Working Mechanism Of Bidirectional Reinforced Composite Foundation And Its Design Method

The new composite foundation treatment reinforced with vertical piles and horizontal geosynthetic reinforcement over piles has been used widely in the geotechnical engineering such as subgrade improvements. The benefit of this foundation treatment is that the piles and geosynthetic reinforcement can give fully play to increasing bearing capacity and reducing settlement of soft soils. Despite a large amount of research and successful field applications, the bidirectional reinforced composite foundation is still not widely used at the same level as conventional methods such as piling, due to the lack of design procedures. Limited work has been done on the working mechanism and design method for road embankment supported by vertical piles with geosynthetic reinforced cushion. The purpose of this thesis is to discuss the working mechanism of this composite foundation and to propose the relative bearing and deformation analytical method. The thesis is funded through the Chinese National863High-Technique Research and Development Project (Contract name "Treatment technique of bidirectional reinforced foundation based on the settlement control for large area and non-uniform soft subgrade in highway construction", Contract No.2006AA11Z104), Chinese National Natural Science Foundation (Contract name "Working mechanism of discrete material pile composite foundation and its design method according to deformation", Contract No.51078138), the Project of Postgraduate’s Innovation Fund Support from Hunan Province and the Project of Excellent National Doctorial Dissertations in Hunan University. The main research contents and research character in this paper are as follows:Firstly, the working mechanism and failure mode of pile composite foundations were studied based on the analyses of bearing and deformation behavior and load transfer mechanism of various vertical piles such as rigid piles, flexible piles and discrete material piles. The lateral resistance effect, membrane effect (or flexible valve plate effect) and vertical stress dispersion effect of geosynthetic reinforced composite foundations were discussed and its different failure modes were also analyzed. On the base of that, the complex interaction among the pile, geosynthetic reinforcement, and the surrounding soil in the bidirectional reinforced composite foundation was studied. The interaction between the composite foundation and the embankment material was also analyzed. Secondly, based on the Vesic cavity expansion theory, an ultimate bearing capacity calculation method for single discrete material piles such as stone columns was proposed. The advantage of the lateral resistance from the surrounding soil and vertical pressure acting on the soils are taken into account in this method. Considering the deformation characteristic of the discrete material pile, an analytical solution for the settlement of the composite foundations reinforced with discrete material piles was presented. From the present solution, the vertical settlement and lateral bulging of the column under any applied loads can be evaluated at any depth. Based on the shear displacement method, a settlement calculation method of composite foundation with cohesive material piles was developed by taking the account of the interaction of piles, and the interaction between pile and soil. In this method, the Mylonakis&Gazetas model was introduced to simulate the interaction of pile and pile, pile and soil. And the elastic-perfectively plastic model was employed to model the nonlinear relationship between the shear stress and shear strain of the surrounding soil. A model and a simple bearing capacity calculation method for the geosynthetic reinforcement-supported embankment on the soft subgrade were proposed as well. The model and calculation procedures considered both the "vertical stress dispersion effect" and "membrane effect" of geosynthetic reinforcement. By treating the geosynthetic reinforced cushion as a foundation beam, the force and deformation analysis of the reinforcement in matrix form was proposed based on the study of the elastic foundation beam theory. And then the bearing capacity and deformation solutions were extended to calculating the bearing capacity and deformation of the bidirectional reinforced composite foundation was consideration of the vertical pile effect.Thirdly, based on the Winkler foundation model and with consideration of the effect of the shear resistances at the top and bottom sides of the geosynthetic-reinforced cushion, a deformation control differential equation for the bidirectional reinforced composite foundation under the vertical symmetric loads was developed. The corresponding power-series semi analytic solutions for the bending deformation and internal forces of the geosynthetic reinforcement were presented as well. In the method, geosynthetic-reinforced cushion was idealized as an elastic foundation beam, and vertical piles and subgrade soils were idealized as elastic springs with different stiffness. By taking the beam-soil interface resistance, and the longitudinal and transversal coupling-deformation characteristic and the geometric nonlinearity of the beam into account, a deformation control differential equation of geosynthetic reinforcement which idealized as a finite foundation beam was established based on the Winkler elastic foundation model. Corresponding analytical solutions for the deformation, rotation angle, shear force and moment of the finite-length beam under symmetric loads was presented using Galerkin method. Based on the Euler-Bernoulli beam theory, fourth order difference equations dealing with the deflection of geosynthetic reinforcement which idealized as a beam under three different soil support conditions and their solutions were presented for a beam resting on a nonlinear tensionless foundation. The nonlinearity of the foundation soil was simplified by elastic-perfectly plastic model.Fourthly, according to the similarity theory, a group of indoor model tests including the untreated soft soil, geocell reinforced subgrade, composite foundation reinforced with stone columns, and composite foundation reinforced with geocell and stone columns were conducted. The test datum including the load test, vertical settlements at different positions, tensional strain of the geocell material, bulging deformation of the stone column and lateral soil pressure around the column were analyzed. Moreover, the bearing and deformation calculated solutions proposed in this paper were also used to analyze the test datum. A new measuring instrument was developed by national seminar to measure the bulging deformation of stone column under vertical loads. A new measuring and testing technique for determining the modulus of the geocell reinforced cushion was developed as well in the model test. Appropriate values of the elastic modulus of the reinforced cushion were suggested for the bidirectional reinforced composite foundation.Finally, according to the purpose and characteristic of the foundation treatment technique with bidirectional composite foundations in the deep soft soil, the design philosophy according deformation control was developed. The proposed semi-analytical deformation solutions were also proposed to quantify the effects of various factors, such as the elastic modulus of the reinforced cushion, cushion height, pile stiffness, pile spacing and embankment height on the behavior of the bidirectional composite foundation. The relation between the load and settlement was set up accordingly. The reference values of the control index were obtained according to the relative criterion and engineering practice. The detailed design procedure and design flow were also presented in this thesis.