Optimization Of Ultra-shallow Segment Of The Large-bias Tunnel’s Crossing Scheme

With the rapid development of expressway network and high-grade highways inthe western region of China, more and more highway mountain tunnels are constructed.However, in the construction process, the highway mountain tunnels meet a lot ofthorny issues.How to pass through the ultra-shallow segment of large bias tunnel safelyand economically is one of the most difficult problem to overcome.If we use the moreeconomical construction scheme, the safety of the tunnel will naturally be quitelow;And adopt a more conservative construction scheme, improve the safety of thetunnel, the cost will rise, so how to reconcile the two factors of the mountain tunnels hasbeen the most important problems that should be solved at present stage. Based on theproject of a highway tunnel in Shanxi Province,this paper studied and discussed thedisaster phenomenon of this type of tunnel construction prone to, the rock pressuredistribution through the different programs across and the characteristics of constructionmechanics,with the method of combining field testing,theoretical analysis andnumerical simulation,and finally researched the space effect of tunnel excavationsurface in the best passing through scheme. Main research work and achievements areas follows:（1）Combined with the construction practice and the achievements from themonitoring measurement of the project of a highway tunnel in Shanxi Province,thepaper enumerats the disaster phenomenon of the tunnel during the period ofconstruction,then analyses its causes.（2）The finite element software, ANSYS,is applied to establish the two-dimensionalnumerical model for calculating and analysing the surrounding rock situation and themechanical characteristics of the super bury section of eccentrically Compressedtunnels in the three different passing through schemes--the unbiased design,the backpressure method design and the large eccentric compression design.（3）This paper calculates the safety factor and the engineering quantity per extendedmetre of each scheme by using software,defines the economic safety factor coefficientratio,obtained the economic-safety ratio in the three different passing throughschemes,and combined with the surrounding rock situation draws the conclusion thatthe passing through scheme in the back pressure method design is the best one. （4）According to the best crossing scheme, a three-dimensional elasto-plasticnumerical model is presented for making construction stages simulation, researching thespace effect of the type of tunnel excavation,emphatically analysing the spatialrestraint effects of the tunnel excavation surface in the process of construction.Thispaper also analyzes and studies the stress characteristics of the tunnel,the deformationlaw of retaining structure and surrounding rock,the rational parameter of tunnelexcavating and plastic zone distribution.