| With the increase in coal resource mining depth,the deformation&damage problems and support difficulties caused by rock rheology of roadway surrounding rock become more and more prominent.It has become the primary mission to solve the long-term stability&safety of roadway and to ensure the safe&efficient exploitation of coal mine.The failure of roadway surrounding rock is essentially caused by the formation and expansion of plastic zone,so it is very important to determine the plastic zone reasonably and accurately.However,due to the uncertainty of in-situ stress,the complexity of surrounding rock media and the limitation of related theory,etc,now there is no good solution to the analysis of surrounding rock plastcic zone in roadway under non-uniform stress field.Meanwhile,as an important property of geotechnical material,rheology is seldom considered in relevant theoretical studies.So it is of great theoretical and engineering significance to study the surrounding rock plastic zone in roadway under non-uniform stress field by considering the rheological characteristic of rock mass.Therefore,the-500m level southern trackage roadway at 81 mining area in Xutuan Coal Mine in Northern Anhui Province is taken as the engineering background in this doctoral dissertation.And by means of the comprehensive research methods of theoretical analysis,numerical simulation,rock mechanics test and field engineering test,the analytical problem of surrounding rock plastic zone in roadway under non-uniform stress field is systematically studied,the influence of rock mass's rheological property on plastic zone of surrounding rock is revealed,a new model of surrounding rock deformation zoning considering rock rheology is established,the advantages and disadvantages of existing approximate analytical methods are analyzed,and a relatively reasonable and accurate approximate solution for the plastic zone in roadway surrounding rock under non-uniform stress field is derived.Finally,the following research results are obtained.(1)The ultimate compressive strength,long-term strength and residual strength of target roadway rock are measured.And based on this,the internal friction angle&cohesion of rock as well as their variation regularities are determined respectively.It is revealed that the strain softening of surrounding rock in the target roadway is obvious and the strain softening effect of rock mass is mainly caused by the reduction of cohesion.The results above can provide a parameter support for the follow-up study.(2)Stabilized roadway surrounding rock is taken as the research object in this paper,and rock long-term strength under a certain confining pressure is considered as the peak stress of stabilized roadway surrounding rock.Then,based on the fact that the peak stress of stable roadway surrounding rock is no higher than the long-term strength under a certain confining pressure and the theory of creep termination trajectory,a new 3-stage strain softening model considering rock rheology is established,and the stress,deformation,displacement and radius of each deformation zone in roadway surrounding rock under uniform stress field are derived on the basis of the new model.Finally,the influence of rock rheology and other factors on the deformation and stress distribution of roadway surrounding rock is discussed by using an example,and the results show that ignoring the influence of surrounding rock's rheological behavior is actually overrating the feature of surrounding rock,so the plastic zone obtained is excessively small,which is bad for long-term stability evaluation and supporting quantitative design of surrounding rock.(3)Under the condition of the ideal elastic-plastic model,the preliminarily evaluation about some frequently-used approximate analytical methods for the surrounding rock plastic zone in roadway under non-uniform stress field is carried out.Therein,the complex variable function method and the small parameter method are not only complicated in solving process,but also have some defects:the incompletely reasonable assumption that surrounding rock plastic zone is a connected domain covering the roadway boundary is generally taken as the precondition of the complex variable function method,while the reasonable range of lateral pressure coefficient in the small parameter method is very limited,which seriously restricts its application in engineering practice.Therefore,the solving process,error analysis and other contents of the stress constructing method and the approximate implicit method are mainly discussed,and the rationality,accuracy and applicability of each algorithm are compared and analyzed based on the numerical simulation results.The results show that there is no doubt theoretical defect in the stress constructing method,but its accuracy is relatively high in a neighborhood of lateral pressure coefficient 1,while outside the neighborhood,only the plastic zone radius on horizontal(or vertical)axis can be ensured,radii on the rest locations are not accurate,or even wildly inaccurate.There are also theoretical defects in the approximate implicit method,and the size of plastic zone obtained by this method has a large theoretical error,however,it can preferably reflect the evolution law for general shape of surrounding rock plastic zone.In view of above,the introducing existing estimation method for surrounding rock plastic zone radius on horizontal axis and the base of the formulas of plastic zone under axisymmetric stress field as well as the constant assumption of the total loads,the approximate analytical solution of the surrounding rock plastic zone of non-uniform stress field on vertical(or horizontal)axis is derived.Then combining with plastic zone shape determined by approximate implicit method,the plastic zone of surrounding rock under unequal compression in two directions is relatively accurately determined.The proposed method can not only eliminate the theoretical error of the approximate implicit method for the lateral pressure coefficient 1 but also ensure the accuracy of plastic zone radius in surrounding rock on the horizontal(or vertical)axis.So it can be treated as the revision and promotion of the original estimation method,and also the modification of the approximate implicit method.Finally,through the comparative calculation of an engineering case,it is found that:if lateral pressure coefficient is 1,plastic zone of surrounding rock behaves as a,round,shape,and the approximate implicit method has a large theoretical error,while the the results gained by the modified method,stress constructing method and complex variable function method are the same,and which are very consistent with those of the finite element method.And if lateral pressure coefficient is in a neighborhood of 1,the plastic zone behaves as an "ellipse",and the results based on stress constructing method relatively accurate,which are in good agreement with the finite element results.Else if lateral pressure coefficient is beyond the neighborhood above,the comer of plastic zone in roadway surrounding rock develops rapidly and it forms a "butterfly" shape finally.At this time,the shape deviations of plastic zone got by stress constructing method and complex variable function method are very large compared to numerical simulation method,so it is appropriate to use the modified algorithm to approximate the solution.(4)The viewpoint that the shape of surrounding rock plastic zone has little relation with the choice of material model is introduced and verified.And on basis of the formulas of plastic zone under axisymmetric stress field,the constant assumption of the total loads,as well as the distributing shape of plastic zone determined by ideal elastic-plastic model,the approximate analytic solution of surrounding rock plastic zone in deep-buried soft rock roadway under non-uniform stress field is derived according to the similarity principle by considering rheology,post-peak strain softening and expansion of rock mass.Finally,based on the application of the above theory to the analysis of an engineering example,the differences between considering rock mass's rheology and not considering it are compared.The results show that plastic zone distributition is relatively reasonable if rock mass's rheology is in consideration;otherwise the plastic zone will distribute in a very small range of surrounding rock in a form of sliding wedge,which means surrounding rock almost only produces elastic deformation under this circumstance and it is very different from practical engineering.So it can be seen that rheology has great impact on the distribution of plastic zone.It is better to take rock mass's rheology into account in theoretical research and engineering practice,so that the long-term stability and safety of roadway surrounding rock can be elsured.(5)Taking the-500m level southern trackage roadway at 8s mining area in Xutuan Coal Mine in Northern Anhui Province as an example,the method and concrete process to solve surrounding rock plastic zone in deep-buried soft rock roadway under non-uniform stress field are given in detail in this paper.And based on the comparison of the theorical results above,field measured results of broken rock zone and numerical simulation results,the scientific nature and reliability of the theory and method in this paper are demonstrated. |
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