| Changning tunnel of Yunnan Changbao expressway is located in Changning County,Baoshan City,Yunnan Province.It is located in Hengduan Mountain Area in western Yunnan.It is a typical tunnel project with severe influence of tectonic stress,joint fissure development and weak and broken surrounding rock.The large deformation and failure of Changning tunnel cannot be controlled by using the traditional support scheme.In the tunnel,the initially supported steel arch frame was distorted and deformed,and the arch part fell seriously.The maximum deformation was more than 2125mm,the arch replacement rate was more than 33%,and the annual footage was less than 100m.Therefore,Changning tunnel has become a difficult project open to traffic,which seriously restricted the safe construction of the tunnel and greatly increased the construction cost of the project.To explore the applicability of NPR anchor cable high stress compensation support technology in highway tunnels in southwest mountainous areas,relying on Changning tunnel,this thesis analyzed the characteristics of surrounding rock and in-situ stress of the tunnel,and analyzed the deformation and failure characteristics of surrounding rock of the tunnel through numerical simulation and similar physical model experiment.The mechanical model of tunnel surrounding rock structural layer was established,the main factors affecting the stability of surrounding rock were clarified,and the large deformation mechanism of tunnel surrounding rock was revealed.Based on the principle of excavation compensation support,a tunnel excavation compensation support scheme system with NPR anchor cable as the core was proposed.Using the method of physical model experiment,the mechanical evolution law of surrounding rock deformation under NPR anchor cable excavation compensation support was analyzed,and the support effect was verified.Five NPR anchor cable support schemes were optimized and tested in the field,and the effect and reliability of the support scheme were verified by remote monitoring.The research conclusions are as follows:（1）In this thesis,the geological structure and surrounding rock structure characteristics of Changning tunnel were analyzed in detail.Through the numerical simulation inversion method,the distribution characteristics of tunnel in-situ stress were obtained.The calculation results were compared with the field in-situ stress test results to verify the accuracy of the inversion results,and it was clear that the tunnel stress field is dominated by horizontal tectonic stress.The mineral composition and micro characteristics of tunnel surrounding rock were analyzed,and the relationship between rock mass strength and jest dip was analyzed through laboratory rock mechanics experiments.Based on the theory of soft rock engineering mechanics,it was clear that the soft rock type of Changning tunnel was high stress jointed expansive soft rock（HJS type）.（2）The deformation and failure characteristics of surrounding rock under the original support design of tunnel were summarized.The similar physical model experiment of the tunnel under the original support condition was carried out,and the variation laws of displacement field,temperature field and stress field and the deformation mechanical characteristics of the support structure were analyzed during the excavation and overload of the tunnel surrounding rock.Through the numerical simulation of discrete fracture joints,the deformation mechanical characteristics of surrounding rock under different influencing factors were analyzed.Based on the above research,it was determined that the soft rock deformation mechanism of Changning tunnel was the large deformation of rock compression bending and interlayer shear slip caused by rock structure and tectonic stress.According to the theory of soft rock engineering mechanics,it was clear that the large deformation mechanism of the tunnel is a composite deformation mechanism of"bedding trend and physicochemical expansion type under the action of self weight stress and tectonic stress"(ⅠABⅡABⅢCA).The mechanical model of tunnel surrounding rock structural layer was established,the main factors affecting the stability of surrounding rock were clarified,and the relationship between the limit displacement of structural layer and rock inclination,lateral pressure coefficient and rock mass strength was deduced.Based on the transformation technology of composite deformation mechanism,the tunnel excavation compensation and support measures with NPR anchor cable as the core were proposed.（3）Based on the principle of excavation compensation support,the determination method of high prestressed NPR anchor cable support strength is proposed,and the excavation compensation support scheme of NPR anchor cable with large deformation in soft rock of Changning tunnel is formed.The physical model experiment of NPR anchor cable excavation compensation support was carried out,the mechanical evolution laws of displacement field,temperature field and stress field of surrounding rock under NPR anchor cable support,as well as the variation process of NPR Anchor Cable axial force and temperature were analyzed,and the experimental results of original support and NPR anchor cable support were compared.The experimental results show that NPR anchor cable can improve the stress state of surrounding rock,homogenize the stress distribution of surrounding rock and reduce the stress concentration under the stress levels of different buried depths,and the support effect was good.（4）Based on the optimization method of gradual weakening of support strength,five support schemes were applied in Changning tunnel.Through the monitoring of surrounding rock convergence deformation,initial support and secondary lining pressure,arch stress,deep displacement,and NPR anchor Cable axial force,the support effects of these five NPR anchor cable support schemes were analyzed.The monitoring results showed that the five schemes could control the deformation of surrounding rock to varying degrees.The deformation tended to converge within 25 days,the maximum deformation was less than 135mm,and other monitoring indicators tended to be stable in about 50 days.Through comparative analysis,the safety factor of the initially supported structure was more than 1.4.The load sharing ratio of secondary lining was between 8.1%-14.1%,which was far less than the value recommended in the code.The reserved deformation of tunnel surrounding rock and the remaining reserved deformation basically showed a positive correlation trend,and the minimum reserved deformation of 350mm was determined.The research results can provide a reference for the prevention and control of large deformation in similar projects. |
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