樁基礎為常見離岸支撐系統之基礎形式，其主要設計載重為水平作用力，本研究主要為以創新之模型試驗，探討樁基礎受水平載重之行為及結構與土壤互制關係。常見求取基樁與土壤互制關係及基樁側向載重傳遞機制為利用現地基樁側推試驗所得，但現地實驗費用昂貴且複雜，本研究主軸為發展一可模擬現地土壤應力狀態下基樁側推試驗平台系統，以了解基樁於土壤中之互制行為，由修改原用於CPT校正之標度槽，利用標度槽可以控制在不同深度之邊界應力的特性，結合非線性數值模擬計算在不同側向載重下之邊界應力，於試驗中進行補償，組成土層模擬器，並降低邊界效應的影響，同時以裝設可量測變形與土樁界面受力之模型樁，探討基樁與土壤互制行為，初期結果顯示其可做為基樁研究之有利工具。

SUMMARY
Pile foundation is one of the most common foundations for offshore structures. The main purpose of pile foundation for offshore wind turbines is to resist the lateral loadings from wind, waves, and current. This study investigates the behavior of pile foundation under lateral loadings and soil-pile interaction by using an innovative model testing technique. In-situ lateral pile load tests have been used to study the load transfer mechanism of pile under lateral loading and soil-pile interaction. However, the complicated soil conditions and high expense have limited the implementations. The purposes of this study are to develop a laboratory testing platform for lateral pile loading test subjected to similar in-situ stress state and investigate the soil-pile interactions. In order to eliminate the boundary effect of model tests, calibration chamber used for CPT tests is used to serve as a field simulator capable of controlling the stress conditions at different layers. Push-over tests on a instrumented model pile is installed in the field simulator are performed to investigate the soil-pile interaction responses. The preliminary results show that the system can be a useful tool for pile behavior study.
INTRODUCTION
P-y curve is common used for the design of the lateral capacity of pile foundation in practice, and can mostly be determined by the lateral load test of pile. Because of the high expense and complicated work of the in-situ test, this study develop a laboratory testing platform. It can simulate the field stress of soil by using FBG sensors (Fiber Bragg Grating sensors) to control the boundary conditions of the calibration chamber. Combining the result of nonlinear numerical simulation, the testing platform can be used to study the load transfer mechanism of pile foundation under lateral loading and obtains good test data for the reference of the design of laterally loaded pile in the future.
P-y curve method was firstly proposed by McClelland and Focht (1958) , after long-term research, it has become the most important method for the analysis of laterally loaded pile. P-y curve is the relation between the soil resistance and the deflection of pile of a certain depth, and can be used to determine the parameter of the nonlinear soil spring in the numerical simulation.
Holden (1977) proposed four basic types of boundary conditions for the calibration chamber system. The stress condition of the situ soil can be reproduced by the combination of four types of boundary conditions. In this study, in order to reduce the influence of boundary effects, the testing platform can control the lateral compensation stress provided by nonlinear numerical analysis during the experimental process. The boundary state is defined as the fifth boundary state (BC5). This boundary state is that the compensated lateral stress provided by nonlinear numerical analysis can be adjusted with reasonable lateral deformation while the vertical stress is kept constant. This kind of stress condition is similar to the in-situ soil stress condition under the lateral load test of pile.
The testing platform consists of the modified calibration chamber, FBG measurement system, instrumented pile and lateral loading system. The modified calibration chamber is consisted of 20 layers steel ring, each layer can change the boundary condition by stress or strain control to simulate different boundary state of soil. The cap of the chamber can provide vertical stress to simulate the overburden pressure of the in-situ condition. The FBG sensor can provide good strain measurement of each steel ring to enhance the accuracy of the system control. In addition, the FBG sensor has both electromagnetic shielding and waterproof property. The instrumented pile with densely distribution of the strain gauges and tactile sensors is then used to obtain soil resistance and the deflection of pile for the study of soil-pile interaction. Lateral loading system can provide lateral load to the pile top with the fixture to prevent from the rotate effect. The maximum load of actuator is 5kN and the accuracy is 1N.
MATERIALS AND METHODS
The main goals of preliminary test included three issues. The first one is the effect of the applied compensated boundary stress, the second is the behavior of laterally loaded pile under various consolidated pressure and the final is the soil-pile interaction by the directly measurements of instrumented pile.
The effect of compensation boundary stress for reducing the influence of boundary is the comparison of pile behavior under applied compensated boundary stress provided by nonlinear numerical simulations and that under Ko consolidated stress. In addition, the p-y relation of this study is also compared with the proposed method of API(2007) method. The second issue used the cap of chamber to control the different consolidated stress to simulate different overburden stress for the study the behavior of laterally loaded pile under various consolidated pressure. The final issue used the tactile sensors and strain gauges to directly measure the soil resistance and the pile deflection and compared the results with the proposed method of API(2007) method for the verification purpose.
RESULTS AND DISCUSSION
Figure 1 shows the effect of the boundary stress the p-y relation. It can be seen that under the Ko stress condition, the effect of boundary stress on p-y relation increased with the increase of lateral load. When under the compensation boundary stress condition, the p-y relation is similar to the result of API(2007) method. The result also prove that the testing platform can eliminate the boundary effect to reproduce a good simulation of in-situ stress condition. Figure 2 shows the behavior of pile under different consolidated stress. Under the loose sand condition, the behavior of pile is similar to the behavior of short pile. When the consolidated stress is high, the behavior of pile presented as the behavior of long pile due to the increasing of soil stiffness. The results of soil resistance and deflection of pile obtained by elastic beam theory, directly measurements of sensors, and proposed by API (2007)are compared, as shown in Figure 3. It can be found the results of different method are very similar. It is also verified the tactile sensor can obtain reasonable and good data in this study.
CONCLUSION
The innovative testing platform can eliminate the boundary effect and improve the quality of the test. The tactile sensor can directly measure the normal stress of pile and present the reasonable measurement for the study of soil-pile interaction. The FBG sensor has both electromagnetic shielding and waterproof property to enhance the accuracy of the system control

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