Study on drainage mechanism of complete path for vacuum preloading based on thermodynamics theory

Abstract

Vacuum preloading has been a widely used consolidation method for soft clay ground improvement since the 1980s. Consolidation theory only explains the radial drainage process from soil to prefabricated vertical drains (PVD); however, the complete drainage path mechanism by which water drains vertically through PVD to the upper horizontal sand drainage layer and eventually to vacuum pumps is still unclear, resulting in controversies about vacuum preloading. A large oedometer test was performed to study the complete drainage-path mechanism for vacuum preloading. During vacuum preloading, the soil’s average internal temperature decreased to 5 °C below initial temperature, with the lowest temperate occurring near the PVD, which was 2 °C lower than the outskirt. A complete drainage path mechanism is proposed based on the phenomenon of internal temperature decreases. Water evaporates only in the PVD, and the vertical movement of water in the PVD is caused by a density difference between the gas molecules that is independent of gravity. Finally, the proposed mechanism was used to explain the controversy about vacuum preloading. For example, vacuum should not decay along the PVD, vacuum acting elevation at the top or bottom of the PVD has no effect on the final vacuum preloading effectiveness, there is no unsaturated zone formed, and the groundwater level does not drop during vacuum preloading.