Study On The Grouting Mechanism And Construction Techniques In Freeze-sealing Pipe Roof

Excavating shallowly buried,supper large cross-section(e.g.over 300 m~2)tunnels with difficult geological and site conditions creates a major engineering challenge.The Gongbei tunnel of the Hong Kong-Zhuhai-Macau bridge has an excavation cross-sectional area of336.8 m~2,representing the largest single tunnel excavation in the world.It was excavated under the protection of“pipe roof+artificial ground freezing+excavation face grouting”providing a new technology for shallowly buried,super large cross-section tunnel excavation.Grouting in the freeze-sealing pipe roof is a key technology during the construction.Due to the special seepage and temperature boundary condition,the traditional grouting technique and parameters are difficult to apply.It requires innovation on theory and construction means.This thesis studies the theory and construction techniques of grouting in the freeze-sealing pipe roof with the Gongbei tunnel as a case project using theoretical analysis,numerical simulation,and filed instrumentation.The major contents and conclusions include:(1)Studied the formation of freeze-sealing pipe roof and ground frost heave control techniques.By applying the Gongbei tunnel as a case project,the soil temperature and ground surface displacement are predicted using finite element(FE)simulation based on the thermal-mechanical coupling theory.The effects of the frost heave control using pre-grouting around pipe curtain and using the frozen thickness limiting tubes are compared.The calculated ground displacement is compared with the field measurement.The frost heave control principle is explained from the distribution of ground temperature and ground surface displacement.The initial temperature and displacement characteristics of the soil around the freeze-sealing pipe roof are obtained under taking the frost heave control of combined the pre-grouting around pipe curtain and the frozen thickness limiting tubes.(2)Studied the grout diffusion and ground improvement mechanism of grouting within the freeze-sealing pipe roof.With the consideration of Bingham's fluid viscosity variation and gravity stress of the grout,the relationship between the fracturing pressure and the diffusion radius of the grout is derived.The effects of the grouting pressure,viscosity coefficient,average diffusion velocity and the direction of the grout migration on the diffusion distance are analyzed.The grouting pressure,and the longitudinal and latitudinal of grouting hole spacing are recommended.Findings from this part of study can provide theoretical basis for grouting model and parameters and provide theoretical support to the successful construction of Gongbei tunnel.(3)Studied the numerical model,the distance of grout diffusion,and the law of the soil displacement within freeze-sealing pipe roof.The single-hole grouting model is simulated using COMSOL FE simulation based on the multi-phase coupling theory.The effects of the grouting pressure,the fracture width and length on the grout migration,and the displacement of soil were calculated.Combining the initial characteristics of the soil simulated by the frost heave model with the grouting mechanism,the grouting model of the Gongbei tunnel within the freeze-sealing pipe roof is proposed.The variation of the grout diffusion distance and the soil displacement are obtained using numerical simulation.Findings from this part of study can provide valuable reference for the construction of tunnels using similar grouting method.(4)Summarized a complete set of construction techniques for grouting within the freeze-sealing pipe roof.With the application in the case project,a complete set of construction techniques,including grouting material selection,grouting pressure determination,grouting process,monitoring program,etc.are developed for grouting within the freeze-sealing pipe roof.Findings from this part of study provide technical support for the success of the grouting during the Gongbei tunnel excavation.