Reliability of bored pile foundations considering bias in failure criteria

The failure of a pile is always defined by a certain failure criterion. Several different failure criteria are commonly used, and the pile capacity values associated with each of these failure criteria can be considerably different. For the sake of international harmonization, it is necessary to calibrate the reliability levels associated with various failure criteria and factors for loads and resistances. This paper aims to evaluate the effects of failure criteria and factors for loads and resistances on the reliability of single bored piles. The bias arising from failure criteria is described by a bias factor, which can easily be accommodated in a reliability analysis. A comprehensive database of static load tests of bored piles is utilized to evaluate the bias associated with several failure criteria. Five limit-state design codes for piles are investigated to illustrate the effect of the bias from failure criteria, the effect of factors for loads and resistances, and their combined effect. The results indicate that the bias from failure criteria has a significant influence on the reliability of piles. Similarly, the use of different factors for loads and resistances in various design codes can also cause considerable differences in the calculated reliability. As a result of these effects, the actual reliability levels of any two design codes, assuming the same nominal target reliability index, can differ considerably.