Research On Influence Of Bolt Supporting Timing And Preloading Force On Surrounding Rock Stability Based On Displacement Gradient

In this paper,a FLAC3D numerical analysis model is established to obtain the fitting curves,which describe the correlation ofdisplacement values of the surrounding rock data points in various directions with its distance from the tunnel surface.The fitting results show that the correlation between the two variables is highly consistent with the correlation described by the exponential function y=A₁*exp?x/t₁?+y₀.The displacement gradient functions of different supporting time and the different pre-stress values in the surrounding rock are obtained by taking the derivative of the fitted curves.The displacement gradient function can reflect the degree of fragmentation and its distribution law in different positions of rock mass after excavation and unloading or supporting.The greater the absolute value of the function,the more obvious the fragmentation and expansion of rock mass.For the deep weak coal rock,the absolute value of the displacement gradient is greater than10,that is,the rock mass has been broken and expansive,and the displacement gradient is greater than 30,that is,the rock mass has been the significant fracture.Therefore,through the displacement gradient function,the range of coal's loosening circle and the range of coal's swelling circle under different supporting time and different bolt prestressing are determined.Then,the most reasonable supporting time and supporting mode are found,and the supporting cost is saved as much as possible on the premise of ensuring the stability of surrounding rock without destabilization.The results show that:a.In this model,the appropriate timing for applying the anchor should be selected in step 600 of the calculation step.Compared with the support immediately after excavation,the elastic strain energy accumulated in the surrounding rock can be released greatly,and the stress level of the rock material has been significantly.Therefore,the application of smaller support force can meet the support requirements.b.The application of prestress not only causes high cost,but also the anchor rod is subjected to high stress of rock mass,and the effects of this kind of supporting method is negative neither in the final total displacement nor in the degree of fragmentation of the entire surrounding rock.Therefore,in this model,the pre-stressing force should not be applied.