| This paper takes the air shaft project between the Fangzhi city station and the Xiangwang station of Xi’an Metro Line 9 as the research background.Due to the urgent construction period and insufficient enclosure conditions,the method of "first shield tunnel,then mining method,horizontal channel and vertical shaft" is adopted in the project.There are the following key technical difficulties in the construction of this project:①the optimization of the excavation scheme of the horizontal channel;②the optimization of the grouting reinforcement and the initial support measures for the gravel stratum;③the optimization of the temporary support scheme for the adjacent existing tunnel segments;④the optimization of the removal scheme for the existing shield segments.In this paper,numerical method and in-situ monitoring method are used for comparative analysis.According to the above key technical difficulties,the optimization scheme is proposed.Finally,the key construction control technologies are as follows:(1)Excavation plan of horizontal channel:to reserve core soil and excavate by four steps method.The interval between step working faces should be controlled within 3m,and the core soil area should be controlled over 70%of the excavation section of this layer.(2)Stratum grouting reinforcement and support technology:in order to enhance the stability of the stratum,to reduce the surface settlement and prevent the groundwater from leaking into the structure,the advance large pipe shed supporting scheme is adopted in the arch of the horizontal channel,and the WSS grouting is carried out within 2m outside the excavation contour of the horizontal channel.After reinforcement,the elastic modulus of the stratum is increased to 1.2～1.5 times of the original soil stratum.(3)Temporary support technology of existing shield segment:When the horizontal channel is excavated to the adjacent existing segment,the bolts of the shield segments within 4～6 rings of the adjacent horizontal channel was loosen firstly,and the stress concentration effect of the shield segment was largely reduced.In the existing shield segments,the reinforcement schemes,such as the temporary steel frame support(6-9m outside the cross channel),low-grade concrete block wall and M2.5～M5.0 mortar backfilling are more conducive to segment breaking construction,which can speed up the construction period and save costs.(4)Breaking technology of existing shield segments:the "three horizontal and four vertical" cutting route was employed to break the segments by using the water drilling machine,the segments adjacent the excavation contour were broken at first,and then the segments at the core.At the same time,the temporary inverted arch support should be closed quickly.(5)Based on the above optimized construction schemes,the in-situ monitoring are carried out during construction period.The results show that the maximum stratum settlement caused by the construction of horizontal channel is-24.3mm,which don’t exceed the control standard value of-30mm.The maximum deformation of the existing shield segment is-13.5mm,which don’t exceed±20mm of the tunnel deformation control standard value.It is indicated that the above key construction technology has good effect on the stratum stability and deformation control of the existing segment,which can provide scientific reference for similar "First tunnel and then shaft method" construction. |
