| Coal seams have undergone complex geological tectonics.Extremely soft and fractured coal seams are widely distributed in South China,southern margin of North China and Northwest China.Under the mining condition in these coal seams,the surrounding coals of roadways is featured characterised by poor self-stability,fast overall weighting and long-time deformation.It is easy to leak and fall in large area during the excavation of extremely soft coal roadway,and it is difficult to implement bolt-mesh support.During the roadway service period,the surrounding coals are prone to severe and large deformation,which requires repeated renovation to meet the requirements of roadway use.The problem of surrounding rock control in extremely soft coal roadway has become an important challenge to restrict safe and efficient mining in many mining areas.However,the existing researches on the basic mechanical properties,long-term creep mechanism,the interaction between surrounding rock deformation and bolt-mesh supports and its response law are still limited.The design of bolting with mesh system in the extremely soft coal roadway is still in the initial stage relying on engineering analogy and experience.Therefore,this paper takes the typical extremely soft coal seam roadway in Zhengzhou mining area as the project object,and studies the mechanical properties and the bolting mechanism of extremely soft coals surrounding roadway.(1)On the basis of testing the basic physical and mechanical characteristics of raw coal,such as strength,deformation and microstructure characteristics,25 groups of raw material ratios were designed in the orthogonal experiment and were carried out on the basis of optimizing and improving the preparation method and technology of similar coal samples.During which,different particle sizes of pulverized coal,gypsum and petrolatum,calcium carbonate,in-situ stress level were selected as aggregate,composite cementing agent,additive and forming pressure,respectively.And range analysis revealed the key influencing factors of each mechanical index.The gypsum/petrolatum ratio controls the density,compressive strength,elastic modulus,cohesion and deformation characteristic.The coarse/fine coal powder(1–2 and 0–0.5 mm)controls the internal friction angle and is the secondary controlling factor for compressive strength and elastic modulus.When fine coal powder/aggregate is about 70%,the strength and elasticity of samples are largest.With the increase of gypsum/ petrolatum ratio from 5:5,6:4,7:3,8:2 to 9:1,the With the increase of gypsum/ petrolatum ratio from 5:5,6:4,7:3,8:2 to 9:1,the deformation characteristics of coal sample materials change from ductility and plasticity to brittleness,and the failure modes include compression failure,wedge splitting,plastic shearing,brittle shearing and brittle splitting.The micro-structure shows that the stability and homogeneity of the samples are better when the ratio of coarse to fine coal particles is about 3:7.Based on the test results,a multivariate linear model between the physical and mechanical indexes of extremely soft coal and the main influencing factors was established.Finally,a extremely soft coal sample which is very similar to the raw coal in physical and mechanical parameters,deformation characteristics and microstructures has been developed.(2)The uniaxial and triaxial long-term creep tests of extremely soft coal samples were carried out by using a multi-channel rock long-term creep test system.The results show that:(i)In the uniaxial long-term creep tests,transient creep,steady-state creep and accelerated creep stages occurred,which lasted 232 hours,and the strain was as high as 3.78%(creep accounted for 91.27% of the total deformation),especially for steady-state creep stage,the creep rate was constant and was as high as 8-10×10-5/h.Besides,the maximum accelerated creep rate was as high as 0.043/h.The deformation time effect and deformation failure rate of extremely soft coal are concluded to be much higher than that of hard rock or soft rock.(ii)In the long-term creep test of very soft coal under single-stage loading with the same axial pressure and different confining pressures,with the increase of confining pressure from 0 MPa to 0.6 MPa(axial stress was 0.96 MPa),the creep strain of extremely soft coal decreased greatly,the ratio of creep strain to instantaneous strain decreased obviously,the steady-state creep rate decreased in an order of magnitude,and the creep duration increased obviously.Especially,an accelerated creep stage is difficult for extremely soft coal to occur at higher confining pressures(currently lasting 2200 hours).Creep properties of extremely soft coal were most sensitive to confining pressure changes in the range of 0-0.2 MPa.(iii)In the long-term creep test of extremely soft coal under triaxial and multi-stage loadings,significant transient creep and steady-state creep stages occur under each level of deviating stress.With the gradual increase of deviating stress,the creep strain increased,especially the ratio of creep to instantaneous strain increases,and the creep characteristics became more significant.More importantly,the steady-state creep rate of extremely soft coal under different confining pressures and various axial pressures was constant and larger than zero(generally over 10-6/h-10-5/h),which would have an important impact on the long-term stability of the project.Under the same confining pressure,the steady-state creep rate increases in the form of power function with the increase of deviating stress.(iv)Even under the same deviating stress,with the confining pressure increasing from 0.6 to 1.6 MPa,the creep and the ratio of creep to instantaneous strain both showed a significant decreasing trend;Moreover,at the same deviating stress,the confining pressure also has a significant restraining effect on the instantaneous deformation of extremely soft coal;In addition,under the same deviating stress,the steady creep rate of coal decreased exponentially with the increase of confining pressure.Finally,an empirical model of steady-state creep rate related to deviating stress and confining pressure was established.(3)According to the long-term creep law of extremely soft coal,the classical Burgers creep model was improved,and the concept of accelerated creep start-up time was put forward,and a six-element nonlinear viscoelastic creep model,which could describe the whole process of transient,steady-state and accelerated creep of extremely soft coal more accurately,is established by connecting the non-linear viscoplastic elements including accelerated creep start-up time element with the improved Burgers model.The corresponding three-dimensional creep equation was derived.Then,based on Levenberg-Marquardt optimization algorithm,the creep test curves are fitted nonlinearly,and the creep parameters of extremely soft coal were identified.The creep test curves were in good agreement with the theoretical curves of the creep model.(4)In order to study the bolting effect of pre-tensioned bolt-mesh system on the extremely soft coal surrounding roadway,and to reveal the interaction and evolution law between deformation of extremely soft coal and loading state of bolt-mesh system.The unit anchorage surrounding rock system in roadway surrounding rock was selected as the prototype of physical model,and a large scale(1:2)true three-dimensional anchorage model test system was designed independently.Under the condition of initial bolt mesh support scheme and the improved scheme,the evolution law of bolt anchoring force and bolted coal deformation in roadway under the influence of excavation disturbance and high stress were simulated and analyzed.The test results show that:(i)In model 1 supported by low pre-tensioned bolt and steel-plastic mesh,both the axial force and surface supporting force of bolt decreased to a certain extent(the maximum reduction of the axial force is about 26%)when subjected to disturbance stress induced by roadway excavation.It is suggested that the bolt should be tightened several times behind the roadway driving face.When subjected to high stress,severe extrusion and outburst surrounding coal deformation occurred rapidly between the bolts.A "hill" type of protruding deformation area(extremely uneven and large deformation area accounted for about 50% of the total area of surrounding coals between bolts)formed,during which,the axial force of bolt increased rapidly at first and then decreased slowly.Based on the test results,the bolting effects distribution of pre-tensioned bolt-mesh support were analyzed,and the instability mode of bolted surrounding coal system was revealed.(ii)In model 2,the pre-tightened force bolt and the stiffness of mesh are increased by 50% and 135%,respectively,the radial stress and residual strength of shallow surrounding coal were obviously increased,which enhanced the bearing capacity of the bolted surrounding coal system.In addition,when the extremely soft coal was supported by bolts with higher pre-tightened force and higher stiffness combined metal mesh,the surface supporting anchor force was larger and could be spread wider and farther into the shallow coal,which significantly improved the stability of the bolted surrounding coal system.The concrete manifestations are as follows: the total deformation of bolted surrounding coal decreased by 31.9%,and the integrity of surrounding coal was significantly improved;the area of extremely uneven and large deformation decreased by 75%;the reduction degree of anchoring force decreased obviously under the influence of excavation disturbance;under the influence of sustained high stress,the axial force and surface supporting force of anchor increased steadily;in model 2,the radial stress of roadway at different depth of surrounding coal was greater than that in model 1,especially the shallow coal.Reasonable pre-tightened force of bolt and metal mesh are very important to enhance the control effect in extremely soft coal roadway.(iii)Higher pre-tightened force of bolt was beneficial to the rapid increase of resistance of bolt,and the working load of surface supporting force was larger;higher stiffness of metal mesh was more conducive to the exertion of bearing capacity of bolt.(4)Under the condition of bolt-mesh support in extremely soft coal seam roadway,there are obvious weak supporting parts between bolts.The application of compensating bolt(or anchor cable)could promote the formation of interaction zone on the surface of shallow coal and anchorage zone,significantly reduce the degree of uneven deformation of bolted coals,and improve the bearing capacity and structural stability of bolted coals surrounding roadway.However,the effect of compensating reinforcement was quite different with different deformation of surrounding coal and different bearing state of bolt.(5)Based on the research results of routine and creep mechanics properties of surrounding coal and bolting effect of bolt-mesh systems,the design principles of bolt-mesh system in extremely soft coal roadway were explored and discussed.It is pointed out that the key to control the deformation of surrounding coal in extremely soft coal roadway by pre-tensioned,lengthening anchorage bolt and metal mesh system was to:(i)pay attention to the formation of interaction zone in surface and shallow surrounding coals to reduce or eliminate the structural loss of anchorage system,thus,the breakthrough of extrusion deformation or fast creep deformation,which would cause the instability of bolted coal system,was suppressed;(ii)pay attention to the improvement of bearing capacity and stability of anchorage interaction zone;(3)pay attention to reducing the influence of steady-state creep and non-linear accelerated creep of extremely soft coal on long-term stability of surrounding coal in roadway.It was suggested to adopt the support form of "high pre-tension-force bolt + small mesh steel-plastic mesh + high-stiffness welded metal mesh + anchor cable for compensating reinforcement in key parts",in which the compensating anchor cable should be applied to weak stability parts of each support unit,and it was suggested that the compensating cable should be applied after the completion of bolt-mesh support and the surrounding coal at the end of transient creep,on the one hand,the deformation and yielding pressure of surrounding rock could be properly released;On the other hand,it was conducive to formation of the synergistic complementary effect between compensating cable and primary bolts,forming a reliable interaction zone in the surface and shallow surrounding coals which could reduce or eliminate the breakthrough of squeezing deformation of anchorage system;on the another one hand,it can further improve the support stress,reducing the steady-state creep rate of surrounding coal,prolonging the non-linear accelerated start-up time,and significantly improving the long-term stability of anchorage bearing structure.The research results could lay a theoretical foundation for long-term stability analysis and bolting design in extremely soft coal or rock in underground engineering.There are 77 pictures,22 tables and 256 references in this paper. |
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