Research On Design Theory And Model Tests Of Multi-element Composite Foundation

With the economic growth and the expansion of infrastructure construction, the technology of ground improvement has developed rapidly. Composite foundation technique is one of the hot fields of ground improvement technology, especially multi-element composite foundation which emerged in last decade, its theory falls behind the application. In order to spread the multi-element composite foundation technique better, it's necessary to study the characteristics of bearing capacity and mechanism of settlement further.In this dissertation, theoretical analysis, model tests and numerical simulation methods have been employed to study the characteristics of load bearing and the mechanism of settlement of the multi-element composite foundation. The main contents are as follows:The state-of-the-art of multi-element composite foundation in the theoretical analysis, model and field tests, numerical simulation, and other aspects were summarized at home and abroad.The efficiency factors of bearing capacity have been studied. The analytical formulas for efficiency factors of bearing capacity of multi-element composite foundation were deduced. Because the design theory and calculation method are immature, the efficiency factors of bearing capacity cannot be selected accurately. From the relationship between efficiency factor of soil and pile-soil stress ratio of single-pile composite foundation, and based on the analytical solution of pile-soil stress ratio of multi-element composite foundation, the general method for calculating efficiency factors of bearing capacity of auxiliary piles and the soil in multi-element composite foundation was presented in this dissertation. By applying the method in practical project, its feasibility was proved.With theoretical analysis methods, the settlement mechanism and the calculation method of the multi-element composite foundation were studied deeply. This dissertation performed the analysis of load-transfer law of single pile in multi-element composite foundation. Based on the above law, the relationship between the settlement of the single pile's top and the load on the sigle pile's top was derived by load-transfer principle in elasticity theory. Relying on the model of pile to pile from Mylonakis and Gazetas, considering the interaction among cushion, pile and soil, the flexibility coefficients of pile to pile and pile to soil were obtained. And according to the superposition principle in elasticity theory, an equation including the settlements and the loads on the pile top and soil top was established. Pile-soil stress ratios of primary pile to soil and auxiliary pile to soil and load-sharing ratios of primary pile, auxiliary pile and soil of multi-element composite foundation could be obtained by solving the equation.A set of apparatus of foundation physical model has been developed and utilized to conduct model tests of multi-element composite foundation. By changing the types of multi-element composite foundation and their design parameters, a number of characteristics of multi-element composite foundation have been obtained, including the characteristics of load-settlement curves, stress of the piles, stress of the soil between piles, pile-soil stress ratio, and load-sharing ratio of piless and soil. Although the model tests cannot precisely describe the characteristics of multi-element composite foundation, they reveal the mechanism of multi-element composite foundation qualitatively. It is concluded that the multi-element composite foundation on steel pipe pile and sand column and the multi-element composite foundation on concrete pile and lime column contributed more greatly than the composite foundation on sand column for improving the silty fine sand. But the multi-element composite foundation of lime column and sand column did not contribute more greatly than the composite foundation of sand column for improving the silty fine sand.The nonlinear finite element model of the multi-element composite foundation established by ANSYS, can simulate the static load tests of multi-element composite foundation under the conditons of homogeneous foundation and the double foundation. Through the comparison of measured and computed results, influence laws of the characteristic value of bearing capacity, pile-soil stress ratios and efficiency factors of bearing capacity of multi-element composite foundation were obtained by changing the design parameters.By comprehensive analyses of analytical solutions, model test results, numerical simulation, field test results and engineering practice, the design theory of the multi-element composite foundation was further improved.Finally, the problems which need further studies were discussed.