| A layered soft-rock tunnel of the Chengdu-Lanzhou Railway under construction is a deep-buried,super-large section and super-long tunnel.It is also a typical high geostress soft-rock tunnel,which produces obvious large deformation during the tunnel construction stage.At the same time,the groundwater in the tunnel is exposed obviously and the flow rate is large.Due to the large deformation and the massive development of groundwater,the construction of the tunnel is slow,the construction period is prolonged,and the construction safety of the tunnel is seriously threatened.Therefore,in order to deeply analyze the large deformation mechanism of the soft-rock tunnel,this paper starts with the geological environment conditions of the tunnel area.The engineering characteristics of soft-rock and the development characteristics of large deformation in tunnel area are deeply analyzed by means of indoor and outdoor rock mechanics tests and deformation monitoring results analysis.On this basis,the influence of geostress and groundwater on the deformation of the soft-rock tunnel is analyzed in detail by means of qualitative analysis and numerical calculation,and the large deformation mechanism and mechanical model of the soft-rock tunnel are studied.The main research results obtained are as follows:1、The tunnel area is dominated by the mid-alpine landforms,and the sandstone,slate and phyllite of the Triassic Zagunao strata,Zhuwo strata and Xinduqiao strata are developed.The tectonic activity is strong,and multiple secondary folds and five groups of dominant joints are developed.The main type of groundwater is bedrock fissure water;the maximum principal stress direction of the tunnel area is NW direction,intersecting with the tunnel axis direction at a small angle,and the maximum principal stress of the tunnel buried depth is about 11~25MPa.2、Thin slice identification and mineral composition analysis revealed that the soft-rock in the tunnel area is mainly carbonaceous phyllite,which contains mineral components such as mica,illite,chlorite and quartz.The mineral illite content which may cause swelling deformation is 5.85%~34.37%.The results of shear test show that the cohesion c and internal friction angle φ of phyllite in parallel schistosity direction are 0.997 MPa and 44.48°,and the cohesion c and internal friction angle φ in vertical schistosity direction are 4.504 MPa and 55.37°.3、The uniaxial and triaxial test results of soft-rock under different water conditions reveal that the softening coefficient of phyllite in the tunnel area is 0.58,and the compressive strength and shear strength of soft-rock decrease overall with the increase of saturated time;and the influence of water on the internal friction angle of soft-rock is obvious.The results of surrounding rock classification indicate that the tunnel mainly develops the surrounding rock of grade IV and V,and is mainly grade V,with only a small amount of grade III surrounding rock developing.4、The deformation of the surrounding rock of the tunnel has obvious spatial heterogeneity.The deformation of a large number of values is mainly affected by the content of soft-rock,groundwater,geological structure,rock mass structure and stress state.The analysis of the large deformation characteristics of the tunnel shows the soft-rock and the broken rock mass structure are the basis of the large deformation of the tunnel,and the groundwater is the main factor affecting the large deformation,which has a significant impact on the large deformation of the tunnel D5K222+360~570 section.5、The groundwater in the tunnel area mainly comes from the seepage recharge of atmospheric rainfall and Daxing Valley.The softening and seepage of groundwater affects the large deformation of surrounding rock.The geostress mainly passes the tangential stress concentration,which makes the soft-rock enter the plastic state and produce plasticitc deformation leads to the creation of large deformations.The numerical analysis shows that the geostress and groundwater have significant influence on the surrounding rock deformation.With the increase of geostress and groundwater level,the surrounding rock stress of each part of the tunnel increases,and the deformation of the surrounding rock and the volume of the plastic zone increase.The stress concentration is higher,the influence of geostress on the deformation of the tunnel vault and the arch bottom is more prominent,and the influence of groundwater on the deformation of the surrounding rock of the vault is more significant;6、The large deformation of the tunnel is generated by the coupling of various influencing factors,but the groundwater is the main controlling factor for the large deformation of the soft-rock tunnel.The softening mechanism of the groundwater is the main deformation mechanism caused by the large deformation of the tunnel.The mechanical mode of the large deformation of the tunnel is mainly the plastic flow of soft-rock,accompanied by the plastic shear slip of soft-rock and the shear slip of the plastic wedge.The numerical calculation results show that the surrounding rock after tunnel excavation has significant plastic extrusion deformation,which verifies the correctness of the qualitative analysis of the mechanical mechanism of large deformation.In view of the main causes of the large deformation of the tunnel,the corresponding support,construction suggestions and the optimization scheme of support measures are put forward,and combined with the engineering practice,the effectiveness and practicability of the support optimization scheme are demonstrated. |
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