Reduction of water inflows into the City Creek Segment of Inland Feeder An''owhead East Tunnel by grouting resulted in the buildup of hydrostatic pressures behind an existing pre-cast concrete primary support system designed only for ground loads. The maximum pre-construction groundwater head along the approximately 2,440 meter long TBM driven tunnel exceeds 335 meters. The pre-cast concrete segmental primary support system was designed as an unsealed (non-gasketed, non·bolted) system. The segments were expanded against the excavated tunnel surface to provide immediate ground support behind the TBM. Reducing water inflows into the tunnel required sealing of the pre-cast concrete segmental primary suppOli to the extent possible through various and extensive grouting techniques. The resulting buildup of hydrostatic pressures behind the concrete segments initiated the evaluation of the design of the installed primary suppOli to carry additional hydrostatic loads. Design changes of the pre-cast concrete primary support system to carry the added hydrostatic heads and allow continuation of the tunnel drive were also considered. The design changes are applicable for the segments to carry a maximum hydrostatic head of 244 meters with excess water head to be relieved as needed. The design changes for the segments had to be compatible with the Contractors existing TBM segment erection system. Additionally, a ground water monitoring program was developed to assure the buildup of hydrostatic pressures did not exceed the load carrying capacity of the already installed primary support system and compromise the safety of the tunnel. The monitoring program included installing piezometers to monitor buildup of hydrostatic head behind the primary suppOli, and a water relief system to limit the hydrostatic head buildup.