Research And Application Of Freeze-sealing Pipe Roof Construction Method

With further advancing of China’s engineering construction, there have been a number of complex projects, such as highway crossing beneath railway, passage passing beneath airport runways, road underpassing sensitive buildings etc.. These projects have strict requirements for their own safety and for controlling ground surface settlement. The freeze-sealing pipe roof construction method provides a new technique and approach for solving these complex engineering problems.The freeze-sealing pipe roof method that the pipe-jacking equipment is used to drive steel pipe sections into soil mass, and the soil between adjoining pipes are sealed using freezing method, is proposed on the basis of the pipe-roofing method. Under the supporting system composed of pipe roof and frozen soil, workers perform excavating and structure constuction. In this method, the pipe roof supports the main loads, and the frozen soil prevents water leakage and soil erosion between adjoining pipes.This dissertation is based on construction practice of subsurface excavating segment of Gongbei Tunnel of Zhuhai Link Road of Hong Kong-Zhuhai-Macao Bridge which is still under construction. The variation regulation of pipe section’s stress due to jacking angle changing is researched by laboratory model test. The interaction between pipes is analyzed through model test and numerical simulation. The research results provide references for forming curved pipe roof. The water-sealing effect of frozen curtain during active and maintenance freezing phase is analyzed by means of numerical simulation method and model test. And also, the freezing heave behavior is studied using numerical simulation calculation. Though the studies on the freeze-sealing pipe roof method, some major findings are listed as follows:1. The freeze-sealing pipe roof method is technically feasible and it provides a new technique for subsurface excavating under complex condition.2. When calculating the force according to empirical formula, various influencing factors should be considered sufficiently. According to the full scale test, the strain in segments is asymmetric in curve pipe jacking and it shows a decreasing tendency along the jacking axis. The segment also presents a slight deflexion.3. According to the results of interaction between adjoining pipes, the subsequently jacked pipe will have an impact on the formers. The settling tank due to pipe-jacking will move from the top of subsequently jacked pipe to the central between former and subsequently jacked pipes. The yield deformation has occurred in the soil mass between two adjoining pipes. And the subsequently jacked pipe may cause extra force on the formers. However, through controlling jacking parameters, timely measuring and rectifying deviation, the interaction can be effectively reduced and the pipe roof can be guaranteed.4. In active freezing phase, the freezing curtain can be formed when only circular common tubes are open. However, it takes a much longer time for freezing soil to reach the designed thickness. The freezing effective can be improved by the special-shaped enhancing tubes. Therefore, the special-shaped enhancing tubes and circular common tubes are suggested to be opened simultaneously during active freezing phase in practice. For long-distance and horizontal freezing, the soil is advised to be frozen by segments in space and by stages in time.5. In maintainable freezing phase, in order to mitigate the adverse influence on freezing effect and facilitate excavating construction, the air temperature in excavating space should be controlled below 25℃. The wind speed in excavating space has minor influence on freezing effect. Therefore, too stringent requirements for ventilation are not necessary.6. The raised-temperature brine-tube has significant effect on freezing effect. The thickness of frozen soil can be controlled effectively when the raised-temperature brine-tube is open. The temperature of brine has great influence on freezing effect. In order to avoid puncturing and control the thickness of frozen soil effectively, the temperature of brine should be controlled around 15℃.7. The maximum value and influence extent of ground surface upheaval and horizontal displacement present linearly increase with the frozen soil thickness. And also, the above and lateral displacements of pipe roof increase linearly with the frozen soil. Therefore, in order to reduce the effect on surrounding environment the thickness of frozen soil must be controlled.8. The position of maximum ground upheaval and horizontal displacement changes hardly with thickness of the frozen soil. The position of maximum ground upheaval always locates the central part in the subsurface excavating section. And the position of maximum horizontal displacement locates around 5m away from the pipe roof. Using the law above, some targeted measures can be applied to control the surface deformation according to the monitoring data of ground surface settlement.