Abstract: The present safety factor of pile foundation judges the safe characteristic of pile-soil system integrally; but the mechanical character of pile is not clear for the end resistance and side friction of pile by using this safety factor. So, based on the bearing mechanism of end resistance and side friction of pile, and different proportion of two resistances to the total bearing capacity, the safety reserve factor of end resistance ( ) and the safety reserve factor of side friction ( ) are put forward; these two different safety reserve factors (SRFs) are objectively judged by strength reduction of nonlinear limit analysis FEM. The inflection points of - curve and - curve, and the previous reduction-factors on the sharp point of V type of - curve and - curve are estimated as the safety reserve factor of end resistance and the safety reserve factor of side friction under the load condition. The analysis of FEM shows that the results of two reduction-factors method are near to the results of static load tests, the load increment method and a safe reserve factor method when the strength parameters of ground (c and ) near to real parameters; and these inflection points of - curve and - curve, and the sharp point of V type of - curve and - curve have the specific physic purport; and the safety reserve factors of pile-soil system (F) by two reduction-factors method are close to the safety reserve factors of single reduction-factors method; and double safety reserve factors of and have some practicality in practice engineering.

Key words: pile foundation, finite elements, limit analysis, strength reduction method, ultimate load, two reduction-factors