Study On Mechanism Of Settlement Reducing Of Steel Pipe Pile With Cushion Under Immersed Tube Tunnel

The project is sponsored by National Key Technology Research & Demonstration Program (the construction technology research and demonstration of Hong Kong-Zhuhai-Macao bridge group project-Long Immersed Tunnel Design and Key Technology of Soft and Hard Foundation off the Coast, NO.2011BAG07B01). Based on a large amount of reference researches aiming at the settlement-reducing pile foundation which is at the bottom of immersed tunnel, this dissertation makes a comprehensive study on load-bearing characteristics of settlement-reducing pile groups made of steel pipe by using the method of combining theoretical analysis and laboratory model test. This research probes into the following questions:the settlement of settlement-reducing single pile and settlement-reducing pile group with cushion layer, pile-soil load share ratio calculation theory, the model test on the bearing behavior of the settlement-reducing pile of immersed tunnel and the statistical analysis of influencing factors, the finite element numerical simulation of settlement-reducing pile group foundation and the analysis of influencing factors, and the application of the settlement-reducing pile calculation theory proposed by this dissertation in practical engineering design. The main work and conclusions are as followed:1) Based on the load transfer method, a calculation theory of settlement-reducing single pile is proposed with the consideration of the influence of pile top with and without cushion on the immersed tunnel settlement. For the pile without cushion, the load share ratio is specified in advance to acquire the pile load and soil load, and the settlement of settlement-reducing pile is calculated by using the method of iterative computation. For the pile with cushion, this research proposed the concept of equivalent pile, and established the compression calculation method for cushion in different location of the pile top and the soil between piles using the settlement coordination of the different regions at the bottom of the rigid immersed tunnel. The share ratio of pile-soil load and the settlement of settlement-reducing pile by using the method of iterative are also acquired.2) Based on the side friction resistance and end resistance of settlement-reducing pile calculating with the load transfer method, a hybrid method which the settlement of the settlement-reducing pile group with cushion is calculated by using the elastic theory method is put forward. The elastic friction resistance of pile is obtained and the distribution of pile friction resistance is linearized through separation of elastic and plastic displacement of pile-soil. On the basis of the superposition of the Geddes stress solution for different friction resistance regions, the additional stress formula in any depth of the soil layer below the settlement-reducing pile tip is deduced. According to the principle that the additional settlement caused by the pile group interaction is in accordance with the pile stress and the displacement is coordination in different areas at the bottom of immersed tube, the iterative method to calculate settlement and load share ratio of settlement-reducing pile groups with cushion is proposed.3) With the consideration of the pile length, pile spacing, cushion thickness and the pile caps,6 sets of preliminary model tests through the uniform design method were accomplished. Each factor spans six levels largely so that it could cover all the value range in the practical project. Through the analysis of the model test data and mathematical statistics, the basic rule of the immersed tunnel settlement which is affected by many factors is acquired, and the references to the further model tests and the project design is also given. In this dissertation, a concept of pile consumption per unit area which is an economic indicator is proposed. This concept can select the most economic feasible scheme of pile foundation from different pile foundation combinations which fit the settlement-reducing and bearing capacity. Through the back analysis of regression model, some parameters of settlement-reducing pile which is in accordance with the requirement of settlement control are determined, and the analysis result agrees with the results of validation test.4) Eighteen laboratory tests are performed on models of 12 (3x4) piles in sand by orthogonal design, and the interaction mechanism among immersed tunnel, cushion layer, steel piles and soil is revealed. The test results indicate the rules of the soil stress distribution at the bottom of the immersed tunnel, the distribution of pile axial force and the pile-soil load share ratio. It also reveals the rule that these factors vary with the load of immersed tunnel. Through the range analysis, variance analysis and correlation analysis of the settlement of immersed tunnel and the bearing capacity of composite foundation, the influences which the pile space, pile length, the thickness of cushion layer and pile cap have on the settlement of the immersed tube and the bearing capacity of the settlement-reducing pile are revealed. The immersed tunnel settlement regression model based on results of the orthogonal design test is established. The optimal pile parameter combination solved by back analysis can provide guidance for engineering design.5) Based on the 3D finite element stimulation of the immersed tunnel foundation for Hong Kong-Zhuhai-Macao Bridge, this dissertation suggests that the length of settlement-reducing pile is about 30m;the cushion layer should be set on the pile top. Considering the fact that the large thickness of the cushion will increase the settlement and the construction cost, the thickness of cushion can be lm. When the pile length and the thickness of cushion are determined, the pile space should be decreased on the basis of combining economy in order to control the foundation settlement, and the pile space with 6D is suggested.6) Aiming at the design of a certain settlement-reducing pile foundation of the immersed tunnel in Hong Kong-Zhuhai-Macao Bridge, single pile stress and pile group interaction is analyzed and calculated through the theory of single and group settlement-reducing pile with cushion presented by this paper. The global stiffness of the immersed tunnel and the pile-soil load share ratio were acquired. In addition, this dissertation also calculates the bearing capacity of settlement-reducing piles and makes a comparison with the load results calculated by using the theory of this paper. The analysis can provide reference for both design and construction.In conclusion, this dissertation aimes at the engineering characteristics of the settlement-reducing pile foundation in Hong Kong-Zhuhai-Macao bridge and makes a thorough research on the settlement of settlement-reducing pile and pile-soil share ratio calculation theory, and studies the relative factors affecting the bearing capacity of settlement-reducing pile by model tests. All these can be applied to the practical project and be used to solve the practical problems.