Theory And Application Of Soft Subsoil Improvement With PVD-DJM Method

Soft soil treatment is one of the common problems during the construction of express way. The improved methods of prefabricated vertical drains (PVD) and dry-cement jet mixing pile (DJM) have been widely used in the soft soil improvement. The effects of the two methods are not satisfactory when faced with the thick soft soil with low permeable coefficient, which reason is that the improved depth with these two methods is limited. This thesis recommend improving thick soft subsoil with long prefabricated vertical drains and short dry-cement jet mixing pile (abbreviated to PVD-DJM method), and research the features of consolidation, settlement, deformation and load transmission of the soft foundation which improved by PVD-DJM method.The reasonable improved depth with PVD method needs to be determined. With equivalent of vertical hydraulic conductivity , the PVD improved subsoil can be transformed into a double-layered ground. Base on the theory of one dimensional consolidation of the double-layered ground, the average degree of consolidation of PVD improved subsoil is calculated. By analyzing the average degree of consolidation of subsoil with different PVD improved depth, the results have been achieved as that the improved depth has a reasonable value, and it is not helpful for the soft soil to accelerate its consolidation when the depth exceeds the reasonable value. And then, factors affecting the appropriate improved depth have been systematically analyzed. kveEffective length of dry-cement jet mixing pile which deals with soft soil foundation is analyzed. Base on the theory of geotechnical FEM, settlement of the DJM method composite foundation under embankment is simulate by finite element program. Chapter Three discusses the determination of the effective length of pile on the settlement of composite foundation, and analyzes the factors affecting the effective length of DJM pile.In order to get the average degree of consolidation of PVD-DJM method composite foundation, the composite foundation should be transformed into the double-layered ground. Base on the solution of the average degree of consolidation of a double-layered ground, the formulas of pore pressure and consolidation degree of PVD-DJM method composite foundation are deduced. Comparing the theoretical solution of consolidation degree of the composite foundation with the numerical solution, the two are basically the same. Base on the above theoretical solution, this thesis discusses the consolidation degrees of the composite foundation with different treatment and analyzes the factors which affect the consolidation degree and the dissipation of pore water pressure of the composite foundation, such as module of DJM, the pile spacing, the length of pile and the discharge capacity of PVD.With the finite element program and the analytic approach of the plane-strain axisymmetric problem, the ground improved by PVD-DJM method is simulated. The load transmission characteristics and the settlement characteristics of the composite foundation under embankment are analyzed, and the factors affecting these characteristics are comparatively researched.