Research On Bearing Characteristic Of Double Direction Reinforced Composite Ground And Numerical Analysis

In recent years, along with the development of the freeway construction, the treatment of soft soil foundation becomes increasingly important, which triggers the development of various soft foundation treatment technology.The double direction reinforced composite foundation method is a new soft foundation treatment technique. This new foundation treatment is widely applied for engineering constructions such as freeway subgrade improvement, filling in the back of the abutment strengthening and widening of existing roads with unfavorable geological conditions. However, the theory about double direction reinforced composite foundation remains immature. This paper researches on bearing characteristic of double direction reinforced composite ground, which includes pile-soil stress ratio, settlement and consolidation of this composite foundation from several aspects of theoretical analysis, numerical simulation and model experiment under the supports of High Technique Research and Development Program of China (863 program)——"The Treatment Technique of Double Diretion Reinforced Foundation Based on The Settlement Control for Large Area and Non-uniform Soft Soil Foundation in Highway Construction"(2006AA11Z104).At first, According to characteristics of interaction of vertical pile, horizontal reinforced cushion and soil, the horizontal reinforced cushion is supposed to be a thin plate, piles and soils are simplified as spring series with different rigidities respectively. A single pile and soils located in the influence area of the pile compose a concentric cylinder. The cylinder is regarded as the typical element to analysis the double direction reinforced composite foundation. A function of the horizontal reinforced cushion deformation is deduced based on a circular thin plate on Winkler foundation, and the formula of the pile-soil stress ratio of the composite foundation is also deduced.Then a biparametric model is adopted to calculate the residual settlement of double direction reinforced composite foundation, in which influences of the horizontal resistant on the residual settlement is taken into account. A formula for calculating the residual settlement is deduced. The results show that the horizontal resistant of reinforced cushion help to reduce the residual settlement, and the effect should be considered.Based on an approximate analytical model, the vertical reinforcement's stiffness matrix considering interaction between pile and pile, pile and soil,soil and soil is determined by shear displacement method, and the horizontal reinforcement stiffness matrix was calculated by element-free of galerkin method. Then co-working system of double direction reinforcements'vertical balance equation was got through vertical displacement coordinate relationships. The application of element-free galerkin method was extended to calculate settlement of two-reinforced composite foundation. This method has some advantages such as no elements is needed, node arrangement is flexible, node parameters is few and high accuracy is got.The whole cross section of pile and soil around pile were regarded as an element to analyses. According to the equivalent relationship of the seepage discharge to the volumetric change, the equations for the consolidation of composite ground with piles were derived using the concept of average excess pore water pressure, and the consolidation deformation of pile was considered. Through the initial and boundary conditions of the composite foundation and the assumption of the equal strain, the equations were solved by separation of variables. The average excess pore water pressure and average consolidation degree of pile and its surrounding soil were obtained, and the degree of consolidation of the composite foundation was acquired. Consolidation theories for composite ground with piles were further improved.Finally, three comparative model tests including"soft soil foundation model test","geocell reinforced gravel cushion on soft soil subgrade model test"and"geocell reinforced gravel cushion+gravel pile composite foundation model test"were designed and carried out under the guidance of similarity theory and similarity criterions. The test results showed that, the geocell reinforced gravel cushion can diffuse and homogenize upper load, the non-uniform settlement was also reduced effectively. The double direction reinforced composite foundation formed by geocell reinforced gravel cushion and gravel pile composite foundation can be used to enhance the bearing capacity of the foundation and reduce its total and non-uniform settlements effectively, the lateral deformation of the embankment was restricted. The geocell gravel cushion and gravel pile constitute a perfection draining system, which is propitious to drainage consolidation of soil around piles. The theoretical results of the pile-soil stress ratio and the settlement coincide with the results from tests.