Studies On Effects Of Excavation On The Load Transfer Of Supper-long Piles And Its Settlement Calculation

The high-rise and super high-rise buildings promote the development of super-long piles and super-deep excavations. The load transfer mechanism of super-long piles is different from that of short piles. The working performance of the super-long piles beneath the excavation bottom is more complicated. It is quite necessary to research the bearing behavior and settlement calculation theory of super-long piles.Firstly, the characteristics of super-long piles are analyzed based on the measured static load test data. It is shown that the shaft resistance of super-long piles mobilizes from pile top to tip. The shaft resistance distributes in a single hump pattern under working load. The hump is usually located at the middle of the pile length, and its location moves upward as the slenderness ratio increases. Under working load, the super-long piles have small tip resistance ratios, most of which are not more than 2%.Furthermore, load transfer mechanism of different length of piles is analyzed by finite element method. According to the working performance of super-long piles, some improvements are made to the traditional settlement calculation method of pile foundation. It is shown that, under working load, the load of the short piles is mainly undertaken by the lower piles shaft, while that of the long piles is carried by the middle and upper piles shaft. The shaft resistance distributes in a triangle pattern and a single-hump pattern for the short and long piles respectively. Therefore, the settlement of super-long piles calculated by single hump distribution is closer to measured data than that of Geddes'solution.The load transfer mechanism of super-long piles beneath the excavation bottom is studied by finite element analysis. The additional stress in foundation soil and settlement calculation method of super-long piles under excavation are analyzed. It is shown that excavation could lead to tension in piles and a decrease of piles stiffness. The shaft resistance and tip resistance of piles beneath the excavation bottom are mobilized asynchronously due to excavation. This is adverse for transferring load to lower pile shaft and mobilizing bearing capacity. The shaft resistance distributes in a triangle pattern and an'R'pattern for the short and long piles respectively. The shaft resistance is mainly mobilized along the upper part of the piles for super-long piles. The friction on the lower part is even negative for piles longer than certain value. The settlement considering the distribution and swelling-recompression of the soil under excavation is closer to measured data than that of Geddes'solution and single-hump solution.The parametric study on piles under the condition of excavation is conducted using finite element analysis. Models of different excavation geometries and different piles lengths are calculated. The reduction of stiffness, maximum tension and its position, and the distribution of shaft resistance are analyzed.It is shown that the reuduction of piles stiffness due to excavation varies with different positions. The maximum, medium and minimum reduction occurs at center, side, corner piles respectively. The different pile stiffness may intensify the disk-shape settlement, which even could cause the fracture of raft and superstructure. Excavation can cause tension in the full length of piles. The maximum, medium and minimum tension occurs at center, side, corner piles respectively. The maximum tension of piles should be taken into consideration in design for deep excavations and long piles.Finally, the distributions of shaft resistance under working load of different pile lengths and excavation geometries are presented. A software is designed based on MATLAB platform, which takes the different distributions of shaft resistance into account in calculating the settlement of pile foundation.