Mechanism And Technology Of Vacuum Combined With Surcharge Preloading

In recent years, highway is developing rapidly in our country. The value of settlement after completion of works is high in the point view of built highway on the soft soil region, which produced a big problem called bumping at bridge-head. Vacuum combined with surcharge preloading is a good method to reinforce soft foundation at bridge-head. The method has been used to improve soft foundation of highway extensively. But the mechanism of vacuum preloading is not clear, it is necessary to study the method.First, the field test of soft ground at the jialichen bridge-head on the highway from Luqiao to Zeguo was performed and the observation results of test site were analyzed. The results show that the increase of vacuum energy can not expand the influence scope of vacuum degree obviously when vacuum degree under membrane is high. The vacuum degree in the PVDs is higher than vacuum degree in the sand drains, and the vacuum degree in the sand drain is higher than vacuum degree in the silt at the same depth in the ground during the initial stage of pumping. The value of pore water pressure dissipation can be divided into two parts. One part is produced by vacuum degree and the other part 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 (or sand drains) is produced mainly by vacuum degree. The dissipation value of pore water pressure does not change along the depth. The influence depth can reach 4~5m below the PVDs. The pore water pressure difference between silt and PVDs (or sand drains) 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 influence scope of soil horizontal displacement is far from the improvement area border 20m away, about one times of PVDs depth. The shear strength increases by 3-6 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.Second, the mechanism and some problems of vacuum combined with surcharge preloading were analyzed. 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 airfrom the end of flexible pipe. Based on the Bernoulli equation, the limit lowering of groundwater table is analyzed, which is about 5~7m.The effective influence depth of vacuum preloading has relationship with PVDs depth and can be more than 10m.Then, combined with the mechanism of vacuum preloading and based on the layer method, settlement calculation of vacuum preloading was analyzed .The study shows that the vacuum degree under membrane equivalent load method does not accord with the mechanism of vacuum preloading. Based on the layer method, vacuum degree difference method and effective stress method are presented. The results show that the final settlement calculated by the vacuum degree difference method is more close to the observation results. Every method should be improved in the practice.Based on the results of testing site, some problems of vacuum preloading was analyzed by Plane Strain Finite Element Method. The relative stable seepage velocity field can be produced with the pumping time. The depth of negative pore-water pressure transferring increases with the PVDs length. The negative pore-water pressure influence depth can reach 4~6m 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 influence scope expands with the PVDs depth. The influence to surrounding environment of the protecting measures is also analyzed by Plane Strain...