Abstract：The practices show that the plastic drainage plates would be bended because of the large settlement of soft foundation treated by vacuum preloading，which makes the longitudinal drainage content of prefabricated vertical drains(PVDs) decreasing then impairs the improvement performance of the deeper layer soil. To decrease the influences，the scheme of twice installation of PVDs was proposed for the ultra-soft foundation，which meant that the shallow-layer soil was treated firstly then the longer PVDs would be installed to improve the whole foundation as the shallow-layer soil had certain bearing capacity. Combined with the vacuum preloading engineering in the reclamation project of Dingshan reclamation area in Wenzhou city，Zhejiang province，the longitudinal drainage content of PVDs was measured under the different bended states to estimate the variation of the vertical permeability of PVDs during the improvement. The Duncan-Chang constitutive model was developed based on the FEM program automatic dynamic incremental nonlinear analysis(ADINA)，then the permeability variation of the bended PVDs and the initial un-dissipated excess pore water pressure were considered in 3D FEM simulation. Comparing the calculated results with the field data，it showed that the settlement and reduction of pore pressure caused by twice installation of PVDs were more than that caused by once installation of PVDs，and the calculated results were more approximate to the field data when considering the influence of the bended PVDs. The conclusions can be useful for the design and construction of the similar engineering.
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2011, Vol. 30 
