Research On Large Deformation Mechanism And Control Measures Of Deep Buried Soft Rock Tunnel

The Central Yunnan Water Diversion Project is a major measure of the national water resources strategy,but the southwest region has the characteristics of complex geological conditions,strong tectonic action and high seismic intensity.During the construction process,tunnels often face disasters such as large burial depth,high stress,rock mass fragmentation,and insufficient supporting measures,resulting in large deformation of surrounding rock and damage to supporting structures.Therefore,it is of great significance to carry out the research on the large-scale deformation mechanism and control measures of deep buried soft rock tunnels for actual engineering construction.Based on the chlorite schist section of the Xianglushan tunnel,this paper analyses the mechanism of large deformation in the tunnel using theoretical analysis,indoor tests and field ground stress test data.In addition,through on-site monitoring data,numerical simulation and other means,large deformation control measures are proposed in terms of tunnel excavation method and support structure,the main research contents and conclusions are as follows:(1)Through the collection and collation of relevant literature on deeply buried soft rock tunnels,the current status of research on the mechanisms of large deformation and control measures in tunnels is summarised and analysed,and the current status of numerical modelling of large deformation in soft rock tunnels is described.Combining the ground investigation data,basic indoor tests and the ground stress characteristics measured by hydraulic fracturing,it is concluded that the tunnel crosses complex geological conditions,poor lithological conditions and high initial ground stress due to excessive burial depth.(2)Combined with the large deformation characteristics of the tunnel,the tunnel is characterised by various forms of deformation and damage,fast initial deformation rate,long deformation duration,large deformation volume and large damage range of the surrounding rock.In addition,the influence of ground stress,rock level and support strength on the deformation of the surrounding rock was investigated by the finite difference software FLAC3 D.The results show that as the ground stress increases,the deformation of the surrounding rock continues to increase and the extent of the plastic zone increases;when the surrounding rock grade is less than Class IV,the deformation of the surrounding rock increases to a greater extent and the extent of the plastic zone increases significantly;for deeply buried soft rock tunnels,only increasing the strength of the initial spray concrete and steel arch does not have a significant effect on the control of the surrounding rock and the extent of the plastic zone does not change significantly.Combining the geological profile,deformation characteristics and numerical results,it is concluded that the plastic shear slip deformation of soft rock under high stress,the softening effect of weak inclusions and the bending deformation of rock layers are the main deformation mechanisms of deeply buried soft rock tunnels.(3)A more suitable intrinsic model for the simulation of deeply buried soft rock tunnels is analysed with the help of numerical results and field monitoring data.On this basis,the excavation method is discussed in terms of large deformation control measures,and the construction parameters are optimised for the excavation method with the best large deformation control effect.The results show that: the deeply buried soft rock tunnel surrounding rock has obvious strain softening characteristics,the H-B yield criterion calculation results are closer to the actual situation,and the error between the results simulated by the M-C model and the actual accumulated deformation values is larger and does not conform to the deformation law of soft surrounding rock.In addition,the deformation curves of typical sections,deformation rates,cumulative deformation of the surrounding rock and the extent of the plastic zone of the surrounding rock were compared with the deformation control effects of pre-excavated core soil excavation,three-stage excavation and underpass excavation,and it was determined that the use of underpass excavation in deeply buried soft rock tunnels can effectively reduce the deformation of the surrounding rock.The best control effect is achieved when the diameter of the lower guide tunnel excavation is 4m,the progress of excavation is 1m and the length of the lower guide tunnel excavation is 3m.(4)Combined with the support parameters of the original tunnel design,the design was optimised in terms of slurry reinforcement range,length of overrun pipe shed,thickness of initial support,spacing of steel arch and length of anchor rod.The results show that when the slurry reinforcement range is 6m,the 10 m Φ108 pipe shed is used for advance support,the initial support is made of Φ8 reinforcement mesh and 32 cm thick C20 polystyrene coarse fibre concrete,the steel arch is made of full section I22 a with a spacing of 1.0m,and the anchor rod length is Φ25 hollow grouting anchor rod with a length of 6m.The control effect of large deformation of the tunnel is relatively good,which has guiding significance for the actual construction.