Reliability-based design of large-diameter bored piles in Hong Kong―Part I: Evaluation of design methods

摘要

Large-diameter bored piles are the most commonly used foundation type in Hong Kong. Many of the piles are embedded in saprolites, either granitic or volcanic, and/or sccked into bedrocks. Several design methods are widely used for estimating the shaft resistance of large-diameter bored piles in saprolites and the toe and shaft resistances of rock sockets. The methods include "rule-of-thumb' methods and correlations with the Standard Penetration Test (SPT) blow count or the Unconfined Compressive Strength (UCS) of rocks. Although these design methods have been in practice for years, a formal evaluation of the reliability of these methods has not yet been conducted. This paper is the first of two papers that aim to comprehensively evaluate the reliability of large-diameter bored piles in Hong Kong. In this paper, common construction procedures and design methods for large-diameter bored piles in Hong Kong are briefly described. Data of the ultimate shaft resistance and toe resistance of large-diameter bored piles from a number of full-scale tests in Hong Kong load tests in Hong Kong are collected. These data are used to evaluate the reliability of the design methods using a first-order, second moment reliability method. The "rule of thumb" methods for the shaft friction in saprolites and the toe resistance in rock socket are found to be associated with a reasonable probability of exceedance, whereas the correlations for the shaft friction in saprolites and rock sockets appear to have a large coefficient of variation and may result in unsafe designs.