Study On Bearing Characteristic Of The Cast-in-Place Concrete Pile With Screws

The cast-in-place concrete pile with screws is a kind of newly-patented technique developed in recent years, which was used successively in some provinces of our country. But on the whole, this technique is at the early stage of development, research work is not systematic. The failure mechanism and bearing behavior of the pile under compressing loads and pulling loads are not clear. A reliable and practical formula to calculate the vertical bearing capacity of a single pile is also lack.This dissertation try to carry through the in-situ static loading test of the pile with screws, and study systematically that the load transferring regulations of this kind of pile under the compressing load based on the test and stress measuring of pile shafts, and analyze that the bearing characteristic of the pile with screws under the help of FLAC^3D.The following is some conclusions.1. With results of the model tests, the circular-pile and the pile with screws were compared in characters such as loading-settlement relationship, distribution regulations of the axial force, friction resistance and the base resistance. The study shows that the friction resistance of the circular-pile is distributed equably along the pile, the axial force of the pile reduces with the depth. With a small stress, the shaft force is almost all taken by the shaft friction. As stress becomes greater, the shaft friction increases more quickly. But in the late period, the base resistance increases faster than the friction resistance, the friction resistance come to the maximum before the base resistance. In the cases of the pile with screws, at the early stage of loading, the friction of upper part of the shaft increases sharply, the lower part of the shaft have almost no stress. With the load increasing, the friction of upper shaft reaches ultimate state, the middle part takes more and more force until reaches ultimate state, the bottom part only uses about 40～60% of its capacity, and the axis forces also decreases sharply.2. The load-transferring regulations of the friction resistance and the base resistance of the pile with screws are analyzed, the load-transferring regulations of the friction can be presented with hyperbola function.3. It is found in practical cases that piles with screws can undertake capacities 1.8～2.2 times of that of circular piles. This is mainly caused by the interactions of screws, soils and pile. The ultimate friction of the pile is relative to shear strength of soils. Also, the base resistance undertakes less force in the case of the pile with screws.4. Some practical examples were introduced and the ultimate capacities were analyzed for the pile with screws. The rest results shows that the vertical bearing capacity is 2～2.5 times of its circular counterpart, which is identical with results of experiment and theory analysis.5. FLAC^3D was applied to analyze the stress and displacement field in accordance with the model tests. The regulation is very obvious that the shaft resistance attenuate from up to down. The z-displacement of the ground caused by the pile with screws decreases with the depth increasing. When the load reached ultimate state, the soils around the screws reached shear failure state. The relationship between the screw-pitch/diameter ratio and the bearing capacity of a single pile is also analyzed. The screw-pitch is an important factor to the bearing capacities; it should be no more than 3 in practical design.6. A formula is presented to calculate the vertical bearing capacity of the pile with screws based on experience. Through the analysis on the practical examples, a formula supposed to calculate the bearing capacity of the pile with expanded branches or plates in《Temporary technique specification for the pile with expanded branches or plates of firepower plant》is modified. Considering the resistance attenuation from up to down, a depth corrected coefficientη_pi is put forward, and a modified formula is given out. The data we got from the modified formula matches the ones that come from experiment. So it's reasonable to use this formula to calculate the vertical bearing capacity of the pile with screws.7. Based on engineering examples, we got the data of the pile with screws of actual sites. With some analysis, it came out that the pile with screws is more efficient, more environmental-friendly and more economical than the traditional type. The data also bring forward the idea of using the hyperbolic method to predict the limited vertical bearing capacity of a single pile. After a comparison between statistics from the predicted data and the measured data, we believe the hyperbolic method is an available and reliable way to get the limited bearing capacity of the pile with screws.