Study On Working Mechanism Of The Technique Of Geosynthetic-Reinforced And Pile-Supported Cushion

In recent years, the geosynthetic-reinforced and pile-supported (GRPS) cushion has been widely used in the ground improvement of expressway and railway of high speed for the advantages of short construction duration, small settlement and fast convergence. The existing research mainly focuses on the pile/column while little work about the geosynthetic-reinforced cushion (GRC) has been conducted. When only one or two layers of reinforcement are placed horizontally inside the cushion, the strength and stiffness of the cushion are not enough to control the subgrade settlement and differential settlement. Meanwhile, for the pile/column with great strength and stiffness, not only the bearing capacity and settlement control ability could not be fully developed, but the pile/column would also thrust into the cushion. To overcome these deficiencies, the concept and realization approach of composite GRC with variable stiffness are proposed. Both the working behavior and mechanism are systematically studied by laboratory test, field test and numerical simulation. The main contents are presented as follows.Firstly, both the working behavior and mechanism of composite GRC with variable stiffness were compared and analyzed by performing finite element numerical simulation and parametric study. The results show that the geocell-reinforced cushion could improve the stress concentration ratio and reduce the settlement and differential settlement at the subgrade surface effectively.Secondly, both the interface behavior between the geogrid and soil and the deformation characteristic of biaxial/triaxial geogrid were investigated by laboratory tensile test, direct shear test and pullout test, especially the effect of test directions. The test results show that the multi-directional tensile strength of triaxial geogrid is of smaller difference as compared with biaxial geogrid with similar weight per unit area and tensile strength both in transverse and longitudinal directions. Due to the effect of the difference of rib deformation during the test, the test directions would impose great influence on the distribution of interface stress and deformation of triaxial geogrid.Thirdly, the technique of GRPS cushion was adopted for ground treatment of Ningan intercity railway in Fanchang. The subgrade settlement, stress distribution and deformation of GRC were all explored by field monitoring. The experimental results demonstrate that the subgrade settlement is reduced effectively. The total settlement is very small and reaches stable value soon. The differential settlement between the pile and surrounding soil is also reduced by the GRC. The embankment load distribution is homogenized by the GRC, thus the soil arching effect in the embankment is greatly weakened. The stress concentration ratio above the GRC is similar to 1.0.Fourthly, both the effect of settlement control and load transfer of geogrid-reinforced cushion and geocell-reinforced cushion was compared by finite element numerical simulation. The effect of elastic modulus, width and spacing of pile wall, soil modulus, pile length and traffic load was also investigated. The results show that the geocell-reinforced cushion could reduce the settlement and differential settlement at the subgrade surface more effectively. The stress concentration ratio and pile efficiency are all improved, leading to full development of the bearing capacity and settlement control ability of the pile wall.Finally, a coupled hydraulic and mechanical finite element numerical model was built to study the time-dependent behavior, including the subgrade deformation and the stress distribution, of the GRPS cushion. The effect of design parameters on the time-dependent working behavior and mechanism was also investigated. The results show that the geocell-reinforced cushion could reduce the excess pore water pressure in the subgrade effectively and shorten the consolidation time. The stress distribution between the pile wall and surrounding soil has not been obviously changed with the subgrade consolidation. The elastic modulus, width and spacing of pile wall have great influence on the performance of GRC both during and subsequent to embankment construction, while the pile length only impose great influence during the embankment construction.