||Large-diameter bored piles are a common type of foundations adopted in Hong Kong. As bored piles are constructed underground, visual examinations of potential pile defects are not feasible. Thus, quality assurance measures such as integrity tests and static loading tests play an important role to ensure the construction workmanship and quality. According to results from quality assurance tests, pile imperfections, especially at the pile toe, are not uncommon. As large-diameter bored piles are usually designed as end bearing piles, engineers concern much about how toe defects affect the pile performance. This thesis aims to investigate the occurrence of pile defects and the effect of pile defects on the behavior of large diameter bored piles. The construction and quality assurance of large diameter bored piles in Hong Kong were first reviewed. Survey studies on the causes and occurrence rates of pile defects were then carried out. Lastly, five centrifuge model tests and corresponding numerical simulations were carried out to study the effect of pile defects on the performance of pile foundations. In the survey study, a questionnaire survey on quality assurance measures and occurrences of potential pile defects was conducted. Besides, several case studies in Hong Kong on potential defects such as toe debris and cracks were carried out. About 86% of the pile toes in the investigated projects are found intact. Moreover, the occurrence rate of toe debris has been decreased gradually with time as a result of the extra attention paid to the construction workmanship and advances in construction techniques. In the centrifuge modeling, two series of static loading tests were performed to simulate large-diameter bored piles 2 m in diameter. The first series of tests included two tests on single piles while the second series of tests included three tests on pile groups. Excessive settlement was measured from the defective single pile and there was a 30% reduction in its ultimate capacity. In the pile groups with defective piles, the applied load was largely taken by piles without defects. The reductions in the ultimate capacities of the pile group with soft toes and the pile group with short piles were 14% and 51%, respectively. When the applied vertical load was large, lateral failure mechanisms could also be induced even if the applied vertical load was concentric. FB-Pier, a finite element program, was adopted for the analysis of the defective single piles and pile groups simulated in the centrifuge. In general, the results from the numerical analysis were in good agreement with the results obtained from the centrifuge tests.