Research On The Vacuumization Method, Treatment Mechanism And Application Technology Of Underwater Vacuum Preloading

Soft soil is distributed in the vast area of the world with poor property. Vacuum preloading is popular soft soil improvement technology in civil, port, communication, and hydraulic engineering. Usually, Vacuum preloading is limited in land because of the complexity of underwater engineering and the uncertainty of mechanism in underwater vacuum preloading. With the development the construction and technology, the research and application of underwater vacuum preloading can be possible and necessary. The study was directed toward the research and development of the mechanism of underwater vacuum preloading technique, the equipment for vacuum pumping and the technological process for implementation of underwater vacuum preloading.During the feasibility study, the existing land-based vacuum pumping equipment was tested in the water and rationally analyzed by way of combining theoretical research with model testing. It was known from the laboratory tests that, the land-based vacuum equipment was capable of producing a vacuum of 95 to 75 kPa and they could be used in underwater vacuum preloading.The working mechanism of the water on top of the sealing membrane was studied through laboratory model tests and theoretical analyses. The pre-loads produced by the underwater vacuum preloading equal the sum of the water pressure on the sealing membrane and the vacuum beneath the sealing membrane. In the laboratory tests, when the water head was 10 m, the whole water pressure (100 kPa) could be converted into pre-loads. During the implementation of underwater vacuum preloading, the infiltrated flow of water from outside the boundary of soil being consolidated wouldn't result in the failure of the soil mass. During the laboratory tests, the change of jetting parameters with the water depth in different circumstances during vacuum pumping was also studied to achieve a series of regular patterns. And the laboratory model tests of underwater vacuum preloading have verified the working mechanism of underwater vacuum preloading and the conversion of water pressure into pre-loads.Basic theory and governing equations of underwater vacuum preloading were firstly introduced. Basic process of underwater vacuum preloading analysis using finite element method was introduced. Then using finite element method, a numerical study on the process of underwater vacuum preloading was conducted. Some key subjects of numeric analysis were discussed such as: selecting principles of constitutive equations of soft soil, the treatment methods for boundary condition of displacement and pore water pressure, the simulation of sealing membrane.Base on the feature of underwater construction, some key technology of underwater vacuum preloading were studied such as: the laying of sealing membrane and filer tube, the installing of vacuum pump. The study was focus on one of the key procedures, the underwater sealing membrane laying technology and a concept of underwater membrane-laying vehicle was presented in this study. It is suggest by monitoring technology of underwater vacuum preloading construction study that the settlement of ground surface can be measured by liquid differential pressure settlement metre. This method obtains the settlement from the differential value of pore water pressure at each measuring point. The cost-effectiveness analysis revealed that underwater vacuum preloading has fairly good applicability and economical efficiency.