This study focuses on the quantification of the effect of limited site investigations on the design and performance of pile foundations. Limited site investigation is one of the main causes of structural foundation failures. Over the last 30 years, most site investigations conducted for infrastructure projects have been dictated by minimum cost and time of completion, rather than meeting the need to appropriately characterize soil properties. As a result, limited site investigations remain common, resulting in a higher risk of structural foundation failure, unforeseen additional construction, and/or repair costs. Also, limited site investigations can result in over-designing foundations, leading to unnecessary cost. Based on the reliability examination method for site investigations, the reliability of the foundation design was analyzed with a probabilistic approach, using the Monte Carlo technique. The levels of spatial variability of the soil were found as notable factors affecting the effectiveness of site investigations. The results will assist geotechnical engineers in planning a site investigation in a more rational manner with knowledge of the associated risks.