Research On Strata Behavior Characteristics At Large-height Working Face With Large-scale Physical Modeling

As a safe and efficient production technology for thick coal seam mining area,large mining height coal mining process(LMHCMP)has been widely adopted since the beginning of this century in China.Unlike conventional fully-mechanized mining process,it presents profound changes in relevant characteristics,such as characteristics of overlying strata movement,mining-induced ground pressure,interactive relationship between hydraulic support and surrounding rock,which brings new challenges to the existing theory on ground pressure and strata control.As the indispensable experimental and verification means in above research field,the existing physical modeling system and testing techniques have been difficult to meet the increasingly complex characteristics represented by thick coal seam mining.On the basis of extensive on-site consulting and investigations,a new kind of physical modeling system with high stiffness,great loading capacity and rotatable frame was developed for coal mine working face,which included the main structure and a complete set of test system.With such an excellent experimental platform,a large-scale physical model experiment was carried out on basis of No.2302 working face with large mining height in thick coal seam in Sihe coal mine,Jincheng coal mining area,Shanxi Province.The movement and broken laws and structural features of strata overlying the coal working face with large mining height were obtained,and the distribution characteristics of the abutment pressure and mining-induced stress filed around the working face were revealed.By integrating the movement and broken laws of strata and related evolution characteristics of mining-induced stress,the essential characteristics of "beam-arch dualism" was revealed at working face with large mining height.The experimental results of large-scale physical model were in well agreement with field measurement.The physical modeling system has a promising application prospect in the research field of ground pressure and strata control in the complex coal seam.The main research results and conclusions are as follows:(1)A new kind of physical modeling system with high stiffness,great loading capacity and rotatable frame was developed for coal mine working face,which included the main structure and measurement system.The physical test rig could hold physical models with different sizes and inclination angles,and the maximum size was 5m×2m×0.4m.The rotatable frame could realize the precision physical model with various dip angles(0-60°),which was accurate to 1°.The two-way loading system from vertical and horizontal directions was applied with bidirectional multi-loop servo control,and the maximum pressure exerted on the model surface could reach up to 2.10 MPa,and the corresponding simulated mining depth could reach up to 2,000m.(2)A high-precision,low-disturbance,and high-efficiency measurement system was developed for physical modeling system,which consisted of three parts,i.e.,stress test device of iso-elastic and low disturbance,abutment pressure test device and roof supporting simulation system,respectively.Its application in the large-scale physical modeling experiment showed that the measuring system was of high sampling precision,high test efficiency,and excellent long-time reliability.i)The stress testing device made of similar material was developed.Its block could perfectly match the physical and mechanical properties of the physical model,and had good deformation coordination with the latter.The external surface of the block adopted special moisture-proof technology,and the coating had good sealing properties,which could help it to adapt to the humid environment during model making and curing.ii)On basis of hydraulic control principle,the roof support simulation system was developed,which included model support,hydraulic control device,pump station system and operation condition monitoring device.The results of calibration test and large-scale physical modeling test showed that the support simulation system had high linearity and good repeatability,and it could reflect the changing process of ground pressure,and it was verified as an ideal test method for the study of interaction mechanism between hydraulic support and surrounding rock.(3)Aiming at the new characteristics of large scale physical model,such as large geometric similarity ratio and significantly improved strength of similar materials,a series of similar materials(SGC)were developed,which were made of raw materials such as river sand,gypsum and calcium carbonate.According to the difference of prototype strata strength,the cementing material could be ordinary gypsum or high strength gypsum.When ordinary gypsum was selected as the cementing material,the uniaxial compression strength of the specimen was 348-1100kPa and the ratio of compression strength to tensile strength was 4.5-7.5.As for the high strength gypsum,the strength of the specimen was significantly improved,with uniaxial compression strength of 730-2375kPa,tensile strength of 74.8-219.7kPa,elastic modulus of 0.20-0.71GPa and cohesion of 0.17-0.47MPa.The workability time window of this kind of similar materials was 5-20min,and the higher the cementing material content was,the shorter the workability duration was.On the premise that the strength of similar material was roughly the same as the required value,the mixture ratio scheme of high strength gypsum was preferred to reduce the content of cementing material and ensure the operability of physical model making.(4)The macroscopic enveloping line of overlying strata was distributed in obtuse angle "A-shaped" during each broken stage.The region between two adjacent "a" was newly formed broken strata.After superposition of previous enveloping lines,they showed a "spiral-shaped" distribution pattern.Taking the working face advancing to the right as an example,the right wing of "A-shaped" region was the periodically broken strata,whose width was approximately the same as broken span of strata.The outer of right wing was strata broken boundary line,i.e.,"\",whose inclination was basically the same value during each stage.The left wing of "?" was the newly broken strata in vertical direction,which inclined to the advancing direction of working face.(5)Based on structural morphology of strata blocks in the large-scale physical modeling,the lower blocks of broken strata presented a state of unordered accumulation,while upper blocks were in form of orderly arrangement,and a structure model of "cantilever beam-sandwiched layer-voussoir beam"(CSV)was established for large mining height working face,which could contribute to determination of theoretical value of hydraulic support working resistance.(6)The mining-induced maximum principal stress field at working face with large mining height was characterized by "stress arch" evolution process.The strata on both sides of the goaf and overlying the goaf formed a collaborative bearing structure.The vertical stress field was characterized by "zonal" evolution process.The strata overlying the goaf was the stress decreasing area,while the strata on both sides of goaf were the stress increasing area.The stress concentration in abutment area was higher than that in the rear area.(7)The broken process of overburden strata was characterized by "beam-arch dualism",that was,after the overburden strata were broken,the accumulation morphology of blocks looked like a beam,while the upper blocks,especially those which were in range of fault zone,presented "stress arch" force transfer mechanism.The broken of thick and hard strata had the crucial influence on the evolution of the macroscopic mining-induced stress field.Taking the broken of two layers of thick and hard strata as an example,the macroscopic evolution process could be summarized as:lower stress arch,lower secondary stress arch,upper stress arch and upper-lower compound arch.After the breaking of high stratum,the upper-lower compound arch appeared above the working face.(8)The reasonable working resistance of hydraulic support was determined to be 12000kN by comprehensive research methods such as physical modeling and discrete element numerical simulation.In the numerical simulation study,the "roof-coal wall-support" was regarded as a collaborative load bearing system,and the stiffness of the support was regarded as a specific constant.The deformation characteristics of the roof and coal wall,and supporting response of hydraulic support under different support schemes were comprehensively evaluated.(9)During the production of No.2302 working face,field measurement and analysis of characteristics of mining-induced ground pressure and working resistance of hydraulic supports were carried out.The field measurement results verified the accuracy of large-scale physical modeling experiment,and the reliability of the new physical modeling system was verified.