Study On The Fracture Characteristics And Roof Control Of Overlying Strata In The Deep Mining Area Of Ordos

As the increase of mining intensity and the extension of mining areas in recent years in Ordos,the coal mining activities gradually go deeper and deep coal mines start to operate and produce.Meanwhile,the strata behaviors of intensive mining faces in Ordos show significant difference with those observed in the deep coal mines at middle and east mining areas of China.To be specific,in the middle and east coal mines the large deformation and apparent rock rheological characteristics are observed for the gates,and the rockburst disasters are frequently found during the period of gate drilling and face retreating.In deep mines at Ordos by comparison,the unbalance between the entry development and face advance under the high pressure becomes unacceptable,and the gob-side entries show tremendous deformation.The strong massive roof caves in fiercely in a larger periodic weighting interval,leading to the rock bump activities ahead of the face.Therefore,due to the high pressure,intensive disturbance and complicated geological conditions in deep coal mines of Ordos,the rock strata show intensive excavation unloading effects,the elasto-brittle-plasticity deformation and failure characteristics and the support units see the sudden increase in loading during weights.The risk of roof caving along the faceline and the iron-bound shield increases significantly.Thus,the mechanism,prediction and control of roof roof failure and movement is of great significance and should be benefiting the safety and effectiveness of the mining operations in Ordos.Based on the engineering background in Nalinhe coal mine at Ordos,this thesis studies the roof movement,failure and control techniques using filed observation,theoretical analysis,numerical and physical modeling approaches.The field monitoring is first conducted to observe the roof failure and entry deformation in the longwall faces of No.3-1 seam.The tri-axial compression tests are performed to analyze the overburden deformation and failure characteristics,and to study the influence of stress level and moisture content on the deformation and failure characteristics of the sandstone and mudstone,which is later classified into three stages i.e.the elastic stage,brittle failure stage and perfect-plastic stage.According to this,a perfect elasto-brittle-plastic theoretical model is developed to describe the failure and deformation of the overburden in the deep mines.The models are used to establish the numerical models using FLAC3 D to simulate the progressive development of overburden failure and face fall.A physical model is also developed to study the overburden structures with the progressive development of the longwall face.Combining the field observation,numerical and physical modeling studies,the overburden movement,failure and structural characteristics are analyzed.An analytical model regarding the failure of roof ahead of and along the faceline is proposed to investigate the influencing factors of the main roof failure and roof control techniques.Important findings are listed below:(1)Field observation,physical and numerical models show that the intensive roof failure and face fall occur in the face area due to the high ground pressure and massive unloading mining effect.However,the failure roof still retains the capacity to support the above strata,which has a significant impact on the movement and failure of the overburden.The overburden areas can be identified as the area supported by solid coal or rock,the shield supported area and gob supported area.The failure of the main roof and the above key stratum results in long and short combined periodic weighting intervals.(2)The deformation tests on the rocks taken from deep mines of Ordos show that the confining pressures significantly impact the strength of the sandstone and mudstone.With the increase of the confining pressure,the peak and residual strength also increase.The deformation of the sandstone and mudstone can be classified in to the elastic stage,brittle failure stage and perfect-plastic stage.Based on this,a perfect elasto-plastic mechanical model is developed to describe the elastic and plastic deformation of the overburden in Ordos deep mines,which also describes the deformation and failure of the sandstone and mudstone.(3)The numerical models regarding the longwall faces in Nalinhe coal mines are developed according to the theoretical models in this research.The progressive development of the failure and deformation of the roof and longwall face is simulated.The load bearing characteristics and long/short weighting intervals of the key main roof stratum are reproduced.The numerical models in this thesis well describes the mechanical behaviors of the overburden and the movement and failure of roofs in deep coal mines of Ordos.(4)The supporting and self-supporting characteristics of the main roof are obtained from the field observation,numerical and physical models.The mechanical model of the main roof supported by the solid coal,the longwall shield and gob caving materials are developed.The numerical codes are developed based on the criterion of supported main roof failure ahead of faceline,and parametric analysis is performed to study the influence of main roof thickness,modulus,weighting interval,support stiffness,gob stiffness on the failure of main roof ahead of faceline.(5)A model of Main roof hanging beam and hinged beam is developed in this thesis considering the support from both shields and gob materials.The criterion for determining the breaking positions of the roof around the faceline is proposed.(6)The roof support from Shield ZY13000/28/62 D and the caving rock fragments in the gob prevents the occurrence of the main roof failure along and ahead of the faceline in longwall at Nalinhe coal mine.As the face advances,the main roof fall occurs typically behind the longwall shield;the iron-bound shield therefore is avoided.This agrees with the field observation.(7)Based on the research results in this thesis,it is proposed that the roof control techniques such as improving the support stiffness and intensity and increasing the face advance rate are recommended in deep coal mines of Ordos.