Study On Roof Cut Off Mechanism Of High Intensity Mining In Western Shallow Seam

The northwest region of China is rich in a large number of shallow seams whose depth is 50 ~ 150 m, these seams’ typical features are shallow depth, thin bedrock and overlying thick loose sand. In the coal mining process, working face roof is often difficult to form a stable “masonry beam” structure. In large-height, high-speed and advancing longwall mining process, roof prone to sink like steps, this situation often causes the roof along the coal wall cutting off, strata pressure behavior is more intense and more complex. Leading to this “crushed to death” or the formation of gushing collapse sand channel, brings a lot of coal mine safety risks. How to ensure the safety of shallow coal seam mining and reduce pressure frame toppers accident, it is one of the coal industry needing to solve scientific problems. Therefore, the study for mine pressure regulation during shallow coal seam mining reveals that the mechanism of the shelf disaster of a wide range of roof cut off and the adaptability of support resistance. It has important theoretical and practical significance. The support compression quantities expressions under the static and dynamic loading before and after the system instability were obtained by the correlation analysis methods of catastrophic theory and rock dynamics. This thesis synthetically uses the numerical simulation, theoretical analysis, physics experiments and other means, for the study of face strata behavior regularity during shallow coal seam mining face in high strength strata behavior regularity, the mechanism and factors of large area roof cut off the shelf disaster, working face and surrounding rock the interaction. Firstly, the formation mechanism of the wide range of shallow coal seam roof cut off and pressure frame disasters by using catastrophe theory analysis method and the necessary and sufficient conditions for the occurrence of the roof cut off was analyzed, and the main factors of the general process toppers pressing frame formed was explained. According to the buried faults, fractures and large joints which decide rock breaking and pressure the actual situation of a top slate body, by using fracture mechanics theory, the mechanical model of main roof with a central oblique crack breaking was established, the expression of main roof crack tip stress intensity factor and the calculation formula of main roof periodic weighting step distance and support working resistance were derivate, and the underlying causes of the roof cut-off and rack was revealed, and throughout the thin plate theory and fracture mechanics analysis method, the spatial and temporal characteristics of high-intensity mining longwall roof collapse was analyzed, by means of numerical simulation, the general rules of shallow coal mine pressure appearing in the conditions of great mining height, speed advance was researched. Combined with fracture mechanics analysis, by using similar material simulation, the whole process of crack and cut-through until the fall when the shallow seam is mined is revealed, the variation of abutment pressure, roof displacement, concealed faults and support working resistance stress in the process of mining is analyzed.The main conclusions are as follows:(1) A mechanical model of the mechanism of roof cutting disaster in large area was established, and then the main factors for instability failure of the model were analyzed and revealed the general process of roof cutting disasters. Aiming at roof step convergence along the coal wall caused roof cutting support crushing disasters during the periodic weighting in shallow seam mining, making the immediate roof for the research, according to the nonlinear variation character of immediate roof under the abutment pressure unstable failure, a system mechanics model composed by the upper roof- immediate roof- powered support-waste rock was established. The instability mechanism of the system under the action of load was studied by catastrophic theory. The necessary sufficient condition for instability failure of the system and the formula of the jump value of displacement of immediate roof were gained. It pointed that bifurcation set equation is sufficient conditions leading to jump instability of the system, only when the control variable p of the system was less than or equal to 0, the ratio of the slope at the inflection point of the stress-strain curve of support stiffness and immediate roof rock mass, K was less than or equal to 1, did it be the necessary condition for instability failure of the system. Stiffness ratio is related to internal properties of the system only, which is the necessary condition for instability failure of the system. The support stiffness is crucial to instability failure of the system when the material parameters of immediate roof rock mass structure are constant. Thereby it plays an important role to increase support stiffness of roof stability within a reasonable range for roof stability. When the support stiffness is constant, the immediate roof rock mass more complete the stiffness greater, and the stiffness ratio K smaller, the system is easier to fail. On the contrary, the immediate roof rock mass more fracture, the stiffness smaller, and the stiffness ratio K greater, the system is more stable. The analysis on the necessary sufficient condition for instability failure of the system shows that failure of the system was related to the material parameters, also related to the upper load and the periodic weighting step of key roof. The engineering geological condition of 1203 face, Shendong daliuta mine, was combined to analyze its main parameters, which are the main factors of the displacement, and energy release △V of the roof during catastrophic instability failure of the system. Through engineering example, the rationality of the theoretical derivation was verified. Application of rock dynamic correlation analysis method, the calculation formula of the support load is derived. It is of positive significance to reveal the mechanism of instability and cut off of the roof of the shallow seam, and to study the size of the load on the top of the roof. By using the theoretical analysis results, the coal mine fully mechanized face roof cutting crushing accident were analyzed, the calculation results show that the support resistance is too small, cannot meet the requirements of support, which caused the accident of roof cutting. It is proved that the rationality of theoretical deduction can provide theoretical guidance for engineering practice. The RFPA numerical simulation software is used to simulate the general process of the roof caving disaster in the process of advancing the working surface.(2) Combing the actual situation of key roof easy to form the cantilever structure leading to fracturing rotary while key roof cutting, when shallow seam was in high intensity mining, a mechanics model of roof with center inclined crack breaking was established by using fracture mechanics. Firstly, a finite plate model of key roof rock beam with center inclined crack was established. According to the complex composite load characteristics of the key roof rock beam, the crack stress intensity factor was divided into simple finite plate model under the action of tensile stress, shear stress and bending moment. The stress intensity factor of the crack tip of the rock beam is calculated by the superposition of stress intensity factor of three simple stresses. It is pointed out that the stress intensity factor of the crack tip is mainly related to the angle, the length of the crack and the thickness of the rock beam. The greater the crack length is, the more easily the roof break. With the increase of the crack angle β, the KI increases and then decreases. When the angle between the crack and the rock beam reaches 90°, the maximum KI is reached, and then gradually decreases. And KII increases with the increase of the crack angle β, when the angle between the crack and the rock beam reaches 45°, the maximum KII is reached, then gradually decreases.The expression the periodic weighting step and the calculation formula of working resistance of support of key roof were obtained further. According to the analysis of the law of the corresponding change with the change of different parameters in the expression of the fracture step l of the key roof, it is k Nown that the variety trend of fracture step l is symmetrical, which decreases and then increases when the crack angle β varies between 0 and 180°. The fracture step l increases with the increase of fracture toughness Kc, working resistance of support Q, horizontal pressure T, while decreases with the increase of the load of overlaying rock seam. Working resistance of support of certain working face was calculated by the expression of support working resistance, verifying the support of the working face meeting the support requirements, and provides corresponding theoretical guidance for engineering practice.Applying thin plate theory and fracture mechanics analysis method in elasticity, combining the situation that seam roof often has geological characteristics of small faults, cracks, etc. taking advantage of the plate theory in elasticity, the mechanical model of cracked plate was established. According to weighing law on the direction of longwall roof in the process of periodic weighting, the destruction of the elastic sheet instability in the process of working face advancing process was divided into crack, plastic hinge and hinge plate buckling failure, the various stages of the genesis and development was mechanically analyzed, roof caving to pressure law on the face length direction was theoretically explained and verified combining with theoretical analysis of engineering practice team.(3) The face strata behavior regularity is researched when shallow coal seam is mined in high-strength. Firstly, by using simulation software UDEC, based on the occurrence conditions and mining technology of Shendong Daliuta 1230 coal seam face, the ground pressure and the rock behavior law when its roof cut off in conditions of different advancing speeds and mining heights was researched by numerical simulation analysis, it points that with the increase of the height of the mining face, overburden rock is difficult to form a stable self-supporting structure, collapsed rock can’t support the overburden rock, the roof always exists in the formal structure of "cantilever beam”, roof collapse more easily fall into the mined-out area, the step pressure decreases, underground pressure increases; with the increasing speed of face advance, periodic roof and first weighting step distance increases, the expanding coefficient of collapsed rock increases, roof pressure increases when the pressure comes.(4) Based on the analysis results of mechanical model of roof with center inclined crack, a further research on the general rule of roof cutting hazards induced by fault cracks as the working face advancing under roof which contains master cracks is conducted by means of similar material simulation. Firstly, the general experimental rules of roof weighting as working face advancing are summarized, the characteristics of roofs in the course of high intensity mining which tends to form the Cantilever structure leading to roof collapse are analyzed. There is a systematical description of the whole process, which namely contains fault activation, crack propagation-coalescence, roof cutting line emergence, strata collapse, and the collapse characteristics of roof strata within different dip angles are compared. The results show that roof cutting performs in an overturning way and load is mainly applied on the back of supports as working face promoting through master cracks 1. When the master cracks 2, overlying strata tend to cutting in a shift-type way along the roof cutting line; high load is applied on the front part of supports, which is easy to cause damage.According to the monitoring data, there are obvious vertical displacements in overlying strata due to the large height mining. When the working face passing the master cracks, sinkage near both master cracks increase dramatically considering the overlying strata collapse in large scale, and the maximum roof sinkage of master crack 1 is larger than that of master crack 2. Compared with master crack 1, the abutment pressure near master crack 2 is higher, which is related to the occurrence and instability of crack. The volume of cutting rock produced by master crack 2 destabilization is larger than that by master crack 1. Working resistance of support is monitored by self-made supports. Monitoring results show that, working resistance of support when working face advancing master crack 1 increase by 33% compared with normal weighing condition. And when working face advancing master crack 2, working resistance of support increase by 64% compared with that in periodic weighting process. According to the stress characteristics of roof cutting when working face advancing 2 master cracks respectively, the dip of crack plays important role in the emergence of roof cutting line. With the increasing of master crack dip angle, the volume of collapse rock mass induced by the crack coalescence is becoming larger, shear failure characteristics of roof are more obvious and the supporting loads are increasing when roof cutting occurs.