| Under the condition of high geostress,layered soft rock tunnel has the characteristics of large deformation of surrounding rock and long duration.The traditional rigid arch support can not fundamentally solve the problem of large deformation support of surrounding rock.At present,how to deal with the large deformation disaster of layered soft rock tunnel has become a difficult problem for engineers.The retractable steel arch is different from the traditional rigid arch structure.The constant resistance deformation section is designed on the arch body,which allows the retractable steel arch to release part or all of the energy accumulated in surrounding rock after tunnel excavation during the expansion and contraction process.It is a potential method to effectively prevent and control the large deformation of soft rock tunnel in layered soft rock tunnel.In this paper,based on the support concept of"rigid and flexible"in soft rock tunnel,on the basis of fully studying the mechanism of large bending deformation of layered soft rock tunnel,the designed pressure relief device is installed on the ordinary rigid arch frame to form a retractable steel arch,and the arch is applied to the support system of tunnel surrounding rock,and its stress deformation law and strength are analyzed by theoretical calculation and numerical simulation The degree characteristic is studied.Finally,on the basis of the actual project,the supporting effect of rigid arch and retractable movable steel arch in large deformation tunnel with bending in layered soft rock is compared and analyzed by numerical simulation method,the suitability of practical application of retractable movable steel arch is studied,and the appropriate supporting scheme of retractable movable steel arch frame is proposed.The main research results are as follows:?1?Through the statistical analysis of a large number of large bending deformation cases of layered soft rock tunnel at home and abroad,it is shown that the basic factor of large bending deformation of layered soft rock tunnel is high ground stress and weak surrounding rock,and the intersection of the maximum principal stress and surrounding rock layer at a large angle is the main cause of large bending deformation.Based on the plate beam bending theory,a multi-layer rock beam model considering interlayer force is established,which further reveals the failure mechanism of large bending deformation of layered soft rock.?2?Combined with the law of large deformation of surrounding rock in soft rock tunnel and the principle of energy release support,the I-steel movable steel arch with three yielding devices is designed.Through the analysis of energy release support process of retractable movable steel arch frame,it is shown that the determination of constant resistance P0 and constant resistance displacement U3 is the key to the support effect of retractable movable steel arch.Therefore,the mechanical model of two hinged arch is established by load structure method to determine the theoretical calculation method of constant resistance P0,and the theoretical calculation method of constant resistance displacement U3 is determined by considering the interaction between surrounding rock and retractable dynamic arch support.?3?ANSYS software is used to study the stress and deformation characteristics of the designed retractable steel arch frame structure.Through the analysis of the deformation and stress distribution characteristics of the retractable steel arch,it is shown that the shrinkage effect of the retractable steel arch is good under the external load,and the constant resistance displacement released by three pressure yielding devices reaches more than 98%of the design value;the stress difference between the constant resistance plate and other parts of the structure is large,which is the weak part of the retractable steel arch structure.Through the analysis of the main factors affecting the deformation of the retractable steel arch structure,such as bolt torque,vertical load and horizontal load,it is shown that under the action of high bolt torque,the pressure release device will reach a higher stress level when releasing the constant resistance displacement;when the constant resistance displacement is released,the stress of the device increases with the increase of the vertical load;the sensitivity of the side wall pressure device to the horizontal load is higher than that of other parts.when the constant resistance displacement is released,the stress of the side wall pressure device increases with the increase of the horizontal load.?4?Based on the Shiziping tunnel,the numerical simulation of three support modes,no support,rigid arch and retractable movable steel arch,is carried out for the large bending deformation mode I?horizontal rock layer?and mode II?vertical rock layer?summarized in this paper.The deformation law of surrounding rock under three kinds of support methods is analyzed.The results show that the change of support form does not change the overall deformation trend of surrounding rock;in mode I,the key deformation position of surrounding rock is located at the vault and tunnel bottom,and the failure mode of surrounding rock is the bending fracture of arch roof and tunnel bottom rock towards the free face;in mode II,the key deformation position of surrounding rock is left wall and right wall,and the failure mode of surrounding rock is side wall rock It is a bending fracture on the free surface.?5?The analysis of the stress and deformation characteristics of rigid arch shows that the maximum axial stress of rigid arch exceeds the yield limit of structure by215MPa,and the deformation of key deformation parts of surrounding rock exceeds the allowable elastic deformation of structure.Therefore,the rigid arch has not achieved rational support effect in the supporting process.The analysis of the stress and deformation characteristics of the retractable steel arch shows that the constant resistance displacement is released by the yielding device,and the energy accumulated in the surrounding rock is released to the outside world while yielding while resisting,so that the axial stress acting on the retractable movable steel arch is far lower than the structural yield limit of 215MPa,which indicates that the retractable steel arch is in normal working state during the supporting process.?6?Through the comparative analysis of the numerical simulation results of rigid arch and retractable movable arch,it is shown that the deformation of surrounding rock in mode I and mode II is greater than that in the case of rigid arch support,and the axial stress of retractable steel arch is less than that of rigid arch,and the supporting effect of retractable steel arch is better than that of rigid arch.?7?According to the classification standard of large deformation and the mechanism of large bending deformation,the supporting scheme of retractable steel arch frame is designed.The constant resistance and constant resistance displacement of the retractable steel arch frame are determined by the theoretical calculation method of constant resistance and constant resistance displacement proposed in this paper.The setting position of pressure relief device is determined by the main parts of surrounding rock deformation in mode I and mode II.The constant resistance displacement value of each pressure yielding device is determined by comparison The deformation of surrounding rock in each part of sub face is determined. |
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