Rheological Mechanism And Control Strategy For Deep Roadway With Loose Coal Mass

With the increase of mining depth of coal resources,the time effect of roadway deformation becomes more serious.For the roadway with very low strength of surrounding coal mass,the phenomenon of large rheological deformation is very common.Given this,this study focuses on the rheological problems of loose coal roadway and adopts the research methods of artificial intelligence,experiments,theoretical analysis,engineering investigation,numerical simulation,and field tests.Based on the inversion model of coal and rock parameters,equivalent loose coal samples were constructed in the laboratory,the rheological characteristics of coal were revealed,and a non-linear rheological model was established.The rheological parameters of coal mass were inverted,and the rheological mechanism of the roadway was revealed.The control method of reinforcing loose coal mass by jet grouting was put forward and its feasibility was tested.The mechanism of reinforcing roadway by jet grouting was explored,which provides a new idea for the control of loose coal roadway.The main contents and achievements of this study are as follows:(1)The model of coal and rock mass parameter inversion was established.Based on analyzing the necessity of parameter inversion,two kinds of physical and mechanical parameters,namely "construction parameters" and "rheological parameters",which are involved in the study of loose coal mass,were inverted using artificial intelligence.Combining Support Vector Machine(SVM)with Beetle Antennae Search Algorithm(BAS),an evolutionary Support Vector Machine parameter inversion model(BAS-ESVM)was further established,and the main steps of the model inversion were determined.The BAS not only carried out parameters of SVM(kernel parameters and penalty parameters)but also optimized the inversion parameters.Using this model,the parameters of coal sample construction and the rheological parameters of the roadway were accurately inverted.(2)The method of constructing coal samples equivalent to loose coal mass on-site was proposed.Given the actual occurrence of typical loose coal seams,it is difficult to carry out physical and mechanical tests of coal and rock by conventional means.The method takes the firmness coefficient of loose coal as a link and aims to equate the porosity and strength of briquette with that of in-situ coal mass.The porosity(9.8%)and the inverse strength(2.5 MPa)of the in-situ coal mass were measured in the laboratory.The method for determining the porosity of the briquette coal was put forward.The forming pressure,time,and moisture content were determined as the key influencing parameters in the process of coal sample construction.The forming process of coal mass was determined and the forming mechanism and factors affecting the effect were analyzed.The three-stage curve in the process of coal mass forming,i.e.initial compaction deformation,plastic deformation,and elastic deformation,was obtained.The five-stage characteristics of briquette failure,i.e.pore-fracture compaction,elastic deformation,stable fracture,accelerated failure,and post-peak failure,were revealed.Based on the BAS-ESVM model,the parameters of briquette construction under the porosity and strength of raw coal were obtained,i.e.the briquetting pressure 23.7 MPa,the briquetting time 33.5 min,and the water content 4.82%.According to these parameters,the coal mass was successfully established.The test strength of the formed briquette is 2.52 MPa and the porosity is 10%,which is very close to the raw coal confirming the rationality and accuracy of the model and parameters.(3)The rheological characteristics of loose coal mass were revealed and a suitable rheological model was established.Based on the constructed loose coal samples,the creep curves of the whole process of the uniaxial rheological process were measured by the step-loading method.The rheological deformation characteristics of loose coal were revealed,i.e."instantaneous deformation,deceleration creep,isovelocity creep and accelerated creep stages".There was a residual deformation after unloading.The four-stage stress-strain of the specimen in the axial direction,i.e.pore-fracture compaction stage,linear deformation stage,fracture-pore development stage,and accelerated failure stage,was obtained.The isochronal stress-strain curve and instantaneous loading deformation modulus of creep coal under long-term creep tests were analyzed.Guided by the rheological element model,an improved CVISC rheological model for loose coal was proposed,and the relevant creep equation and its difference form were deduced.The calculation method of added viscous element parameters was proposed,and the rheological parameters of loose coal were identified.The rationality and correctness of the proposed model were verified by numerical model analysis.(4)The rheological parameters of loose coal mass were inverted and the rheological mechanism of coal roadway was revealed.Based on a typical case of roadway engineering in the loose coal seam,the rheology of the roadway was analyzed,i.e."large rheological characteristics of deceleration in the early stage and large rheological characteristics of constant velocity in the later stage".It is determined that the long-term rheological characteristics of weak and loose coal mass in the sidewalls are the key factors for roadway instability and failure.The proposed non-linear rheological model for loose coal and the range of relative rheological parameters were selected based on theoretical analysis.Through the combination design of orthogonal rheological parameters and three-dimensional numerical simulation of the roadway,the roadway displacement with time was obtained.Based on the in-situ rheological displacement data,the rheological parameters of coal mass around the roadway were obtained by the "BAS-ESVM" model.The forward calculation verified that the inversion rheological parameters and the overall model are appropriate and correct.Furthermore,by analyzing the evolution of horizontal and vertical displacement,maximum and minimum principal stresses and plastic zone expansion of surrounding rock of the rheological roadway,the failure mechanism of loose coal roadway was revealed,i.e.long time of unstable deformation,accumulation of high stress in coal mass,and the extension range of plastic zone over the support range.(5)The technique of strengthening the rheological roadway by high-pressure jet grouting was put forward and the effect of pile-forming by hole-enlarging in loose coal mass was tested.The fundamental of controlling the rheological roadway is to improve the properties of loose coal mass.It is proposed that the coal mass should be deeply modified by high-pressure jet grouting technology to restrain the rheology of the roadway.The coal breaking mechanism of shear,tension,and internal damage of high-pressure jet grouting was analyzed.The reaming range and key influencing factors of jet grouting in coal mass were discussed.The mechanism of cement pile formation and modification of coal mass was analyzed.The feasibility of jet grouting in deep soft coal seam was discussed.The related equipment and model of jet grouting were selected.The effect of jet flow and pressure on coal mass was analyzed.The rationality of equipment was verified in advance on the ground.Two sets of jet grouting processes were determined.The field test results show that the high-pressure jet grouting technology has a limited reaming range for hard mudstone,but it has a better effect on reaming piles in loose coal.The size is about 400 mm?500 mm,which meets the requirements of pre-reinforcement and support.However,the test site and some jet grouting parameters still need to be optimized.(6)The scheme of reinforcing and controlling loose coal roadway by jet grouting was explored and its controlling rheological mechanism was analyzed.Based on the existing state of jet grouting pile in the loose coal seam,the consolidation body of coal-cement composite was constructed in the laboratory.The reasonable ratio of coal to cement 1.3 was determined by theoretical calculation.The mixture of coal-cement was designed and its slump was measured.The modification effect of cement slurry on coal mass was analyzed by macro and micro methods.It is considered that coal-cement consolidation is a composite material with excellent strength and ductility,compared with concrete and coal mass.The size effect of mechanical parameters and rheological properties of the material were determined by experiments.The design idea,principle,and key technology of roadway reinforcement by jet grouting were put forward.The control scheme of the rheological roadway in loose coal seam was established,i.e."jet grouting modification and reinforcement as the main body,combined with shotcrete and U-shaped shed reinforcement" and the relevant parameters were determined.The basic control idea is summarized as "shallow surface modification,pre-reinforcement,enhancing bearing capacity,side-laying resistance,flexible and rigid multi-support".A three-dimensional numerical model with jet grouting reinforcement was established,and the constitutive model and related parameters were reasonably selected.The mechanism of the schemes by jet grouting on controlling the rheological deformation of the roadway,stress optimization of surrounding rock,and expansion of plastic zone was explored.The comprehensive analysis shows that the technology of jet grouting can significantly reduce the deformation of the roof and sidewalls,with a maximum reduction of 69% and 78%,respectively.It can reduce the stability time of the roadway,from 60 days to 15 days.It can optimize the stress concentration of surrounding rock and the coefficient can be reduced by 35%.The plastic zone of the roof can be reduced by 84% and the plastic zone of the sidewalls can be reduced by 42%.This study provides new attempts to study the rheology of loose coal roadway and could have significance in the future.