Abstract: The construction technology and quality of pile foundations in thick sand layer sites are significantly affected by geological conditions. To study the influencing factors to the quality of pile foundations in thick sand layers and analyze the applicability of detection methods, this paper uses the cross-hole sonic logging and the core drilling method to test the cast-in-place piles in the core tube section of a certain project. It is found that the abnormalities detected by the cross-hole sonic logging are mainly concentrated in the thick sand layer positions, while the corresponding defects verified by the core drilling method are mainly mud or sand inclusion. Based on the comprehensive analysis of geological conditions, it is believed that the main cause of the defect is that the high-pressure water-sand layer was disturbed during the hole formation of the pile foundation, causing the hole to collapse and then invading the pile body. The construction process has poor adaptability to the site. Both core drilling and acoustic transmission methods can be used to reveal the stratigraphic conditions. However, core drilling often terminates due to sand inrush or hole deviation, making it difficult to cover the entire pile length, while acoustic transmission can accurately evaluate the quality of the entire pile. In the early stages of construction, using acoustic transmission to inspect key parts of the foundation pile can identify defect characteristics, allowing for early engineering treatment or adjustment of construction techniques to prevent the expansion of engineering risks and economic losses.