Study On Testing Analysis And Prediction Methods For Bearing Properties Of Super-long And Large-diameter Steel Pipe Piles Under Vertical Load

The super-long and large-diameter steel pipe piles have been more and more widely used in large-scale Sea-Crossing Bridge and large scale deep water piers projects due to its higher bearing capacity, relatively simple pile driving process, smaller amount of dumping, and good ability of anti-bend. Currently, determining the design parameters of actual project according to the empirical parameters in national standards is somewhat conservative, and often results in a great waste of resources. A small number of projects have conditions to determine bearing capacity and other parameters through the static load test, whose cost is very high. The current accumulated text and research data is less, and the calculating and determining of settlement is a relatively weak part in the field of pile research, and there is rare systemic research data. So, researching bearing characteristics of super-long and large-diameter steel pipe piles is of great theoretical and practical significance.On the basis of the typical static load test of three very large-scale Sea-Crossing Bridge, this paper deeply analyzes the bearing properties of super-long and large-diameter steel pipe, points out the bearing types and characteristics of super-long and large-diameter steel pipe piles, and has got the load settlement relationship by curve fitting using Matlab software.This essay takes advantage of finite difference method to analog calculation. It gets the Q-s curve, the distribution of shaft axis force and the lateral friction resistance, and compares with the static load test results of actual project. It also analyses the bearing properties of super-long and large-diameter steel pipe piles in isotropic soil with finite difference method. It has systematically discussed the factors which can influence the relation of Q-s curve and distribution of lateral friction resistance, including pile-length, pile-diameter, and ratio of length to diameter, elastic modulus ratio of pile to soil, elastic modulus ratio of under and beside pile. The following conclusions are obtained:Firstly, the difference between simulation results and the static load test is small, which shows it is appropriate to use finite difference numerical method to analyze the bearing properties of super-long and large-diameter steel pipe pile under vertical load and predict the bearing properties of the pile under working state.Secondly, as the pile-length increases, the ultimate bearing capacity of the pile is increasing and the settlement is reducing, but the rate of bearing capacity to increase and settlement to reduce is decreasing. When the pile-length exceeds to a certain value, it's unobvious to improve the bearing properties if the pile-length continues to increase. As the pile-length increases, the lateral friction resistance increases, and the load ratio of the lateral friction resistance increases but the tip resistance decreases. If the same load affects on the top of pile, as the pile-length increases, the lateral friction resistance decreases, but the position of the ultimate bearing capacity doesn't change.Thirdly, as the pile-diameter increases, the ultimate bearing capacity of the pile is increasing and the settlement is reducing, and also the rate of bearing capacity to increase and settlement to reduce is increasing. That is to say changing pile-diameter is better than pile-length to improve the bearing properties. But increasing the pile-diameter will increase the cost. Therefore, we should choose the appropriate pile-diameter. As the pile-diameter increases, the load ratio of the lateral friction resistance decreases but the tip resistance increases. If the same load affects on the top of pile, as the pile-diameter increases, the lateral friction resistance decreases, and the position of the ultimate bearing capacity moves down.Fourthly, when the pile-diameter is unchanged, as the ratio of length to diameter increase, the ultimate bearing capacity of the pile is increasing and the settlement is reducing, but the rate of bearing capacity to increase is decreasing. When the pile-length is unchanged, as the ratio of length to diameter increase, the ultimate bearing capacity of the pile is reducing and the settlement is increasing, but the rate of bearing capacity to reduce is decreasing. This indicates that it's not significant to increasing the ratio of length to diameter of super-long and large-diameter steel pipe pile to improve the bearing properties. Therefore, we should consider the ratio of length to diameter and the economic benefits in practical engineering, and choose appropriate pile-length and pile-diameter.Fifthly, when the pile elastic modulus is unchanged, as the elastic modulus ratio of pile to soil decreases, the bearing capacity of pile is increasing, and the settlement is reducing. When the soil modulus exceeds to a certain value, the improvement of bearing capacity and settlement is reducing. As the elastic modulus of the soil increases, the load ratio of the lateral friction resistance increases but the tip resistance decreases.Sixthly, when the elastic modulus of soil beside pile is unchanged, the larger the elastic modulus ratio of soil under and beside pile is, the greater the bearing capacity of pile is and the smaller the settlement is. Also the load-sharing ratio of tip resistance and the lateral friction resistance are increasing. It is to say that tip resistance can strengthen the lateral friction resistance. When the elastic modulus of soil under pile is unchanged, as the elastic modulus ratio of soil under and beside pile decreases, the bearing capacity of pile is increasing and the settlement is reducing.Meanwhile, in order to verify the above conclusions, the paper use intelligent methods and numerical methods to research the bearing property predicting methods of the super-long and large-diameter steel pipe piles, mainly as follows:(1) By analyzing the static load test data of the super-long and large-diameter steel pipe pile, it has stabled pile bearing capacity model SVM-Q, ICASVM from three aspects, which are the pile length, soil parameter, pile-soil parameters. The results show that these two model can accurately predict the shaft resistance and bearing capacity of the super-long and large-diameter steel pipe pile, and illustrate that the two models are feasible and effective; and the results can well explain the variation of the pile length, soil parameters and pile-soil parameters of the super-long and large-diameter steel pipe pile on shaft resistance and bearing capacity.(2) By analyzing the bearing capacity and settlement data of the super-long and large-diameter steel pipe pile, it extends the two factors bearing capacity variation and settlement variation which reflect the changes of the settlement on bearing capacity in the process of solving the settlement. The two factors increase the expansion of the data processing content, and make the model more accurate. On the basis, the models SVM-S, ICASVM-S, TANEXP-S for solving settlement are established, and the three models are used to analyze and calculate the influence on settlement caused by different combinations of the three factors bearing capacity, bearing capacity variation and settlement variation. The results show the above three kinds of model can accurately predict the pile settlement, which can well explain the variation of bearing capacity of the super-long and large-diameter steel pipe pile on the settlement. (3) The change of soil parameters and settlement of the same area with bearing capacity has some common features. Through the study of the data of a pile of the same area, this paper use support vector machines and independent component analysis to establish support vector machine model which reflects the soil characteristics and support vector machine model which reflects the change of settlement; then, on the basis of this it obtains the corresponding soil characteristics by the data of other piles, and establishes model LearnPre-Q predicting the frictional resistance of different pile of the same area and predicting model LearnPre-S predicting settlement. Experimental results show that the proposed LearnPre-Q and LearnPre-S prediction is accurate, and verify the algorithm feasible and effective; at the same time the results is the same with the results of Chapter 2 and Chapter 3. It can better explain the variation of bearing capacity and settlement of the super-long and large-diameter steel pipe pile.