Shaking Table Test Studying Large-scale Soil-pile-complex Structure Interaction

As a supporting project of the Beijing Olympics and key project of Tianjin Eleventh Five-Year Plan, the engineering and construction of Tianjin Station transportation junction requires deep study on all aspects of the structures. As an important component of the lifeline engineering, the seismic performance of the transportation junction has become an important issue of the city earthquake disaster prevention and mitigation engineering. And therefore, a shaking table test studying large-scale complex structure -pile- soil interaction is carried out on the transportation hub of Tianjin station. In the test the scaled down model was established based on a typical section of Tianjin Station. The model is large and multiplex which is made of piles, underground structure and groung structure. The major work and conclusions are as follows.1. In developing the similarity ratio of the dynamic model test, it is found that it is hard to make the model's physical parameters completely similar to the prototype's of the complex structure -pile- soil interaction system. Hence, the similarity ratio based on soil's predominant period is adopted in the shaking table test to design the model soil, which enables partial similarity between the physical model and the prototype of soli-pile-structure interaction system. It is proven that the method is reasonable by the test resutls.2. The finite element numerical model to get the soil's predominant period is established, through which it is found that the traditional fluctuation theory formula ( T = 4H/v_s) solving soil's predominant period shows a certain amount of error with its scope of applicability. The effects of the soil property, size of soil domain and boundary conditions on the model soil's predominant period are studied though a series of soil modal analyses. It is found that the soil would be affected less by increasing model soil's ratio of the width and height, selecting reasonable boundary conditions and model soil's mixing proportion.The parameters needed in the formulas solving model soil's dynamic shear modulus and damping are obtained through data fitting of the model soil's dynamic triaxial tests. The method solving model soil's maximum dynamic shear modulus by data fitting is proposed. At the same time, by comparing dynamic C &φwith static C &φ, it shows that the model soil's value of C reduces obviously and value ofφ increases a little during dynamic loading. Thus, it is recommended that parameters of dynamic C &φbe used in the finite element numerical model.3. The following measures are adopted in setting up the model, which include the model being divided into two parts, fabricated separately and assembled together finally; the piles being made of hollow pipes filled with iron ore to balance the weight; the micro concrete-filled brass tubes being used to simulate the concrete-filled steel tubes; using foam plastic to seal both opening ends of underground structure to reduce the stiffness effect; replacing the original steel truss roof with glass plate, etc. It is proven by the test results that these measures are effective.The soil box is fabricated and verified though both test data and numerical analysis. Research shows that the frictional boundary applied at the bottom of the soil chamber using dividing strip embedded inside of the model soil works very well. It is also proven that the flexible boundary made of foamed polystyrene applied at the side walls perpendicular to the seismic direction works well to Taft seismic wave and artificial seismic wave but doesn't work so well to Tianjin seismic wave. The sliding boundary using oil lubricated polyvinyl chloride film attached onto the longitudinal inner side surface of the soil chamber works to a certain extent although with relatively large deviation.4. The other test preparation work includes the measuring points arrangement, the model soil filling, model assembling and loading steps determining etc. Some new practices are adopted. For examples, accelerometers in the soil are buried after digging a little well inside the filled soil rather than being buried when filling the soil and double oven heating is used to dry the soil and sawdust in their moisture measurement.5. During the data analysis of the shaking table test studying complex structure-pile-soil interaction, the structure's earthquake response is fully studied based on the acceleration, maximum deformation, maximum normal strain and maximum dynamic soil pressure etc. The findings are summarized as follows:1) It is found that the frequencies inducing the maximum earthquake response on different parts of the structure are different. The response is also influenced by spectral characteristics of seismic waves and its vibration frequency. The earthquake response is drastic when the Tianjin seismic wave is applied.2) The seismic waves are filtered a bit when spreading upward. The amplification for the seismic waves by soil-structure is big when the magnitude is small, while it slows down and even reduces when the magnitude increases underground.3) The maximum deformation becomes bigger while the structure's height increases, and it changes obviously at the interface between the pile and the underground structure and at the soil's surface.4) The maximum normal strain of the underground structure's column is big in the middle but is small at both the ends. Structure's normal strain is affected a lot by the residual normal strain, and this effect attenuates from piles, underground structure to ground structure.5) Also, the soil's dynamic pressure is affected by soil's residual dynamic pressure and the effect increases with the soil depth. The profile of the maximum dynamic soil pressure indicates bigger value at both the ends but smaller in the middle when the soil's depth increases. The maximum dynamic soil pressure occurs at the soil surface. The total soil pressure is affected obviously by the maximum dynamic pressure and it reduces first and then increases when the depth goes down.During the process of loading, the maximum story drift angles on the ground structure meet the Code requirement on the whole. Only a part of the whole structure's normal strains go beyond the limit strain and only part of the foundation soil enters plasticity. Besides, even at the end of test, the structure doesn't collapse. It shows that the structure design is in accordance with the principle of seismic structure design.