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To investigate the tensile strength and fracture properties of sedimentary rock buried at different depths in Shendong coal field,the acoustic emission( AE) parameters of sedimentary rock specimens at different depths in Shendong coal field were monitored by AE monitoring system during the Brazilian splitting experiment. The variation of tensile strength,model I fracture toughness and the ratio k( tensile strength with respect to fracture toughness) of rock specimens with respect to the buried depth was discussed. The damage evolution,failure features and precursory features of specimens were also analyzed. The results indicate that the tensile strength and the fracture toughness increase with the increase of the buried depth and have power function relationship to the buried depth.In general, the tensile strength and the fracture toughness of rocks buried depth above-300 m are lower than 0.5 MPa and 0.15 MPa·m1/2, while those buried depth deeper than-300 m are higher than 1.0 MPa and 0.15 MPa·m1/2,respectively.The ratio k also increases with the rise of the buried depth,and has a logarithmic function relationship with the depth.The k values of rocks with the buried depth over-300 m are 2-3. The k values are 3-6 for the rocks with the buried depth close to-400 m,while the k values are relatively discrete for the rocks with the buried depth deeper than-500 m.In addition,the failure modes were observed similarly existing in the tested specimens,tension-shear mixed failure is the dominant pattern.However, the shear failure occurs at different time for the specimens with the various buried depths.For example,the shear failure begins to appear in the post-peak stage for the rocks with the buried depth less than-300 m, while the shear failure appears in the pre-peak stage for the rocks with the buried depth larger than-300 m.The failure precursors of the tested rocks appear normally before the peak strength, except the difference of the triggered stress. The precursors of the specimens( except medium-coarse sandstone) with the depth less than-300 m appear at the stress reaching 70%-90% peak stress. For the specimens with the buried depth larger than-300 m,the precursors appear at the stress larger than 92% of the peak stress.This indicates that the warning time of the specimen failure decreases with the increase of the buried depths. [ABSTRACT FROM AUTHOR]

The mechanism of the initial fracture instability of main roof in a fully mechanized longwall face is critical to the selection of hydraulic support and the control of surrounding rock. The fracture process and structural instability mechanism of main roof were analyzed by establishing the roof fracture mechanical model. The results show that the internal maximum principal stress σ1 and the maximum shear stress τmax increase gradually, with the decrease of the thickness-span ratio of main roof from the initial state to the limit state. When the thickness-span ratio is close to 0. 2, both σ1 and τmaxincrease sharply, but the increase rate of σ1 is greater than that of τmax, and the main roof is thus easy to break at the top of both ends. The limit thickness-span ratio n1 of main roof is normally greater than 0. 2 when theinitial fracture appears at the main roof. The n1 of hinge structure of main roof that does not appear sliding instability increases linearly with the increase of the rotation angle θ. The hinge structure is easy to occur sliding instability at the beginning of the initial rotation when the rotation angle is 0° ～ 3°. The critical value of the rotation instability of the hinge structure is n1 equals to 0. 624 on a condition that only the roof thickness-span ratio dominates. When the n1 is less than 0. 624, however, the rotation limit balance angle βjbecomes larger than the minimum value β1 allowed by the swing balance angle, and the hinge structure occurs rotation instability. In addition, the method for determining the dynamic load factor of the longwall face when the main roof structure appears slipping instability was obtained. When the separation layer Δh between immediate roof and main roof is 0, the minimum value of the dynamic load factor Fsd should be applied. When Δh is non-zero, Fsdincreases with the increase of Δh, but also can be reduced with the increase of the ratio of immediate roof to support stiffness ks, the ratio of immediate roof weight to main roof transfer load m. Finally, the liability of the theoretical analysis was verified by the analytical calculations for the initial weighting of different fully mechanized longwall faces. [ABSTRACT FROM AUTHOR]

The reverse fault will slip with workface excavation in footwall. As the stresses loading on the combined sample were equal in the X and Y direction, the stress and strain along the fault,as well as acoustic emission signals,were monitored during the coal excavation. The analytical results show that stress decrease zone,stress increase zone and in situ stress zone occurred around the gob and their evolution will cause fault slip. Compared with normal fault, reverse fault has higher slipping risk under the same mining condition. As the workface locating in the footwall of reverse fault advances forward,the ratio of vertical force and horizontal force caused by mining increases from 1 to 6 ,which means the slipping risk of reverse fault rise. The strain along the fault face is not even,and the strain of coal seam is higher than that of rock strata. The energy of 84% of the acoustic events caused by coal mining are between 10-13 J and 10-10 J and their focus mainly concentrate in the roof strata and unlikely transpass the fault face which means the fault can cut-off the stress disturbance of mining. The conjunction of coal seam and fault face tends to cause large acoustic events which means this area has high slip risk. Compared with the in situ seismicity in Zhaolou Coal Mine,the spatial location of seismic focus and experimental acoustic focus have similar features to some extent, which could prove the feasibility of the experimental method adopted in this paper. [ABSTRACT FROM AUTHOR]

In order to study the Coulomb stress variation along a fault face and the tremor distribution features under the mining disturbance,the elements which have effects on Coulomb stress of fault face were theoretically analysed on the basis of the geological and mining conditions of 1303 top-caving workface in Zhaolou Coal Mine,China.With numerical simulation and laboratory test,the Coulomb stress of different location along a reverse fault face was calculated as the distance between mining face and fault face decreasing.Additionally,the in-situ tremor data was collected and statistically analysed.The results show that the Coulomb stress (f) of a fault face point is determined by the mining disturbance (including σx,σy,τxy and τyx),the angle of the fault (θ) and the frictional coefficient (μ) of the fault face.If the mining disturbance and frictional coefficient are fixed,the most dangerous angle of the fault for sliping could be obtained by df/dθ=0.Stress disturbance circle and displacement disturbance circle are formulated as the coal excavating,and the symmetry of the stress disturbance circle and the continuity of the displacement disturbance circle are broken because of the existence of fault.Numerical simulation data and laboratory test data all show that the upper zone of the fault face is influenced by the mining firstly,and the slipping liability of fault increases first and then decreases as the workface mining forward to the fault face.The daily frequency of tremors was about 20 per day as the workface located in footwall and increased to approximately 35 per day when the workface located in the hanging wall.However,the average energy for each tremor declined from 22 kJ before workface reach the fault to 4 kJ after the workface passes the fault,which demonstrates that the tremor disturbance is severe as the workface moving forward to the fault,what is more,the concentration zone of tremor focus transfers from upper zone of the fault to down zone when the mining face mining through the fault face. [ABSTRACT FROM AUTHOR]

Based on the P-wave velocity test,CT scanning and three-dimensional reconstruction technology,the distribution heterogeneity of cracks and mineral inclusions in coal sample,obtained from the No.14 coal in Xinzhou Mine,was researched,and the failure characteristics of coal samples loaded in different directions effected by its heterogeneity were also investigated,according to the uniaxial experiment when the coal samples loaded in various directions.Results indicate that the distribution heterogeneity of microstructures in coal sample leads to the difference of failure characteristics loading in different directions.The primary fractures and mineral inclusions are distributed and extended along the bedding plane,the average uniaxial compressive strength,total AE count,and other parameters are loaded perpendicular to bedding plane higher than that parallel to bedding plane,at the same time,the transverse cutting effect of cleats makes the parameters of mentioned above and the regularity of AE in the loading process loaded perpendicular to cleats lower than the coal samples been loaded parallel to the cleats.The UCS and the P-wave velocity of the coal sample have an exponential function relation,and the effect of heterogeneity inside the coal sample on the UCS can be reflected by the heterogeneity coefficient,and the heterogeneity coefficient has a negative correlation with the UCS of the coal. [ABSTRACT FROM AUTHOR]

Full-mechanized mining method leads to large size gobs and serious stress and displacement disturbance on surrounding rock mass,which would cause a fault tensile-slip motion that induces high energy seismic tremors and further cause rockburst around the mining face.To solve this problem,based on the mining and geological background of 1303 full-mechanized workface in Zhaolou Mine,the tectonic and mining induced stress as well as their main controlling elements were analyzed.The relationship between tensile-slip displacement along fault face and mining procedure was studied with analogue model,and the characteristics of seismic tremors were analyzed statistically.The results show that in the circumstance of mining the footwall coal seam of reverse fault,the tensile-slip motion of fault is determined by several key parameters,such as the ratio (k) of vertical force (Pv) and horizontal force (Ph) caused by mining activity,the dip angle of fault (θ) which represents the geometric features and the friction angle of fault (φ) which represents the mechanical features of fault.Additionally,the mechanical circumstance of the fault face should satisfy the stick-slip requirements as to cause seismicity.The stress and displacement disturbance induced by mining appear first in low overlaying strata of coal seam and then develop to high strata as the excavation processing,these would further cause the tensile-slip motion of fault.Seismic tremors mainly concentrate around the mining gob.With the increase of gob size,the daily frequency and energy of tremors around the mining face rise steadily,and the location of tremor focus propagates to upper strata.The tensile-slip motion of Fd96 fault is the main cause for the tremors occurred in 1303 mining face,followed by the fracture and movement of overlay strata,and the tensile-slip motion of FZ14 fault has least influence on tremor activity. [ABSTRACT FROM AUTHOR]

For shallow-depth mining-induced pressure characteristics of Shendong mines,combining with 126 cases of shallow buried coal face pressure measurement data,the regression analysis and probability statistics analysis were conducted regarding the single and coupling influences on the maximum mining-induced pressure,pressure distribution and pressure step caused by overlying strata lithology,mining height,buried depth and face length.At the same time,KSPB software and overburden rock hardness coefficient method were used to quantify the strata lithology.The results show that (1) Under the condition of no,single and multiple key stratums,the strata lithology,mining height,buried depth and face length have the relationship of linear,linear,negative logarithm and double logarithmic function with maximum mining-induced pressure,and different strata lithology conditions have different parameters；(2) Pressure distribution has the characteristics of the quadratic function on face length direction.When face length increases,the pressure distribution parameter -a decreases with the form of negative logarithmic function,different strata lithology conditions have different parameters,and strata lithology,mining height and buried depth have little influence on pressure distribution；(3) Under different strata lithology situations,in working face,first and periodic pressure step have the relationship of exponential function with strata hardness coefficient,and mining height,buried depth and length of working face are related to probability distribution,and the union values of strata hardness coefficient of expected value and probability statistics value are the face first and periodic pressure step.Comparative analysis and engineering practice show that experience regression formula combining with probability statistics,within a certain residual,can accurately estimate the shallow buried coal face maximum pressure,pressure distribution and the pressure step of coal face,providing a reference for the selection of hydraulic support and the control of face roof in similar geological conditions. [ABSTRACT FROM AUTHOR]

The energy dissipation law of coal under different impact loading conditions was experimentally investigated by using a Split Hopkinson Pressure Bar. The indirect dynamic Brazilian disc split tension tests for coals were carried out. The total absorption energy density, total dissipated energy density and damage variable for different bedding angles under different impact velocities were analyzed and discussed. Meanwhile, the post-failure fragments of coal samples (0-0.2 mm and 0.2-5 mm) were collected and quantitatively analyzed to characterize the dynamic failure features for coal samples. Based on the experimental results, it was found that the damage variable of air dry coal specimens with the same bedding angle nearly linearly increases with the increase of impact velocity, and the damage variable of overall saturated coals increases exponentially with the increase of impact velocity. Except the bedding angle of 45°,14.1% to 31.3% fragment reductions of 0-0.2 mm and 33.7% to 53.0% reductions of 0.2-5 mm were observed for the saturated coals compared to dry ones. For the bedding angle of 45°, the water saturation did not reduce fragments. [ABSTRACT FROM AUTHOR]

In terms of deep long mining face pressure distribution situation, on the basis of pressure arch hypothesis, principal stress shell theory and M.Promojiyfakonov theory, this study summarized the rules of overburden rock broken to build a geometry model of mining fraction zone, deduced formula of mining face fracture zone, verified the model and formula by numerical simulation, and analyzed the influence factors of face fracture zone height and mining pressure size. The results show that the stope fracture zone model is reasonable and corresponds with the spatial distribution characteristic of stope overburden fracture zone. Working face fracture zone formula exists correctly and conforms to the mine pressure distribution of high middle low both sides as the “parabola” shape. Minimum face pressure ratio and its change rate have a negative correlation with the working length, and basically are not affected by mining height and mining depth, the distribution characteristic of mining face fracture zone and mining face pressure have a high consistency. Mining face fracture zone height and mining face pressure, linear correlation with the mining height and rock hardness coefficient have a less correlation with face length and mining depth. [ABSTRACT FROM AUTHOR]

As a new round of energy revolution is pregnant and the Internet plus era and intelligence development are intensified,the new concept and key technologies of precise risk identification and monitoring early warning of typical dynamic disasters in coal mine are proposed by summarizing the main problems of coal mine typical dynamic disasters prevention in China and investigating how coal mine monitoring early warning copes with the advent of a new round of science and technology innovation.By means of technologies including disaster precursor information acquisition and sensing,multi network fusion transmission and multi-source mass precursor information extraction mining,precise risk identification and monitoring early warning of coal mine typical dynamic disasters is proposed as a new future mining mode and method integrating intelligent mining technique with precursor information depth perception,precise risk identification and monitoring early warning of typical dynamic disasters in coal mine.The mode is based on the concept of precise coal mining and evolution mechanism of multiphase multi-field coupling catastrophic.It is able to promote the transformation of the coal mine monitoring early warning from a traditional empirical and qualitative to a precise and quantitative sector,which could comprehensively improve risk identification and monitoring early warning ability of China’s coal mine typical dynamic disasters.The research proposes four key scientific problems and eight major research directions for precise risk identification and monitoring early warning of coal mine typical dynamic disasters,and focuses on the research progress of twelve aspects based on basic theory research,key technology development and early warning platform construction.Therefore,it directs the technology route for online monitoring,intelligent identification and real-time precision warning of coal burst,coal and gas outburst and coal-rock gas compound dynamic disaster. [ABSTRACT FROM AUTHOR]

At present,some mines have reached the end of their life cycle,do not meet the requirements for safe production,or suffer from a lack of financial viability forcing them to face closure or abandonment.If such a mine is closed or abandoned,it will not only cause a huge waste of resources,but also lead to subsequent safety,environmental,and social problems.It is very difficult to exploit and utilize the remaining resources of such closed/abandoned mines in China.In addition,it is impossible to copy the utilization patterns used abroad in foreign waste mine resource handling.Therefore,the overall country strategy,foundation theory,and key technologies lie ahead.Five key scientific problems are posed with a view to improving the exploitation and utilization of closed or abandoned mines in China,namely an analysis of the coupling mechanism of high-efficiency conversion and utilization via underground coal gasification,environmental protection mechanisms for underground space storage based on precise and intelligent control,intelligent precision development modes for mine water and unconventional energy based on multi-field coupled theory,renewable energy development and micro-grid energy transfer modes in closed/abandoned mines,and industrial tourism development based on ecological restoration and environmental support.The work has important political and economic strategic connotations to the development and utilization of closed/ abandoned mine resources and a strategic path under the new normal state of coal resources is proposed.It is of significance to those seeking to improve the efficiency of exploitation and utilization of abandoned mine resources. [ABSTRACT FROM AUTHOR]