| Backfill mining is one of the effective methods to mitigate the deformation of the land surface subsidence as well as reduce the risk of water and sand inrush.It is a difficult and urgent task to investigate the controlling effect of the backfill on the stability of the surrounding rock and the mechanism of subsidence reduction from the perspective of the dynamic temporal-spatial evolution of the surrounding rock mass-backfill combination system.This study focuses on the characteristics and mechanism of the temporal-spatial evolution of stress-displacement in the surrounding rock mass-backfill combination system.The model of the surrounding rock mass-backfill combination system was firstly constructed.Based on scale model test,the dynamic evolution of stress and displacement of surrounding rock mass-backfill combination system was analyzed relying on the spatiotemporal process.Then,a visualization model of the space-time cube of the stress and displacement was established.The correlation degree and state of the stress-displacement was quantitatively characterized and measured based on the symmetric KL divergence framework.The stability criterion of the combination system was obtained based on the energy dissipation.Main achievements are as follows:(1)The model of surrounding rock mass-backfill combination system for backfilling mining was constructed and classified according to the engineering behaviors.The model includes the material composition,rock structure and geological occurrence environment based on the engineering background of the Taiping Coalmine in Shandong province.The surrounding rock mass-stable backfill(SRSF)and surrounding rock mass-disturbed backfill-stable backfill(SRDSF)were selected as the main topic for the subsequent research.(2)The samples of backfills for 2 years in the mining area of the Taiping Coalmine were taken,and the comprehensive mechanical and hydraulic stability properties as well as micro pore structure characteristics of the underground stable backfill were obtained by laboratory analysis and test.The uniaxial strength of backfill is 7 MPa and the maximum tensile strength is 1.01 MPa.The triaxial strength of backfill exceeds 10 MPa.Groundwater weakened the backfills' resistance in cyclic loading and unloading.The permeability is extremely low,and the hydraulic conductivity is close to that of clay with a strong water durability,which allows the backfill to be an effective aquifuge.The microscopic results show that the content of clay mineral in the backfill is 11%,and the ability of resisting water erosion is strong.The backfill belongs to porous structure,which is mainly composed of mesopores and micropores.This kind of structure reduces the permeability of the backfill,and increases the specific surface area and pore volume by radial needle rod-shaped Aft,and makes the pore structure more stable by network structure.(3)The dynamic evolution characteristics of the stress and displacement of surrounding rock mass-stable backfill combination system(SRSF)were obtained by the model test.The change of vertical stress gradually transfers to the vertical and horizontal direction of roof and the advance direction,respectively.The propagation area is limited from the horizontal and vertical directions.The rock layers 28 cm(model size)above the coal seam roof and 16.8 cm below the coal seam floor are macro stress shells with increased stress.The advanced distance of influence of vertical and horizontal stress are 30 cm and 35 cm,respectively.The shape of roof subsidence curve evolved into "single peak","double peak"-"nearly flat bottom".The coefficients of roof and floor subsidence and uplift are 0.19 and 0.014,respectively.The maximum displacement of roof and floor exponentially increases with mining process.The evolution process of roof displacement transfer coefficient ? was divided into four stages: slow transfer,accelerated transfer,uniform transfer and stable transfer,while the ? of coal seam floor was mainly wave accelerated transfer.The fracture evolution form was asymmetric,which was conic shape,parallelogram shape,humped shape higher in the front,humped shape lower in the front.The fractal dimension can be divided into four stages: rapid dimension increase in the early stage,short-term dimension decrease,and dimension increase in the middle stage and stable dimension.The fracture ratio and area exponentially increase with mining process.The fractal dimension can be divided into four stages with the height evolution of the water-conducting fractured zone: the sharp increase before initial subsidence of roof,the short decrease after the initial subsidence,the slow growth during the fracture development and the stable stage.(4)The dynamic evolution characteristics of the stress and displacement of the surrounding rock mass-disturbed backfill-stable backfill(SRDSF)were obtained through the scale model test.The stress of the coal seam floor increased and decreased synchronously with the mining process,and the turbulence of stress distribution increased.The advanced distance of influence of vertical and horizontal stress is 40 cm and 35 cm,respectively.After mining,the subsidence and uplift coefficients of roof and floor are 0.1896 and 0.0084,respectively.The shape of roof subsidence curve evolves into the “double peaks” higher in the left,“double peaks” higher in the right,“nearly flat bottom”,and “flat bottom” with right inclination.The maximum displacement of roof and floor increases linearly with mining.The evolution of displacement transfer coefficient ? of the coal seam roof can be divided into four stages: transfer lag,accelerated transfer,slow transfer and stable transfer.The form of fractures evolves into three stages: humped shape lower in the front,trapezoidal shape,humped shape higher in the front,and trapezoidal shape.The evolution of fractal dimension can be divided into three stages: initial stable stage,increase stage and stable stage.It also can be divided into three stages with the development of water-conducting fractured zone as the slow growth,obvious growth and keeping stable.The stress shell in the coal seam roof and floor of SRDSF were transferred from 48 cm and 16.8 cm to 68 cm and 28 cm,respectively.The shell base in the coal pillar will be stable,but the shell shoulder or the shell top might be instable.Meanwhile,the coal pillar and the backfill remained stable to transfer the load.The height of water-conducting fractured zone of SRSF and SRDSF are 9.25 m and 20 m(in prototype),respectively,and the ratios of the height to the mining thickness are 2.10 and 2.27,respectively.(5)The visualization model of space-time cube of stress and displacement of SRSF and SRDSF were established.The temporal-spatial distribution characteristics of stress and displacement were displayed intuitively from multiple angles.The heterogeneity of stress and displacement in SRDSF is enhanced.The absorption and transfer of load by backfill need buffer in temporal and spatial,and the displacement extends temporally and spatially.(6)The concept of displacement entropy was proposed,and the quantitative representation and measurement framework of symmetric KL divergence for stress and displacement correlation and state based on information entroy and mutual information was established.The correlation of stress and displacement of SRDSF is stronger and more chaotic than that of SRSF.The backfill,the coal seam roof and floor are in harmony with displacement and evolution,and the stress plays a major role in controlling the displacement.The hot spot of entropy indicated that the state of the combination system is more chaotic.The height of the overburden failure of SRDSF extends from 28 cm to 48 cm above the coal seam roof.The stability criterion based on energy dissipation was established according to the associative calculus eauqtion of stress and displacement.(7)The dynamic evolution of stress and displacement of SRSF and SRDSF were verified by the field measurements.The stress and displacement evolution were respectively characterized by the pillar pressure and the displacement between roof and floor.There is no obvious periodic pressure after the initial pressure,and the change of the pillar pressure is slight,which reduced the stress concentration phenomenon of the combination system.The advanced distance of influence of SRSF and SRDSF are 8 m and 10 m,respectively,which is consistent with the model test results,and the backfill and the roof become a whole to produce cooperative displacement and reduce the degree of uneven deformation of SRDSF.The paper has 169 figures,26 tables and 192 references. |
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