| With the implementation of the new Western Development Strategy of China and the Belt and Road strategy,many of tunnels in China are built in the soft rock strata with strong tectonic movements.The soft rock tunnels under high geostress have the characteristics of large deformation,long deformation duration,strong rheological response and great difficulty for supporting.It is a major problem faced by tunnel construction in the world,and it also brings unprecedented opportunities and challenges to the technical development of tunnels and underground engineering in China.Aiming at the difficult problems in the construction of high geostress soft rock tunnel,this paper analyzes the progressive failure mechanism of strain softening viscoelastic-plastic deformation of typical soft rock tunnel of Chengdu-Lanzhou Railway.Based on the classification of large deformation,the deformation control criteria,the large deformation control system of different grades and the reasonable construction timing of permanent lining are systematically studied.The main results are as follows:(1)Chengdu-Lanzhou Railway is located in a region with well-developed tectonics and complex in-situ stress environment,which has a typical stress field of?H??h??v,with lateral pressure coefficients generally greater than 1;saturated uniaxial compressive strength of phyllite is less than 30 MPa,and the surrounding rock is severely crushed and unstable.Under the action of high tectonic stress,it shows significant characteristics of squeezing large deformation Influenced by stratum lithology,in-situ stress,geological structure,rock mass structural characteristics and groundwater,the failure of surrounding rock shows remarkable progressive characteristics.After excavation,a very large loosening circle forms instantaneously by enormous stress gradient,which makes the surrounding rock in the state of post-peak failure,and the surrounding rock undergoes large initial deformation rate under dilatancy and crushing deformation.With the adjustment of stress,the loosening circle develops deeper,the strain softening of the rock mass is further aggravated,the residual strength is further reduced,and finally a very large loosening circle is formed,and the dilatancy and crushing deformation of the rock mass are controlled.The improvement of residual strength is crucial for deformation control of surrounding rock.(2)Using the Strength-Stress ratio and relative deformation under a certain supporting resistance as the classification index of large deformation,the large deformation can be divided into four grades:slight,medium,serious and extremely serious.The reserved deformation of single-track and double-track railway tunnels is 10-20 cm,25-35 cm,35-45 cm,45-60 cm,20-30 cm,25-40 cm,40-60 cm and 60-80 cm respectively under the condition of slight,medium,serious and extremely serious large deformation.Based on the reserved deformation,according to the proportion of deformation in each phase of the construction process,the deformation spatial effect during the construction process is analyzed,and the deformation control benchmark of soft rock tunnel with high geostress based on the construction process is proposed.(3)The bolt and construction method are the key measures to control the large deformation of soft rock tunnel.The bolt should be strengthened in the control of large deformation tunnel in soft rock.We need to adopt control technology of"selecting bolt type reasonably,configuring special mechanized construction,optimizing bolt parameters,combining long with short rockbolts,fast anchoring and early bearing".The excavation method of large section tunnel should based on the principle of"reducing the excavation area,minimizing the disturbance of surrounding rock,connecting the steel girder smoothly and tightly,enclosing the initial support without delay,breaking through the space restriction of long anchor construction in multi-step excavation,and making the anchor bolt take effect in time" to control the surrounding rock pressure and deformation effectively.At the same time,it can also improve the speed and the efficiency of construction.The supporting strength should be strengthened gradually based on the grade of large deformation,together with the long rockbolt,the excavation control method of large section,and the technological optimization such as enlarging the advanced supporting range and strengthening the longitudinal connection of steel frame.Finally,the large deformation control technology of high stress soft rock tunnel is put forward,which includes strengthening bolt,large section excavation,optimizing technology,dynamic adjustment and grading control.(4)The surrounding rock of soft rock tunnel has remarkable rheological characteristics.Affected by the large deformation magnitude and long defrmation duration of surrounding rock,the permanent lining of soft rock tunnel should be constructed after the deformation of surrounding rock is basically stable,so as to ensure the long-term stability of permanent lining.Taking into account the influence of rheological deformation of surrounding rock over a three-year construction period,permanent lining is applied when the deformation of surrounding rock reaches 95%of the ultimate displacement during the construction period,which is equivalent to the subsidence rate of arch crown is less than 0.1-0.2 mm/d and the convergence rate of side wall is less than 0.5 mm/d in minor and medium deformation sections,or the subsidence rate of arch crown is less than 0.4 mm/d and the side wall convergence rate is less than 0.6mm/d in severe and very serious deformation sections. |
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