A Novel Method For Perception Of The Overlying Strata Stress Based On Dynamic Pressure Differential Index And Its Field Application

With the historic development of the construction of the intelligent coal mine in China,the accurate data of comprehensive multiple factors from the coal mining face provide a new development opportunity for the prediction and pre-warning of the dynamic mine hazards.The real-time measurement of the shield pressure in the fully mechanized longwall face offers a huge amount of data for analyzing the mechanical behavior of the overlying strata and the ground control.However,these data measured from the field cannot accurately capture the real roof pressure in the open face area and the strata mechanical characteristics,due to the differential in support setting forces,the opening of the yielding valve and the cyclic shield operations.The purpose of this thesis is to investigate the local characteristics of the overlying strata and the method of sensing stress development in smart longwall working faces.The understanding of the stress characteristics in the hazard process is still unclear because it is still difficult to measure the stress in the overlying strata,and the mechanism of the pre-warning evaluation system for the dynamic hazard prediction is inadequate,a new concept termed the Dynamic Pressure Differential Index(DPDI)of the longwall shield is proposed,based on which a sensing approach for strata stresses is developed.Using the analytical analysis,the numerical modeling method and the engineering practice in the field,this thesis presents a method applicable to the intelligent longwall face of coal mine for the perception of the overlying strata behavior and stress and the comprehensively early warning of the dynamic disasters.The application of this key technology is also investigated in a systematic manner.The main contents and findings are listed below:(1)The characterization phenomena of the support dynamic pressure differential are proposed according to the traditional mine pressure analysis on the shield support working resistance,which can be used to reflect the weighting position and intensity of the roof more accurately.Based on the time-to-space conversion of the proposed characterization phenomena,the model of the dynamic pressure differential index of shield support is developed.The overlying strata mechanical behaviors before and after the shearer cut and the corresponding model of the dynamic pressure differential index of shield support are analyzed from the spatial variation angle of the longwall face mining cycle process.Further,the mine pressure theoretical model of the shield Dynamic Pressure Differential Index is established based on the Elastic Mechanics and the Mechanics of Materials.From this model,the back analysis mechanism of the dynamic pressure differential index of shield to the local performance of the strata is explained.(2)The numerical modeling approach is utilized to study the spatial development of the Dynamic Pressure Differential Index in the longwall face,and the corresponding evolution of this index under the influence of the strata parameters and shield movement.When the roof is in elastic during the face advance,The dynamic differential pressure index increases as a quadratic function of the length of the cantilevered top plate and decreases as the bulk modulus and shear modulus of the coal seam and bottom plate increase.However,when the roof slab is in a plastic state,the increase rate of the dynamic pressure differential index continuously grows with the decrease of the roof elastic modulus.The effect of support movement on the dynamic differential pressure index shows a negative correlation between this and the setting force.During the initial contact between the canopy and the roof,the Dynamic Pressure Differential Index declines with the delayed support distance of the shield;In the final support phase,however,the index increases slowly as the support distance is delayed.(3)The mine pressure application method is developed based on the Dynamic Pressure Differential Index of the shield.The periodic mutational points in the Dynamic Pressure Differential Index series are utilized to determine the change in the characteristics of the overlying strata over face.By contrast,the magnitude of the strata stresses during different periodic roof weightings are estimated from the Dynamic Pressure Differential Indexes corresponding to the same the cantilever beam length at different weightings,and the coal face concentration stress is determined by the Dynamic Pressure Differential Index within a single roof weighting.The Dynamic Pressure Differential Index should be optimized based on the cutting depth of the shearer in the field during the initial contact of roof and support canopy,and optimized according to the influence of the yield valve during the final stage of support before the shield advance.The authors of this thesis propose a method for data acquisition,transmission,processing and storage for the necessary analysis,starting from the time series of support resistance,shear position and hydraulic pump loading state,while ensuring data security.(4)The application and verification of the Dynamic Pressure Differential Index system are performed in the mine pressure characterization of the intelligent longwall faces.The intelligent longwall face in Longtan mine with high content is selected to apply the technology in the thesis.The dynamic pressure differential indexes under different setting forces and during the delay support are analyzed.According to the influence of the neighboring shields’ stiffness differential to the calculated Dynamic Pressure Differential Indexes for different shields,the development of the Dynamic Pressure Differential Indexes under different setting forces and delayed shield movement process are analyzed and validated,which shows good agreement to the theoretical analysis.The strata characteristics at different areas of the longwall face are obtained from the Dynamic Pressure Differential Index serial data.The gas emission characteristics from the coal mass are used to validate the accuracy of the identification of the stress concentration ahead of face and the damage and fracturing of the roof.(5)The back analysis of the mine pressure is performed based on the Dynamic Pressure Differential Index serial,and its application in the field of the pre-warning of dynamic hazards is conducted.From the Face 31201 at Shigetai coal mine with fierce dynamic roof movement,the application of the hazard early warning is analyzed combining the Dynamic Pressure Differential Indexes,the surface movement measurement,the microearthquake measurement and the ground pressure measurement.The disaster pre-warning technology is developed combing the cumulative energy from the microearthquake events during a single weighting and the Dynamic Pressure Differential Indexes,which is based on the precise division of periodic weightings and strata stress characterization from the indexes.Using the assessment of the strata stress from the Dynamic Pressure Differential Indexes,the stress distribution along the longwall face can be determined.The threshold value of the dynamic roof impacting is determined as the maximum cumulative energy during periodic strata fall,which achieves the goal for early warning of the strata dynamic impacting.