Study On Capacity Behavior Of Single Uplift Piles

Uplift piles are the main style of uplift foundations. Compared with researches oncompressive piles, researches on uplift piles are deficient and main focus on the capacity but notthe deformation. Nowadays, designing uplift piles always follows compressive piles justmultiplying a uplift factor. Nevertheless, it lacks to fulfill calculating the true capacity of upliftpiles .The mechanism of capacity mobilization between tensile piles and compressive pilesremains a puzzle. This thesis carries out some studies concentrating on these two problems. Themain contents and conclusions are abstracted as follows.1. An extensive literature review for the existing theories is presented. Most of studiesfocused on the capacity but few of them did their works about mechanism between uplift andcompressive piles, as well as the deformation difference.2. The thesis analyses the theoretic behavior of uplift piles. The instance analysis showsthat preconsolidated stress of soil can elevate the pile skin friction in an excavated area. Areasonable formula calculating the ultimate capacity in preconsolidated areas is proposed basedon the experimental esults. For uplift piles, except for those with low slenderness, thedeformation of shaft dominates the pile head displacement. There are power function, exponentfunction and hyperbola function which can fit the Q-S curve of uplift piles. It is concluded thatthe hyperbola function can fit best among the three functions.3. The numerical analysis shows that, under the same condition, the resistance of upliftshaft is lower than that of compressed shaft, and the ratio is 0.84 on average. Through comparingthe radius displacement, it is found that the soil is "active" when pile subjects to tensile force, onthe contrary, soil being "passive" when pile subjects to the compressive force. This discrepancyis likely to result in the difference of normal stress on pile-soil interface which makes the ratioless than 1. The diameter of uplift pile affects the load transfer actions. The pile-soil relativedisplacement which can mobilize the ultimate shaft capacity is 1.2% of diameter on average.4. Finally,the 3-D finite element method is applied to simulate the uplift pile with somesimplified condition, such as the soil treated as an elastic continuum and only unisonousdisplacement between pile and soil. Numerical results prove that the 3-D model can simulate thepile-soil interaction when the upload is not more than the working load.