Shengzhi Wu

In the construction of tunnels under existing stations, it is necessary to control their settlement. When there is a pile foundation in the existing station, the pile cutting has a significant impact on the settlement of the existing station. To determine the influence of existing piles on the settlement of subway stations, a reasonable pile-cutting time is proposed. Based on the Chengdu Metro Line 9 underpassing the existing Line 1 hatchery station, the settlement law of the tunnel underpassing the existing Line 1 station is analysed via a numerical simulation. Furthermore, the deformation and stress characteristics of the existing piles, pipe roofs, and tunnel linings and the supporting effect on the existing station are discussed. It is concluded that the cutting of existing piles causes a change in the tunnel bearing system, thus resulting in a certain deformation of the station. The influence of different pile cutting times on the settlement of the existing station is then analysed, and it is clarified that the tunnel support stiffness is significantly enhanced after the construction of the secondary lining. At this time, the settlement of the existing pile station is significantly reduced. Finally, through a field investigation, the effect of surface grouting, pipe shed, and multilayer lining on the settlement control of the existing station while the existing pile foundation exists is determined. This research can provide a reference for the settlement control and foundation underpinning of existing stations at ultra-small distances in underground excavation tunnels.

Settlement control of existing stations has consistently been a key issue in tunnel construction. Intermediate soil has a significant influence on the settlement of the existing station as a connector between the new tunnel and the existing station which should be considered when selecting settlement control measures. To clarify the relationship between the state of the intermediate soil and the settlement and control measures of the existing station, this study investigated 49 new tunnel projects under existing stations and elucidated the relationship between the settlement and the settlement control measures of the existing station and the intermediate soil. The relationship between the failure of the intermediate soil and the settlement of the existing station was further analyzed by numerical simulation. Lastly, a simple mechanical model of intermediate soil failure was constructed to determine the stress state of the intermediate soil in constructing a tunnel under the existing station to guide the formulation of settlement control measures for the existing station. Hence, when the intermediate soil is completely destroyed, active control measures, such as jack or grouting lifting, should be implemented; when the intermediate soil is partially damaged, passive control measures should be undertaken. Thus, this study can provide a reference for settlement control of tunnels under existing stations.

The pipe roof is an effective means to control the surface settlement during the construction of the super-shallow buried subway station. The transverse pipe roof method can not only reduce the construction work surface, but also improve the construction efficiency by multi-stage flow construction. Based on the transfer station project of the Olympic Sports Center of Jinan Rail Transit Line 3, the ground settlement law caused by the excavation of ultra-shallow buried subway station under the action of transverse pipe roof is analyzed through field measurement. The mechanism of pipe roof controlling ground settlement and the influence of steel pipe spacing and steel pipe length on the bearing mechanism of pipe roof are analyzed by numerical simulation. The following conclusions are drawn. 1) The surface settlement curve caused by the excavation of the small pilot tunnel in the lateral direction of the station is a wavy U-shaped, and the surface settlement curve becomes a smooth U-shaped after the soil is broken. In the longitudinal direction of the station, the surface settlement is jagged, and the surface settlement above the pipe roof is less than the surface settlement between the pipe roofs. 2) After soil excavation, pipe roof can bear the load of surrounding rock and restrain the settlement of soil above, and soil arch effect is formed between pipes to restrain the relaxation of soil between pipes. With the increase of the spacing of the steel pipe, the height of the soil arch increases. After exceeding a certain spacing, the micro-soil arching effect disappears and the surface settlement increases. 3) The length and stiffness of pipe roof will affect the bearing capacity of pipe roof and there is an upper limit. When it exceeds the upper limit, increasing the length or stiffness of pipe roof has little effect on the control effect of ground settlement. The research can provide reference for shallow buried excavation station and other projects.

When constructing open excavation and adding stories on the upper part of an existing station structure, the new foundation pit contacts with the existing station at zero distance; subsequently, the construction has a greater impact than that of other close constructions. To explore the cross-impact of new and existing structures on open excavation and story-adding above the existing station, in this study, through numerical simulation and field measurement, we analyze the deformation law of the existing station and foundation pit retaining structure in the excavation and story-adding construction above the existing station. The following conclusions are obtained. The excavation of the upper foundation pit leads to the overall uplift of the existing station, and the sidewall exhibits an internal extrusion trend, which is caused by an increase in the horizontal load on the station caused by the unloading of the vault and expansion of the formation plastic zone. Affected by the existing station structure, some foundation pit retaining structures are end-suspended piles; the bottoms of the end-suspended pile slips are evident. This study offers control measures for the primary support connection between the pile bottom and initial retention and protection of the existing station, deep hole grouting reinforcement of the stratum behind the pile, and reserving earth berm at the pit bottom to reduce the deformation of the end-suspended pile. The research results can provide a reference for constructing the added story of a subway transfer station.