Laboratory and Field Experimental Study on the Vacuum Preloading Method with Pumping and Discharging

Negative vacuum pressure hardly reaches deep soils due to the drain board bending and serious blockage with the existing vacuum preloading methods (VPMs), thus resulting in poor reinforcement relative to practical engineering applications. To address this issue, this paper proposes a vacuum preloading method with pumping and discharging (a new dredger filling foundation processing technique based on vacuum preloading). This new VPM is developed through technological improvement and plastic drain board innovations in traditional VPMs. The new VPM uses a plastic vertical drainage board with double drainage channels, and the core board is in a “tic-tac-toe” shape with a hollow centre and square pipeline channels. It can execute air and water drainage. Vacuum transfer holes were set at two grooves, every 2-3 m from the central pipeline of the core board. Grooves at two sides of the core board and filter membrane were rolled together to improve drainage. In addition, the vacuum pipe, tube connector, and core board centre of the vertical plastic drainage board were connected directly and securely. A stereoscopic vacuum transmission system composed of a horizontal and vertical drainage system was built. In this vacuum transmission system, the transfer route was shortened, and the loss of vacuum along the route was decreased. The negative vacuum pressure was transferred to different soil depths through the central pipeline of the core board to accelerate the dissipation of pore pressure in soil mass and prevent vacuum pressure loss caused by drainage board bending, thus improving the reinforcement effect. Dredger filling silt foundation reinforcement by VPM and VPM with pumping and drainage were compared in laboratory simulations and field tests with different drainage boards. Test results show that the loss of vacuum pressure along the drainage path was relatively smaller in the new physical vapor deposition (PVD), and the pressure transfer efficiency was increased. Deep soil mass was reinforced effectively by using the new PVD. After reinforcement, the physical and mechanical properties of soil layers were improved. Moreover, soil strengths were remarkably improved, with sharp reductions in natural moisture content and porosity. Then, the transfer law of vacuum on different drainage boards and the reinforcement mechanism of VPM with pumping and drainage were analysed. Research conclusions show the superiority of VPM with pumping and drainage in terms of effectiveness and soft foundation reinforcement. This study provides a theoretical basis for the application and development of the new VPM.

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