Study On In-situ Test Of Uplift Capacity And Nonlinear Deformation Analysis Of Vertical Single Uplift Piles With Equal Section

Currently, designing uplift piles always follows compressive piles just simply multiplying an uplift factor. Nevertheless, many field tests prove that this designing is too conservative, but sometimes even dangerous. However, compared with researches on compressive piles, researches on uplift piles are mainly focus on the capacity but not the deformation. Furthermore, the most of studies are interesting in the"capacity"of the side resistance and neglect the differences of characteristics of skin friction mobilization which can significantly affect the ultimate uplift capacity. Lastly, the design of uplift piles is consumptive economically, just adopting higher concrete mark of shafts, higher reinforcement ratio of shafts, longer length and wider diameter of piles. Aimed at the problems mentioned above, this paper carries out studies by in-situ test, theoretic analysis and numerical simulation.In-situ test is an effective method to recognize the capacity of uplift piles. The in-situ test in the paper includes three uplift piles and a compressive pile. Four piles are all instrumented with strain gauges to measure the axial load distribution and unit skin friction at all load increments. Base on the testing data, the crack of shafts, the evaluation of uplift coefficient, the mobilization of skin friction, the deformation of shaft and the ultimate skin friction are discussed. The results show that the critical pile-soil relative displacement of uplift piles which is needed to mobilize the ultimate skin friction is smaller than that of compressive piles. Under the same load, the deformation of uplift shaft is much heavier than that of compressive shaft. Base on the testing data, the crack of shafts, the evaluation of uplift coefficient, the mobilization of skin friction, the deformation of shaft and the ultimate skin friction are discussed. The results show that the critical pile-soil relative displacement of uplift piles which is needed to mobilize the ultimate skin friction is smaller than that of compressive piles. Under the same load, the deformation of uplift shaft is much heavier than that of compressive shaft. When subjected to the ultimate load, the head displacement of uplift piles is mainly supplied by the deformation of the shaft, but that of compressive piles is mainly contributed by the toe settlement of the pile due to the magnitude of deformation stiffness between the two piles. A formula to calculate the elongation of uplift pile is proposed based on some reasonable hypotheses.The concept of slender uplift piles is put forward according to the designing principle by deformation control and the dynamic procedure of skin friction mobilization. Piles when L/D>30 can be defined as slender uplift pile. The concept of skin friction mobilization factor of slender uplift piles and a formula to obtain the ultimate capacity of slender uplift piles are also given.Based on the load-transfer method, the static-load-balance method which is particularly suitable to simulate the capacity behavior of uplift piles is brought forward. Considering the tensile nonlinear characteristics of the reinforced concrete shafts, using the uniform bilinear load-transfer function, this paper analyses the deformation of uplift piles in uniform stratum and the effects of the concrete mark of shafts, the reinforcement ratio of shafts, the length of piles, the diameter of piles to the uplift capacity. Correspondingly, Design recommendations are also proposed.