subsidence value is relatively large in the internal side of the inflection of the surface movement and deformation; in the area surrounding the inflection, the curve develops more rapidly, thus a steep cure, while in the areas further away from the inflection, the subsidence deforming curves convergence slowly but with further extending area. Reflected in the rock movement index, the surface subsidence index is relatively large, reaching 0.94, with a large tangent B, subsidence speeds, and dynamic deforming values, resulting in severe surface movement.(5)Reliability of the parameter in the calculating process of mining subsidence is analyzed for the first time by adopting the Theory of Reliability, and the dynamic determination of expected parameter by random variable is for the first time put forward in connection with only fixed value being chosen for random variables with certain distribution laws. In addition, the estimated parameter variables for mining subsidence is improved by adopting random simulation method, which has been proved to be in relatively good in accordance with the real value of Seam 1101.(6)Mining subsidence has significantly influenced the physical features of the surface soil; Water and nutrition losses become severe in the upper and middle parts of the farmlands in the subsidence area, while soil nutrition is centralized in the low subsidence area; Regarding physical properties of the soil, within two years after the formation of subsidence, soil bulk density is affected the most, followed by water contained in the soil and its porosity. Regarding the chemical features of the soil, soil available p is most affected, followed by available potassium, and total soil nitrogen and organic matter, with acidity and basicity least being affected. As far as the soil fertility is concerned, after the stabilization of subsidence(1-3 years), necessary soil conditions are able to be provided for the growth of plants; with regard to farming operation, only operational load is added Therefore, for the farmlands in the subsidence area, proper fertilizing and water protecting measures should be taken despite the existing destruction, so as to continuously bring into play the potential productivity of the farmland.(7)Interrelation between the mining conditions and soil subsidence destruction is interpreted, and the law is pointed that physical changes in the soil is resulted from its geometrical deformation in that horizontal deformation and movement may result in the change in the tensile stress and compressive stress of soil, which, in turn, results in the change in volume weight and extent of porosity. Development of cracks constitutes the most direct reason for the change of water in the soil, besides which, rising height and developing level of the water flowing fractured zone in the upper rocks are also important. The change in the soil fertilizing quality due to the imbalance of surface runoff, resulted from soil macro formation change, brings about water losses, thus leading to the movement of nutrition to the basin bottom.(8)In consideration of the specific conditions of Sima Coal Mine, several suitable reclaiming modes and ecological designing suggestions are raised for project references.