In Taipei Basin, groundwater head drawdown of Chingmei gravel layer is a measure often adopted for maintaining stability and retarding damage of large  scale deep excavation. Huge groundwater pumping amount, long operational duration, and expensive electrical and dewatering facilities associated with man ' s activities significantly affect project costs. The composition of Chingmei formation varies throughout Taipei Basin, and thus the hydrological parameters differ significantly in regions. To meet the practical requirements, the reliability method was applied such that those uncertainties can be evaluated from the perspective of probability. Results of reliability analyses of the piezometric head drawdown of Chingmei formation for deep excavations indicated the probability of failure or factor of safety being less than one varies with uncertain conditions whereas the mean values of the factor of safety are only insignificantly affected by these uncertainties. In addition, results of analyses showed design results are most sensible to transmissibility and piezometric head. Thus, design reliability can be improved by mitigating the uncertainties of these parameters.