Study On Strengthening Characteristics And The Change Laws Of Groundwater Level Of Vacuum-combined Surcharge Preloading

Vacuum combined surcharge preloading is a more effective soft-foundation improvement method. The settlement and stability problems are well solved by this method, and the time limit for a project is shortened. Now, this soft-foundation improvement method is widely used in several kinds of civil engineering projects, such as express highway, port, wharf and so on, and favorable effect is obtained. However, there are still many problems on the research of improvement mechanism. These problems have restricted its wide application. Based on former research and plentiful experimental data, the change laws of underground water level, vacuum degree, pore-water pressure and soil shear strength during reinforcing are deeply researched by theoretically analyzing and numerically calculating. The main works in this paper are listed as follow:Firstly, mathematical calculation model of seepage discharge is used, and the quantity of percolation of drain pile, vertical seepage and the infiltration from the area outside reinforced zone are calculated. According the correlative theories of underground water dynamics, the depth of groundwater is confirmed. The relation between seepage discharge and groundwater level and the change laws of the quantity of percolation and groundwater level are analyzed. According to the work principle of water level gauge and the observation method through water level pile, the disadvantages of traditional observation method of groundwater level are analyzed, and the amelioration method is put forward.Secondly, based on the researching results of groundwater level change laws and local observation data, the effect on vacuum degree and pore-water pressure with the change of groundwater level are deeply studied. New explanation of vacuum degree, and the concept of vacuum suction and the effective transferring depth of vacuum degree are brought forward. According to the reinforcement mechanism of vacuum combined surcharge preloading, the reasons of the change time of pore-water pressure lagging behind loading time are analyzed.Thirdly, according to the reinforcement mechanism of vacuum combined surcharge preloading, analyzing the change laws of soil saturation degree, and drawing a conclusion that it is possible to change into unsarurated in the groundwater level descendent area during vacuumizing. At the same time, the preconditions are confirmed.Fourthly, the formation principle of hard crust during vacuumizing is analyzed. The effect on foundation settlement deformation of this hard crust is analyzed by numerical calculation.