| It’s illustrated from a large number of domestic and international projects that rock mass failure in situ begins with the change of load in the vicinity of excavation. The change of stress distribution in rock mass leads to deformation at discontinuities and weak parts, which triggers discontinuities creep deformation, discontinuities development, macroscopical fractures surface and perforation slip surface generation. Based on indoor physical simulation, field discontinuity observation, damage mechanics, material mechanics, elastic mechanics and Numerical Manifold Method(NMM) code, this thesis aims to investigate the effect mechanism of discontinuities on longwall working face roof failure. Its main content includes effect of discontinuity density on main roof threshold span, the effect of discontinuity dip angle and extraction advance direction on first weighting step,periodic weighting interval,water-flowing fractured zone development height and the roof strata caving angle. A formula for calculating the main roof threshold span under the effect of discontinuities is deduced. Indoor physical simulations with preexisted discontinuities are conducted. Back analysis by NMM modeling is performed to study the distribution of discontinuities and the best fit discontinuity distribution is applied to predict the first breaking of main roof. This thesis provides theoretical references for longwall face roof breaking mechanism and makes new attempts on rock engineering foundations. Through borehole imaging and hydraulic support stress monitoring in Dongzhouyao coal mine and Wangzhuang coal mine, good results showing theoretical derivation is effective. The major conclusions of this thesis are as the following:1) Based on damage mechanics, the effect of discontinuity density on rock mass elastic modulus and poisson ratio is analyzed in both two-dimensional element and three-dimensional element. In the two-dimensional element, both the ratio of effective elastic modulus and intrinsic elastic modulus and the ratio of effective poisson ratio and intrinsic poisson ratio decline with the increase of discontinuity density. To be specific, both the ratio of effective elastic modulus and intrinsic elastic modulus and the ratio of effective poisson ratio and intrinsic poisson ratio decrease from 1 to 0.411 when intrinsic poisson ratio is 0.3 and the discontinuity density increase from 0 to 0.5. In the three-dimensional element, both the ratio of effective elastic modulus and intrinsic elastic modulus and the effective poisson ratio decline with the increase of discontinuity density. Specifically, when intrinsic poisson ratio is 0.3, the ratio of effective elastic modulus and intrinsic elastic modulus decreases from 1 to 0.536 and the effective poisson ratio declines from 0.3 to 0.176 when the discontinuity density increases from 0 to 0.5. Based on the relationship of effective elastic modulus and effective poisson ratio with discontinuity density, combing beam mechanical model and the plate mechanical model, theoretical solutions of the effective load on main roof and main roof threshold span with discontinuity density are deduced.(2) An example calculation on the load on main roof and main roof threshold span is given and a sensitivity analysis is conducted. The sensitivity analysis indicates that the load on main roof and the threshold span decrease with the increase of discontinuity density. Further study reveals that effect of discontinuity on main roof threshold span increases with the decrease of longwall working face width, which means the effect of discontinuity on short-wall working face is more significant than that on longwall working face.(3) Based on the 82002 working face in Meiyukou coal mine 412 panel, indoor physical simulation models are built. The discontinuity trace is marked on the channel steel on the sides in advance. Then the simulation material is slit by steel ruler according to the discontinuity dip angle and length. Mica powder is spread in the slitting narrow gap evenly and the simulation material is tamped. This makes the discontinuity in the simulation model. Five simulations are performed: intact rock mass simulation, advancement direction parallel with 30 ° discontinuity dip direction simulation,advancement direction opposite to 30 ° discontinuity dip direction simulation,advancement direction parallel with 60°discontinuity dip direction simulation and advancement direction opposite to 60°discontinuity dip direction simulationThrough comparative analysis on the effect of discontinuity dip angle and the advancement direction on first weighting step, periodical weighting interval, water-flowing fractured zone development height and the roof strata caving angle, the simulations obtain following conclusion: 1) Discontinuities in the main roof strata arouse decrease of first weighting step and periodical weighting interval and increase of water-flowing fractured zone development height. The increase of discontinuity dip angle leads to the increase of caving height of roof. Roof cutting is more likely to occur when the dip angle of discontinuity is steep. 2) During first weighting, when the dip angle is small, the advancement direction which is opposite to discontinuity dip direction arouses increase of caving height. When the dip angle is large, the advancement direction which is parallel to discontinuity dip direction arouses decrease of the first weighting step. 3) During periodical weighting, when the dip angle is small, the advancement does not show evident effect on the periodical weighting interval. When the dip angle is large, the advancement direction which is opposite to discontinuity dip direction arouses decrease of periodical weighting interval. 4) When the advancement distance is certain, the advancement direction which is opposite to discontinuity dip direction arouses increase of water-flowing fractured zone development height and the increasing effect increases with the increase of discontinuity dip angle. 5) The advancement direction which is opposite to discontinuity dip direction arouses steeper roof strata caving angle, which means roof cutting is more likely to occur when advancement.(4) By NMM simulation, a back-analysis for tunnel deformation is conducted to study the discontinuity distribution and the result is applied to simulate the main roof breaking. First, in-situ stress imposing methods(the initial stress option, equivalent uniform load on boundaries and combination of the initial stress option and equivalent uniform load) in NMM are studied by comparing the convergence rates and an optimal method is proposed. Besides, an interface programme, based on the function of DL code of the two dimensional Discontinuous Deformation Analysis(DDA) software package, is developed to generate synthetic discontinuity trace in NMM. According to the dip angle, dip direction, rock bridge, length and spacing of discontinuities, NMM models are built and applied. Based on the non-connected discontinuities simulation by NMM and deformation monitoring data in-situ in Meiyukou coal mine, a back-analysis is conducted. The modeled rock mass structure is based on field mapping performed during tunneling in the coal mine. By applying in-situ stress by the proposed optimal method and generating discontinuities by the developed interface programme, 144 simulations with different combination of discontinuity length and discontinuity spacing are performed. According to the actual position of monitoring station in the tunnel, measuring points are set in NMM models and the convergence displacements are considered as the steady deformation of the tunnel. In order to take into consideration the effect of all of the measuring points simultaneously, the root mean square of the displacements in each simulation is calculated and compared. The best fit discontinuity distribution is obtained from the least root mean square error between the numerical and measured results. The obtained discontinuity distribution is then applied to study the main roof breaking in 82002 working face in 412 panel.(5) Through borehole image observation in the gateway of Wangzhuang coal mine and Dongzhouyao coal mine, the discontinuity density in main roof strata is calculated. Based on the theoretical analysis results, the effect of discontinuity density on main roof strata elastic modulus and poisson ratio is calculated. The threshold span of 8101 working face in Wangzhuang coal mine is calculated. There are some errors in but the result is closer to the in-situ data. The threshold span of 8301 working face in Dongzhouyao coal mine is calculated and the errors are within permission... |
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