Vacuum pressure distribution and pore pressure variation in ground improved by vacuum preloading

This paper presents the difference between vacuum pressure and pore pressure reduction for vacuum preloading projects. The experimental results show that the pattern of the fluid flow under vacuum pressure can be classified into three categories-a single-phase water flow, an air-water two-phase flow, and a single-phase air flow. The field test results show that the vacuum pressure reaches the highest value at the ground level and the measured gradients of the vacuum pressure in the vertical direction are approximately 11 kPa/m. It is demonstrated that (i) the treatment area of vacuum preloading cannot be sealed and does not need to be airtight, (ii) the air-water mixture is drawn out from the treatment area under vacuum pressure and the groundwater level drops owing to the presence of air in practice, and (iii) there is an air-water two-phase flow in the unsaturated zone during preloading. The study shows that (i) the vacuum pressure is only a part of the pore pressure reduction along the depth of improving soil; and (ii) the vacuum pressure induces the soil to undergo isotropic consolidation, whereas the pore pressure reduction that is greater than the atmospheric pressure induces the soil to undergo one-dimensional consolidation.