Surrounding Rock Controlling Technologies And Supportsstability In Steepy Inclined Fully-mechanized Caving Stope

Extraction methods and relevant matching technologies in steeply inclined seam were systematically researched in China since 1950 s. The controlling of surrounding rock and supports’ stability in this geological condition has been the most important technical problem in the process of mining. The dip angle of coal seam changes the mechanical characteristics of surrounding rock, and decreases the vertical load and increases the tangential load, which leads to different caving sequences along inclination. Meanwhile, the slippage and filling of caving roof in lower level and tangential load result in the obvious differences in caving timing sequence of roof and relationship between supports and surrounding rock. Especially towards panel with complicated geological conditions such as “triple-soft” coal seam, larger mining height, faults and tectonic stress, slippage of floor, rib spalling, partial roof fall and worse supports’ stability result in difficulties in face advancing and affect the safe production of panel. Hence, it’s urgent and necessary to solve the key problems about activity rules of surrounding rock, controlling principles of supports’ stability in steeply inclined panel, supporting design of surrounding rock and developing technologies of fully-mechanized mining equipment.The differences of overlying strata movement rules between greatly inclined panel and nearly horizontal panel in fully-mechanized caving along the strike firstly lie in the load disparity of roof. Secondly, it’s because of the structure discrepancy after roof broke. Therefore, it’s necessary to research the roof stress and structural style before and after caving to study the controlling technologies of surrounding rock. Taking fully-mechanized caving “triple-soft” panel 1201 of Dayuan coal mine in Shanxi as engineering background, starting from roof stress and structural style, comprehensive technical methods, such as theoretical analysis using structural mechanics, numerical simulation using FLAC3 D, similar simulation indoor and in-situ engineering practice, were adopted to systematically study the failure and instability rules of roof, strata behaviors and overlying strata movement regulations, technologies on surrounding rock controlling, relationship between support and surrounding rock and stability controlling of supports. The main achieved conclusions and innovative results are as follows.(1) Uniform stress model of roof along inclination before initial breaking in greatly inclined seam was established in structural mechanics to analyze the stress and deformation characteristics. The results show that the maximum of compressive stress in steeply inclined stope locates the lower surface of the lower beam and its value is about 36% larger than the upper. The maximum tensile stress locates the upper surface of upper beam and its value is 26% larger than the lower. The non-uniform stress of beam and the rock characteristics of higher compressive resistance but lower tensile strength lead early failure in the upper roof and cause roof weighting.Meanwhile, cubic parabola form of caving zone along inclination and “three section” characteristics(densely packing section, non-uniform packing section and dynamic load section) were proposed. In densely packing section, the structure of overlying strata is long cantilever supported by caving gangue. This section has lower periodic pressure. In non-uniform packing section, caving zone has higher developing layer and asymmetrical filling degree along inclination. In dynamic load section, the structure of this section is mainly short cantilever. It has smaller support load and obvious characteristics of dynamic load.(2) According to the structural characteristics of steeply dipping stope after roof caved, the mechanical model of inclined masonry structure was established and instability forms of slippage and deformation were analyzed. Results show that the larger the dip angle is, the smaller the rotation angle of rock is. The larger the dip angle is, the smaller the deflection of bite point and deformation instability degree in below end is. The slippage stability of masonry structure of inclined rock is obviously affected by dip angle of coal seam. The larger dip angle could easily lead slippage instability of upper rock, which results in dynamic load of upper supports. In conclusions, the masonry structure in steeply inclined stope bases more on slippage instability of upper masonry rock.(3) Similar simulation experiment was designed to simulate overlying strata movement and strata structure in steeply inclined stope. Results show that the caving form of inclined stope is asymmetrical. Roof in upper end caved sufficiently while the bottom is not due to filling effect. Inclined masonry structure is formed in both upper and bottom end of steeply dipping stope. But the bottom structure remains stable easily, and the upper structure easily appears slippage instability, which leads suspending roof in the upper end and affects the stability controlling of upper surrounding rock and supports.(4) Numerical simulation results shows that the distribution of advanced abutment pressure in upper, middle and lower of panel is obviously different when panel advances 60 meters. In the first place, the peak of abutment pressure in both middle and lower panel is larger than that of upper end. In the second place, there is no obvious difference in the acting position of abutment pressure. The position of peak is 12 meters ahead the coal wall. There’s no obvious difference in its acting range between middle and lower panel. The range is 40 meters from the coal wall. The acting range of abutment pressure in upper end, which is approximately 54 m, is larger than that of middle and lower panel. Finally, the coefficients of stress concentration in upper, middle and lower end are respectively 1.39, 1.38 and 1.33, which means the stress concentration level and strata behaviors are more severe. Therefore, there needs to take measurements to enforce the controlling of strata behavior and surrounding rock in middle and upper panel.There is obvious difference in the distribution characteristics of roof displacement along inclination between steeply inclined coal seam and horizontal seam. It shows obvious feature of asymmetry. The curve of vertical displacement has a maximum value in the middle of panel. But the largest roof deflection occurs in upper end of panel. The vertical displacement in upper end of panel has a larger range about 7 to 9 meters. If the center distance of support is 1.5 m, it demonstrates the surrounding rock in a range about 5 or 6 supports has severe activity. Taking rational technical measurements to control the activity of surrounding rock in this segment, not only aims at the control of surrounding rock and supports’ stability, but it is also the key point of controlling of supports’ stability in middle and lower panel.(5) Through theoretical analysis, the factors that affect the floor stability of steeply dipping stope can be concluded as floor lithology, rock structure, function of water and mining technology. Mechanical model of floor slippage was established through limit analysis theorem. The weight of gliding rock, which remains the floor balanced, increases with the increase of residual cohesive force and friction angle. With the double effects of tunneling and extraction, the cohesion of direct floor decreases fast, only leaving residual cohesion. Hence, the floor in upper end is easy to glide, which has serious influence on the stability of support. So, it’s necessary to reinforce the supports in the panel.Numerical analysis of floor gliding using strength reduction method demonstrates that there exists local shear zone in floor, which is potential gliding face. In the shear zone, floor has maximum deformation rate. After the effects of extraction, floor appears to be weakened cohesion and intensified friction force. The strength of rock decreases obviously. Therefore, after the face advances, it has great probability for floor slippage to influence the stability of equipment and roof. In panel 1201 of Dayuan coal mine, anchor cable installed into the floor of upper end functions as fixing sliding body pile. After floor consolidation using anchor cable in numerical model, calculation results show that potential sliding block of floor remains stable. In practical application, floor was reinforced by diamond mesh combined with anchor bolt and crosstie. It succeeded in preventing large scale accidents of floor slippage and assured the stability of “roof-support-floor” system.(6) Mechanical model of top coal failure in upper end was established. The analytical solution of stress distribution of top coal was obtained. The top coal and ventilation roadway in the upper end were reinforced through fan-shaped distribution anchor cable to prevent the failure to upper roadway in top coal drawing and achieve termination coal drawing successfully.(7) Mechanical model for analyzing the stability of support in steeply inclined fully-mechanized caving stope was built. Criteria that prevent support from sliding, toppling and reversing were proposed to provide basis and theoretical and technical support for guiding the stability controlling of support in different geological conditions. Mechanical boundary conditions of sliding, toppling and reversing of single support were analyzed to obtain the rational roof pressure for maintaining single support stable. Based on the mechanical analysis to single support, the influences of suspending roof of multiple supports on the stability of support were obtained through calculation, and rational roof pressure could guarantee the stable condition of support. The instability of supports occurred in the process of dropping and moving supports. As long as rational management measures are adopted, the stability of supports can be ensured. According to the mechanical analysis to the sliding, toppling and reversing of supports in steeply inclined fully-mechanized caving stope, targeted improvements were proposed from the aspects of type, side guard plate, four-bar links and bottom design. Firstly, two leg yield support was adopted instead of four leg to improve the stress state of support and to prevent posterior prop from pulling out. Secondly, side guard plate is designed to be double-active and top beam is designed to be shielded hinge to control the decline of tail beam. Thirdly, four-bar linkage is designed to be double side guard plates in both front and back of beam to keep support stable. Finally, the bottom is designed to be entirely closed to prevent drilling into floor.(8) Through the analysis of the relationship between support and surrounding rock, the formula to calculate working resistance can be obtained. Substituting to specific geological conditions of panel 1201, the rational working resistance of supports in upper, middle and bottom end of panel were respectively 4620 kN, 4580 kN and 1864 kN. This conclusion approached to practical situation and satisfied the operating requirement.Through the import of dynamic load coefficient A(m), the distribution characteristics of working resistance of supports in different position along inclination. Data from actual measurement show that the time sequence of periodic pressure is different along inclination. The extent of dynamic load along inclination is “upper end >middle end >bottom end”. The slipping instability of inclined masonry structure in the upper end leads greater fluctuation degree along inclination. There appears obvious character of dynamic load. But the roof weighting led by deformation instability in the bottom end is smaller. Therefore, the slipping instability of inclined masonry structure in the upper end is the main reason that leads heavy roof weighting in both upper and middle end. The measured data verified the proposal of dynamic load in upper end of greatly inclined stope.(9) Based on the conclusions from analysis to support stability, technical measurements to prevent the supports in the panel from gliding, toppling and slanting were proposed. The stability of supports could be ensured through comprehensive technical actions such as rational moving sequence of supports, rise mining, increasing setting load of supports, ensuring the stability of adjacent supports before moving supports, enlarging the width of side guard plates and taking full advantage of the side guard and bottom props to adjust the position of supports. Coal cutting machine were kept stable through multiple measurements such as cutting coal from top to bottom, operating the reamers in vain from bottom to top and supplying oil to front scraper conveyer for pushing it to the coal wall. Through pressure peak combined with antiskid top picks, the condition of scraper conveyer was adjusted and its stability was guaranteed.