Formation Of Negative Skin Friction And Its Time Effect Of Single Pile

The pile foundation is one of the oldest foundations. According to the references, the pile has been used for thousands of years. With the high-speed construction of city and the erection of high-rise, various types of piles have been presented. The pile foundation shows it's unique fascination in the aspect of base processing, at the same time more strict design plan was needed in the pile foundation design.Because of the complex of pile-soil interaction, the study on this subject is far from clear so far. With the plentiful use of pile foundation, many problems were presented and theory lagged behind the practice, espescially in the aspect of pile negative skin friction. The inadequacy in understanding of pile negative skin friction brings some engineering problems after some pile foundation project putting into use. Based on the in-situ static loading test of friction piles in xi'an district, the paper thorouly details the negative skin friction mechanism and its time effect of single pile.At first, the paper analyses the load transferring mechanism of single pile and summarises five different load-transferring functions. Meanwhile combining with elastic theory and load transferring theory, the paper establishes improved model of the pile-soil interaction, which perfects the existing load transferring theory.Furthermore, the paper discusses mechanism, forming condition and distributing of negative skin friction respectively. Based on the improved model of pile-soil interaction, the general expression of the negative skin fricition is reached. The deciding method of the negative skin friction which is aroused by external load, the piezometric variation of two free drainage boundaries and Loess collapse is gotten under the two conditions of considering time effect and T→ ∞.Assumed T→∞ and given shear moduli of the soil is constant, the expression of the negative skin friction, the axial force and the neutral point of single pile are deduced. Finally, a case of pile foundation of power staion in Baoji city is studied. Appling the above-deduced expression of negative skin fricition, the axial force and the neutral point of single pile to this case, the paper forcasts the negative skin friction as well as the neutral point of the single pile. The forcasted result is well consisitented with the test result, which shows the method presented in this paper is effective and valuable.