Test Study on the Movement Law of Gas、Water and Soil in Soft Foundation by Vacuum Combined with Surcharge Preloading

Bumping Vacuum combined with surcharge preloading is a good method to reinforce soft foundation.But the mechanism of the vacuum combined with surcharge preloading is not clear,it is necessary to study the method.The field test of soft ground of the highway from Jiujiang Bridge to JiangMeng city was performed and the observation results of test site were analyzed.The results show that the vacuum degree in the PVDs is higher than the vacuum degree in the silt at the same depth in the ground.The value of pore water pressure dissipation can be divided into two phases.The first is produced by vacuum degree drop and the second is produced by groundwater table lowering.The pore water pressure dissipation in the silt is produced mainly by lowering of the groundwater table and the pore water pressure dissipation in the PVDs is produced mainly by the vacuum degree drop.The dissipation value of pore water pressure does not change along the depth.The pore water pressure difference between silt and PVDs increases and the soil consolidation velocity is accelerated during the embankment construction.The increase of vacuum energy can not accelerate the surface settlement when the flow is small.The influenced scope of soil horizontal displacement is 24m away far from the improvement area border,about the same depth as PVDs.The shear strength increases by 1.5～6 times after improvement with vane shear test and the resistance increases largely with CPT.The soft foundation treated by the vacuum combined with surcharge preloading is very effective.