Characteristics of Stratum Structure and Fracture Evolution in Stratified Mining of Shallow Buried High-Gas-Thick Coal Seam by Similarity Simulation

The stratified mining of super thick coal seam is a process of repeated disturbance of the top roof, especially in the lower stratification, the upper complex rock layer has a greater settlement space, resulting in great changes in the strata structure and fissure distribution. The main coal seam thickness of Rujigou Coal Mine exceeds 20 m, due to the high gas content of the coal seam, it is prone to spontaneous combustion, and the stratified mining method is adopted. When a small-size section coal pillar (less than 10 m) is used, the complex rock structure evolution and fissure development characteristics during the stratified mining of shallow buried thick coal seam will directly affect the movement of gas transportation between the working face and the goaf and will directly affect the safety of the working face. Taking Rujigou coal mine as engineering background, this paper analyzes the breaking structure, fracture development, and evolution law of overlying strata in different layers and different sections of coal seam when the buried depth is shallow, and the extra-thick coal seam is stratified mining. The results show that in the process of stratified mining, the overlying strata break, in addition to the whole trapezoidal failure structure, will also form a local F type fracture structure, and with the stratified downward mining, the F type fracture structure will continue to move up and disappear until it is compacted. The “V” type and “U” type subsidence characteristics of different strata overburden are presented after mining in stratified working face of extra-thick coal seam, and the subsidence amount is approximately symmetrical distribution along the middle line of goaf. In the mining process of the lower part of the layer, the end broken rock block is easy to slip along the hinge point by the hinged rock beam structure, and the sliding instability occurs. In the process of stratified mining of ultrathick coal seam, the main fissure of overburden is mainly longitudinal fissure, and it is very easy to form through with the upper layer and will finally connect with the surface under the condition of shallow buried depth. The inclined cracks connected with the adjacent goaf are formed above the coal pillar of the section, which becomes the passage of gas migration in the goaf. The research conclusion shows that for the stratified mining of high gas thick coal seam, special attention should be paid to the treatment of the gas on the stratified working face. In addition to the conventional gas treatment measures such as coal seam prepumping, the buried pipe pumping in the mining area can also be adopted, which can effectively reduce the gas concentration of the working surface.

A technique for diagnostics of micro-inclusions in the objects of soil and geological origin: validation and practical application

A method for diagnosing microinclusions in the objects of soil and geological origin is developed on the basis of the Atlas of microinclusions in soils (hereinafter referred to as the Atlas). The validation procedure and an example of the practical application of the developed technique are considered. A review of the content and structure of the Atlas, which contains 37 types of microinclusions is presented. Diagnosis of microinclusions is carried out using the Atlas key which is a sequence of identifying diagnostic features (transparency, shape, color, gloss, fracture, structure, as well as density, magnetic properties, hardness, brittleness) for different types of microinclusions. A scheme for determining the nature of a microinclusion (red brick) using the Atlas key is given as an example. The validation procedure of this method consists in experimental verification of the reliability of testing and evaluation of the reproducibility of test results under different conditions: when examining samples of various complexity by several performers at different times using different stereo microscopes. We used 17 soil samples that were previously (2 – 8 years ago) tested and stored as control samples in a laboratory collection of site-collected samples. One part of the samples was examined in usual forensic soil examinations, the other was studied in the framework of participation in the procedure of interlaboratory proficiency testing under the ENFSI (European Network of Forensic Science Institutions). Two experts performed independent studies of control samples at different times. It is shown that the composition of the complex of microinclusions (by type and number) in each of the studied samples coincides with the composition of the corresponding control sample. The experts performed 108 tests, and there were no erroneous results which indicates the reproducibility of the test results and the competence of the experts. An example of the practical application of the developed technique is given.

Characteristics of Pore and Fracture of Coal with Bursting Proneness Based on DIC and Fractal Theory

Coal is a complex heterogeneous and anisotropic material conformed with fractal characteristics. The pore and fracture characteristics have important influence on the dynamic disasters including rock burst and gas outburst, however, the relationship between them is not accurately investigated due to inadequate research method. The pore and fracture distribution of coal with different bursting proneness were obtained by comprehensive application of MIP, LTNAD, SEM, and X-ray CT, and then fractal theory and DIC were used to research the pore and fracture characteristics. The result indicated that the modification of MIP result by LTNAD result could effectively eliminate the adverse effect of coal matrix compressibility, exactly reflect the distribution of pore and fracture in coal, and the pore distribution of coal with different bursting proneness were quite different. Gray scale image from SEM and 3D reconstruction technology based on X-ray CT could show the geological structure, fracture structure, and pore structure characteristics of coal. The study of LTNAD, SEM, and X-ray CT showed that these methods complemented each other, the coal had fractal properties, and the fractal dimension value had a positive correlation with the bursting proneness of coal sample.