| Hanoi-Hai Phong expressway is Vietnam's fist freeway. Soft soil widely distributes along the road. Due to high compression, poor permeability and long solidifying deformation time of soft soil, Ground base will appear excessive settlement and lack of stability against sliding when being imposed load. It could not only reduce the usage value of the road, but also often lead the embankment to destruction rapidly after construction. If don't carry out treatment on soft subgrade or inappropriate treatment, it would increased the quality and the cost of the project, and postponed the project deadline, and may even led the project to break down. Therefore, how to treat the soft subgrade on Hanoi-Hai Phong expressway is an important technical problem to improve the construction quality.Depending on Hanoi-Hai Phong Expressway, the paper made the EX9 project as experimental section. The EX9 project is located in Hai Phong, it is the typical representative of the Hanoi - Hai Phong Expressway. As the result of plain ground, river pond distributing widely, high groundwater level and mainly being paddy farming areas, so one of the EX9 project's most important problem is soft foundation. Local geological condition is different from our country's that is why it can't copy our experience. We should carefully study on soft foundation treatment technique to find out the method which is suitable for the project. Aiming at accelerating the consolidation, decreasing settlement after constructing, hunting for the more economic and effective soft foundation treatment on Hanoi - Hai Fhong Expressway, the paper has done a series of study work and achieved some results.(1)The engineering geological exploration had been taken on along the EX9 experimental road. Along five kilometer subgrade in length, had drilled thirty-five boreholes in all. Twenty-one of them were in subgrade centerline, eleven of them were in section, and three of them were in box-culvert. And having taken on standard penetration test (SPT), cross board test on site, cone penetration test (CPT) as well as soil sample and water sample test indoor. Based on the data of these tests, the subgrade ground was divided into four layers and five sandwich layers. The 2a and 2b sandwich layers are soft soil layers. Due to low bearing capacity and easy deformation, they aren't suitable for building subgrade. Therefore some soft foundation treatment methods should be adopted. Based on corresponding norms of Vietnam, sand drain method and plastic board drain method were used. There were twenty-one sections need to be treated. Twelve sections of them adopted sand drain method which was 2080m in length, and nine sections of them adopted plastic board drain method which was 2920m in length.(2)Based on the design requirement, had designed the construction program which was suitable for the actual situation, had chosen the construction machinery, and had worked out the detailed construction quality control program. For sand drain method, the construction machineries were crawler pile driver and tubular driver. The construction process was summed: to place the machinery→to hit the casing→go on hitting the casing→to pull the casing→to pull out the casing→completed. For the plastic board drain method, the construction machinery was flashboard machine which was composed of rack and excavator. The construction process was summed: to post the flashboard machine→to insert pipe boots→to sink the casing→to hit the plastic board to design elevation→pull the casing→to cut the plastic board→to check and record the situation→to move the flashboard to the next position.(3) The perfect monitoring program on embankment settlement had been designed. The main content of monitoring on soft subgrade during construction and completion was the vertical displacement and the horizontal displacement of ground. To choose rational monitoring machineries depending on actual situation, the main monitoring machineries are observation wells, GT204 piezomrters, MI601 inclinometer, surface settlement plates and wooden stakes. Two types of monitoring programs had been determined which were economical and effective. One was internal settlement monitoring of the embankment, another was surface settlement monitoring of the embankment. The dynamic monitoring could ensure the security and stability of embankment during the entire process of construction and operation. At the same time, the analysis could provide the basis for testing the difference between the design program and the actual situation.Depending on a large quantity of work and study, the paper proposed the soft subgrade treatment methods, the construction control standards and the settlement monitoring programs. It has an important theoretical research value and practical guiding significance for expressway construction, in particular soft subgrade treatment technology in Vietnam.
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