The Research On Bearing Characteristic And Settlement Of Composite Foundation Bidirectionally Reinforced By Stone Columns And Geocell

As a new soft foundation treatment method, the composite foundation bidirectionally reinforced by stone columns and geocell is favored for civil engineers for its better service behavior and widely applied in engineering practice such as soft soil improvement in highway, subgrade strengthening in railway and oil tank engineering and so on, obtaining favorable reinforcing effect. However, the theory of the composite foundation remains immature. In this paper, the bearing characteristic, consolidation analysis, pile-soil stress ratio and settlement calculation of the stone columns and geocell reinforced composite foundation have been discussed based on several aspects of theoretical analysis, model experiment and engineering practice. This paper is supported by the High Technique Research and Development Program of China (863program)——"The treatment technique of bidirectionally reinforced foundation based on the settlement control for large area and non-uniform soft subgrade in highway construction"(2006AA11Z104), the Natural Science Foundation of China (51078138) and the financial support program of topic selection of thesis for the degree of doctor of engineering in hunan university.Firstly, the working mechanism of the two-direction composite foundation is studied based on the analysis of working mechanism of geocell and stone columns. The calculation methods for the bearing capacity of the geocell reinforced mattress and stone columns reinforced composite foundation are respectively summarized. The calculation method for bearing capacity of the composite foundation bidirectionally reinforced by stone columns and geocell is investigated with consideration of their interaction on each other and the consolidation character of the stone columns. According to axisymmetric elastic theory, the formula for the pile-soil stress ratio of stone columns reinforced composite foundation is obtained by choosing different pile and soil elements. Then the formula for the pile-soil stress ratio considering time effect and three dimensional axisymmetric consolidation model of gravel pile is derived based on the analysis of the time effect of pile and soil stresses. The rationality of the solution of the pile-soil stress ratio is verified by an engineering example.Secondly, in view of the deformation characteristics of the composite foundation bidirectionally reinforced by stone columns and geocell, the deformation mechanism is discussed and the existing settlement calculation methods are summarized. Regarding the difference of stiffness of gravel pile and soil is not obvious, the composite foundation can be regarded as a composite soil. The geocell reinforced cushion can be regarded as a beam resting on a Winkler foundation which can be called the geocell reinforced beam. Considering the tangential frictional resistance between the geocell reinforced cushion and gravel pile composite foundation, the settlement calculation model of the composite foundation bidirectionally reinforced by stone columns and geocell is established, the governing equation is deduced and its corresponding analytical solution is obtained based on the differentiator series method. Thus the settlement calculating formula of the composite foundation is derived. The example analysis shows that the analytical solution is right and the parameter analysis shows that the normal and tangential spring stiffness of the composite soil have quite great influence on deformation of composite foundation, mechanical and geometric parameters of geocell reinforced cushion such as elastic modulus and height are less affected.Then, based on the elastic foundation beam model for the settlement calculation of composite foundation bidirectionally reinforced by stone columns and geocell, the separate-step calculation concept is introduced. Firstly the deflection of geocell reinforced beam subjected to all sorts of vertical loads is considered and the deflection solutions of geocell reinforced beam are derived and summarized. Then considering the actual deflection curve of geocell reinforced beam, the relationship between tangential relative displacement and the deflection is established. The solution of tangential relative displacement is acquired. The subgrade reaction and tangential frictional resistance are obtained and applied to the geocell reinforced beam as loads so that the final deformation of geocell reinforced beam is developed. Thus the settlement calculating formula of composite foundation bidirectionally reinforced by stone columns and geocell is derived. The results calculated by separate-step calculation method is proved to be consistent with previous analytical solution in an example, which indicates that the separate-step calculation method can be used in the engineering practice.Based on the beam model for the settlement calculation of the two-direction composite foundation formed by geocell reinforced mattress and gravel piles and the principle of minimum potential energy, the governing energy equations for geocell reinforced beam considering tangential frictional resistance are obtained and solved by an appropriate displacement function. Then the formula for the settlement of the two-direction composite foundation is acquired. The analysis of examples indicates the results calculated by the energy method agree well with those by the analytical solution proposed above. The validity and rationality of the solution are verified and the methodological system of the settlement calculation for two-direction composite foundation gets improved further.Lastly, according to the theory of similarity, a group of indoor model tests including the untreated soft soil, subgrade reinforced by sand bag wells, geocell reinforced subgrade, composite foundation reinforced by gravel piles, composite foundation reinforced by geogrid and gravel piles and composite foundation reinforced by geocell and gravel piles are designed and completed. The obtained data from model tests is analyzed. The bearing and consolidation characteristic and deformation mechanism of subgrade reinforced by sand bag wells, geocell reinforced subgrade, composite foundation reinforced by gravel piles, composite foundation reinforced by geogrid and gravel piles and composite foundation reinforced by geocell and gravel piles are discussed by the analysis of the loading test, stress distribution, pile-soil stress ratio and water pressure. The settlement calculation method proposed above is verified by the model test data. The test results shows that the geocell reinforced mattress can spread and homogenize the loads, reduce the settlement and the non-uniform settlement of subgrade, the gravel pile can effectively accelerate the consolidation of subgrade and the stress concentration effect is clear, and the composite foundation formed by geocell and gravel piles can greatly increase the bearing capacity of subgrade, reduce the pile-soil stress ratio, accelerate the consolidation of soft foundation and reduce the settlement and the non-uniform settlement. Moreover, the results calculated by the theory method proposed above agree well with those from the model tests, which means the theory in this paper are rational and feasible.