Intake Tower Seismic Upgrade with Post-Tensioned Anchors: Design and Construction Aspects

The South Fork Tolt Dam Intake Tower was designed and built in accordance with the standards of practice current in the early 1960's. Thirty years later, analysis based on modern structural design criteria and updated seismic hazard data concluded that the tower was highly vulnerable to earthquake damage and needed strengthening. Due to the critical importance of the water supply, the Owner mandated that the reservoir remain in service and that water quality be protected during the repair work. The innovative seismic upgrade design called for six high-capacity post-tensioned anchors, installed into holes drilled with high accuracy down through the thin walls of the tall, slender structure. Specialized construction techniques and equipment were employed to successfully complete this challenging project, which required some of the largest and longest post-tensioned anchors ever used to seismically strengthen an intake tower structure.