Analysis On Construction Mechanics Of Urban Large Span And Small Clear Distance In-situ Expansion Project

With the development of national economic growth and the growth of therequirement for the daily traffic volume, four-lane highway tunnel which suited to thefour-lane high road cannot meet the needs of traffic. In some domestic large andmedium-sized cities, congested traffic problem has become more and more severe, sothe discussion of tunnel renovation and expansion has become a new research topicduring the field of geotechnical engineering. Based on the technical material from theChongqing Airport Road Yuzhou Tunnel in-situ expansion project which has alreadyfinished, this paper analyzes the surrounding rock and retaining structures constructionmechanics, and the construction methods, according to the expansion project of urbanlarge span and small clear distance tunnel. Contents and resulting achievement of thispaper are shown as follows:①Considering the influence factors of tunnel natural and geographical conditions,traffic demand and approaching construction, it’s necessary for the Chongqing AirportRoad Yuzhou Tunnel in-situ expansion project to choose the one side extension schemewhich belongs to the in-situ expansion type.②According to the tunnel hydrogeology, exploration report, design specificationof highway tunnel and existing engineering experience, this paper chooses availablemechanical and physical parameter values of surrounding rock and makes use of thefinite element analysis software MIDAS/GTS to do the three-dimensional numericalsimulation. The comparison results of monitoring data of the Yuzhou Tunnel and thethree-dimensional numerical simulation results show that, it’s available to use the finiteelement analysis software MIDAS/GTS to simulate the Yuzhou Tunnel in-situexpansion project.③According to the eleven monitoring segment models, this paper selected fourmodels which are representative and close to the monitoring data. At the same time,according to the surrounding rock stability discriminant method in the nearest ten years,this paper adopts more mature method——yield approach index. Based on thedisplacement and equivalent stress intensity, this text gives the mechanicalcharacteristics of the surrounding rock and retaining structures during the constructionprocesses and the conclusion of the surrounding rock stability and the economic andreasonable mechanical and physical parameter values of the retaining structures. ④This paper builds the three-dimensional finite element model at the differentlengths of bolt to the four representative construction periods, and does the bolt lengthoptimization analysis. According to the surrounding rock displacement, stress, yieldapproach index and the bolt stress, the bolt design length of Yuzhou Tunnel can beoptimized to the3.0m.⑤From the perspective of destructiveness of tunnel structure system, according tothe existing urban large span and small clear distance tunnel construction methods andstudy results, this paper simulates the bench method, layers of peeling method andbackfill method by two-dimensional finite element method under the different gradesurrounding rock. With the minimum damage during the construction process, backfillmethod shows good mechanical properties. Comprehensively considering themechanical properties and the construction technology, bench method is better to beused in the IV class surrounding rock, and backfill method is better to be used in the Vclass surrounding rock.