Mechanism Of Vacuum Combined With Surcharge Preloading On Highway Soft Clay And Its Numerical Simulation

Bumping Vacuum combined with surcharge preloading is a good method to reinforce soft foundation . The method has been used to improve soft foundation of highway extensively. But the mechanism of vacuum combined with surcharge preloading is not clear, it is necessary to study the method.(1) The field test of soft ground of the highway from Jiujiang Bridge to JiangMeng city has been performed and the observation results of test site have been analyzed. The results show that the vacuum degree in the PVDs is higher than 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 groundwater table and pore water pressure dissipation in the PVDs is produced mainly by 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 far from the improvement area border 24m away, about one times of PVDs depth. The shear strength increases by 1.5～6 times after improvement through vane shear test and the resistance increases largely through CPT. The soft foundation treated by vacuum combined with surcharge preloading is very effective.(2) The mechanism and some problems of vacuum combined with surcharge preloading have been analyzed. The mechanism of vacuum preloading has been discussed by contrast of vacuum preloading and heaped preloading. The cause of vacuum degree formation under groundwater table is discussed in the paper. The testing results show that vacuum degree above the groundwater table is produced directly by pumping air, and that vacuum degree below groundwater table is produced mainly by dewatering and by escaping of sealed air from the end of flexible pipe. The effective influence depth of vacuum preloading has relationship with PVDs depth and can be more than l0m. The negative pore-water pressure influence depth can reach 2～3m below the PVDs in the silt when the well resistance is small. The value of vertical displacement and horizontal displacement increases with the depth of PVDs. The influenced scope expands with the PVDs depth. The influence to surrounding environment of the protecting measures is also analyzed by Plane Strain Finite Element Method.(3) The embankmen consolidation the characteristic of the three typical soft base sections of the highway from the bridge of Jiujiang to the of Jiangmen city have been simulated by Plane Strain Finite Element Method which adopts BIOT consolidation theory and Pair yield surface model putted forward by academician Shen ZhuJiang. There methods which are equal preloading with PVDs, bumping Vacuum combined with surcharge preloading, bumping Vacuum combined with surcharge preloading and stir piles have been simulated. Surface subsidence, horizontal displacement , pore water pressure and its variation tendency on space and time have been analyzed. The effects of surface subsidence , horizontal displacement on west river embankment have been analyzed also.