In a sewage system development project, pipeline construction is the key work owing to pipeline construction shares high cost compared to other works. Furthermore, pipeline construction usually suffers unforeseen site conditions and results in tremendous delays. Identifying potential delay causes is the basis for resolving delay problems before they actually occur. As the budget of sewage systems is increased annually in Taiwan, identifying delay causes and preventing them from occurring are essential for successful sewage system construction projects. Through examining fifteen real cases and validating via a questionnaire survey, this study concluded that moving existed conduits later than planned schedule, unforeseen site conditions and delayed construction planning report approval are the top three significant causes for pipeline construction projects. Identifying the causes is valuable for reference to succeed in future similar projects.