Research On The Plans And Construction Methods Of The New-built Subway Works Passing Under The Existing Subways

In Beijing, construction concept change and route plan adjust caused lots of newbuilt subway works passing under the existing subways, which are of high risk. Basedon several cases of this type in Beijing subway line6, this paper firstly studied thedetermining process and critical factors of the scheme of the stations and auxiliaryprojects, with the methods of case studies and mathematical statistics. The relavantchemes of crossing the existing subways, construction methods and technical measureswere summarized, compared and analyzed to establish a set of effectivedecision-making system and construction methodsfor the new built subway workspassing under the old ones. Lastly, the applicability of research results was verified byapplication in Beijing subway line7under construction.By analyzing the schemes of crossing the existing subway under differentconditions, the importance of scientific urban line network planning was recongnized.With summarizing the schemes of transfer station, the pratical design ideas of tunnelpassing under the existing subway was proposed for the situations without reservedpassing condition. Auxiliary works are suggested to be included in the integrateddesigns of transfer stations from the perspective of architectural design, which providesa new design concept for the protection of existing lines.Bench method of horseshoe shaped section and cross diaphragm (CRD) method offlat top and straight wall, two commonly used tunneling methods crossing the existingsubways, are compared and studied. The results show that comparing with CRD method,the settlement can be controlled within the allowable range, the schedule is cut in half,the construction costs is saved to30000Yuan per meter by using the bench method.By analyzing the deformation of the existing subway caused by above twoexcavation methods, it shows that upper tunnel excavation is the main step causing thedeformation of the existing subway. And the maximum deformation generally occurs inthe top of the new tunnel or between the two new tunnels.Monitoring data of the project, in which the subway line7passes the existingsubway line10using bench method, verified above research results and revealed therationality of prediction of numerical simulation. It also indicates that the method of the deep-hole grouting is very effective for improving ground conditions and controllingstructures settlement and the action of soil located between the new tunnel and theexisting line can decrease structure floating caused by high grouting pressure.The technical management system and deformation control standard weresummarized and discussed for the new-built subway works passing under the existingsubways in Beijing.