| With the depletion of shallow coal,it is an inevitable trend for coal mining to develop to the deep.Its deformation destruction characteristics are different from shallow mining,and the deep roadway is seriously deformed,which restricts the safe and efficient mining of deep coal.The deformation of the roadway surrounding rock comes from the shear deformation of the fractured surrounding rock in plastic zone,and the control of deformation of surrounding rock is transformed into how to control the development of the plastic zone and the continuous creep of the fractured surrounding rock.At present,the supporting problem in deep mining has not been solved well,and there is still a lack of effective support technology.Therefore,the basic understanding of the formation mechanism of plastic zone,the effect of bolt on the broken surrounding rock in plastic zone,and the reasonable support method have a role in promoting the safe and efficient mining of the deep coal in China.Based on a roadway in Pingmei No.6 coal mine,this paper systematically studies the mechanical mechanism of the formation of plastic zone,using comprehensive research methods such as theoretical analysis,laboratory test,numerical simulation and field engineering test.The controlling effect of bolt on accelerating creep of rock mass is studied,and the anchoring mechanism of bolt on deformation of post peak fractured rock mass is discussed.This paper points out the stability control principle of deep roadway and the anchoring principle of bolting design based on the distribution law of plastic zone,and puts forward the flexible bolt bolting technology.The research results of this paper are as follows:1.Expounding the mechanical mechanism of formation of the plastic zone(1)Based on the M-C strength failure criterion,the boundary equation of plastic zone is obtained by substituting the kirsch solution into the plastic condition.According to the boundary equation,it is concluded that the lateral pressure coefficient of surrounding rock affects the shape of plastic zone.When the lateral pressure coefficient decreases from 1.0 to0.3,the radius of plastic zone of roof and floor and the radius of plastic zone in ribs show the fluctuation trend.With the increase of the lateral pressure coefficient,the radius of the plastic zone in roof and floor gradually decreases,and the shape of the plastic zone gradually changes from circular to elliptical.When the lateral pressure coefficient continues to decrease,the plastic zone presents a"Butterfly"shape.(2)The principal stress direction of surrounding rock will change under the influence of geological structure or mining.The stress direction will affect the development direction of maximum depth of plastic zone,or say that the stress direction affects the orientation of the butterfly leaf of"Butterfly"plastic zone.Buried depth,internal friction angle and cohesion only affect the development depth of the butterfly plastic zone,and do not change the development direction of the butterfly leaf.(3)When the value of lateral pressure coefficient is equal to 1.0,the direction of maximum principal stress is parallel to the tangent direction of roadway,while that of the minimum principal go through the radial direction of roadway.When the value of lateral pressure coefficient is not equal to 1.0,the direction of maximum principal stress is not parallel to the tangent direction of roadway,while that of the minimum principal doesn’t go through the radial direction of roadway.As a result,the direction of shear failure of surrounding rock changes,and the smaller the lateral pressure coefficient is,the greater the change of the direction of principal stress is,which leads to the peak stress curve gradually changes from a closed circle in the figure to an unclosed near straight line in the figure.2.Researching the controlling effect of bolt on surrounding rock(1)The strain parameter is used to express whether the rock mass enters the acceleratedcreep,and the nonlinear clay pot with strain triggering is introduced to describe whether the rock mass enters the accelerated creep.When the total strain of the model is less than the critical valueε_a of strain triggering,the nonlinear accelerating creep element is similar to the rigid body element and does not play any role,while the total strain of the model is greater than the strain triggering critical valueε_a,the nonlinear accelerating creep element is in an active state.The nonlinear accelerating creep element is connected in series with burger model which can better describe the decay creep stage and stable creep stage,and an improved burger model which can describe the three stages of rock creep is obtained.(2)The mechanical model of anchorage system at the interface between the anchorage foundation and surrounding rock is established.It is concluded that the ultimate shear strength of the anchorage foundation and the interface determines the maximum anchorage force that the bolt can provide.Assuming that the bolt is an ideal elastic material,the interface strength between the anchorage agent and the bolt and the surrounding rock is also strong enough.With the development of the creep,according to the boundary position of the fracture zone and the relative position of the bolt anchoring section,the change of the anchoring force with time is divided into three stages,namely,the anchoring force stabilization stage,the weakening stage and the residual anchoring force stage.A plastic element with damage coefficient of anchoring force is introduced to characterize the gradual damage process of anchoring force after the anchorage foundation stress exceeds the yield limit,and a creep model coupled with surrounding rock when the anchorage foundation is located in the elastic zone and plastic zone respectively is established.The creep control mechanism of bolt support for surrounding rock can be summarized as two aspects:first,to share the load of surrounding rock;second,to increase the stiffness of surrounding rock and enhance the resistance to deformation of surrounding rock.It is necessary to meet the two criteria at the same time in order to give full play to the supporting performance of bolt,lengthen the supporting time of bolt and maintain the stable working state of the roadway.The bolt load cannot be over the breaking load of the rod,and the anchorage foundation is far away from the plastic zone.3.Proposing the anchoring mechanism of post peak fractured surrounding rock(1)A large-scale three-dimensional similar simulation test-bed is developed,which can simulate the real three-dimensional stress,pre-fracture the complete specimen through the pre reaction frame,and install different numbers of bolts.(2)When the bolt is not installed,the specimen shows brittle failure.While the bolt is installed on the specimen,its stress-strain curve shows a better yield platform,shows a larger plastic flow phenomenon,and even strain hardening characteristics,which shows that the anchorage specimen can maintain a higher bearing capacity when deformation occurs.With the increase of the support density,the bearing capacity of the specimen increases,and the stress of the corresponding pull rod also increases,while the support resistance provided by bolt increases firstly and then decreases,but the total support resistance of the specimen increases,which indicates that the support strength is not enough when supporting one bolt,and there is a certain excess of support strength when supporting three bolts.(3)With the increase of the bolt pretension force,the bearing capacity of the specimen increases,the growth rate of the peak strength of the specimen decreases gradually,and the effect of the pretension force on the growth of the peak strength decreases gradually.When the pretension force is small,the support resistance provided by the bolt is small.When the pretension force is increased,the support resistance provided by the bolt increases sharply,and the stress of pull rod increases gradually with the increase of the pretension force.With the increase of bolt density and pretension force,the volume strain of the specimen decreases gradually,the curve shows a concave shape,and the volume strain reduction rate decreases gradually.It shows that the reduction degree of bolt density and pretension force to the volume strain of the specimen is limited.Increasing bolt density blindly cannot reduce the volume strain to the maximum extent,but increase the support cost and support difficulty.(5)When the vertical displacement of the specimen is close to each other,the bolt density isdifferent,and the failure mode of the specimen is different.The specimen will experience the cycle process of local stability-instability-restability under the action of bolt until the whole system appears balance.However,the bolt density is equal to that of the equivalent confining pressure applied to the test piece,and the load to be overcome in case of transverse displacement of the test piece is also large.Therefore,the peak load of the pre cracked test piece is large,and the probability of secondary fracture is greatly increased.The high density of bolt is equivalent to the high equivalent confining pressure applied to the specimen,and the load to be overcome in case of lateral displacement of the specimen is also large.The peak load of the pre-fractured specimen is large,and the probability of the second fractured is greatly increased.Therefore,the fractured specimen with high bolt support density has serious fragmentation and small block size.4.Putting forward control method and technology of surrounding rock stability in deeproadwayThe control of roadway surrounding rock should be changed from deformation control to stability control,that is to say,the roadway should be allowed to have certain deformation so that the bolt can adapt to the deformation characteristics of the surrounding rock and give full play to the material characteristics of the bolt.The reasonable support design should be based on the depth of plastic zone in each position of surrounding rock to coordinate the different support design,ensure that the anchorage foundation in each position is located in the elastic zone,give full play to the material properties of bolts,and avoid excessive support caused by local strong support and support failure caused by local weak support.Therefore,the flexible bolt is used to replace the conventional cable,to overcome the defect of the ordinary cable with small elongation,to anchor the fractured surrounding rock in plastic zone into the deep stable elastic zone,and to provide greater support resistance.5.Field engineering testBased on the butterfly plastic zone theory,a new supporting scheme is designed for the test roadway.The flexible cable is used to replace the ordinary cable,and the anchorage depth is designed according to the shape of plastic zone.The monitoring results of mine pressure in the test tunnel show that,the new support scheme can effectively reduce the deformation of surrounding rock.Although the new support scheme does not fundamentally eliminate the separation of the surrounding rock,the flexible cable can well adapt to the subsidence of the roof without anchorage failure.The deformation of surrounding rock during the monitoring period meets the requirements of the safety production of the mine.Therefore,the use of flexible cable with large extension can meet the requirements of ensuring the safe production of the tunnel and reducing the maintenance cost of the tunnel. |
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