Investigation of Roanoke Rapids Dam: Predicting Future Response

Roanoke Rapids Dam is a mass concrete gravity dam with a crest length of approximately 3050 feet and a maximum structural height of approximately 72 feet. The South Non-Overflow Section (SNOS) of the dam is approximately 581 feet long and was designed to curve in plan toward the upstream, in order to align with the incoming rail line for construction purposes. During recent investigations by Gannett Fleming, Inc., cracking was discovered on the upstream face of the SNOS that has been attributed to Alkali-Silica Reaction (ASR) in the mass concrete. In support of these investigations, URS developed a finite element model and performed a series of analyses to estimate the future response of the SNOS to continuing ASR. First, calibration analyses were performed using measurements from survey monuments, piezometers, inclinometers, and visual observations of the dam. As the data on the SNOS was limited prior to 2000, these preliminary analyses were used to establish conservative parameters for predicting past and future response of the dam to the ASR. Second, several analyses were performed to predict the future response of the existing (unmodified) SNOS. These analyses were used to help establish whether or not there was a need to rehabilitate the existing dam and when rehabilitation measures should be constructed. Finally, several rehabilitation alternatives for the SNOS of Roanoke Rapids Dam were evaluated. These analyses supported the recommendations of Gannett Fleming to install post-tensioned anchors in selected SNOS monoliths to improve the long-term structural stability. The analyses also supported the conclusion that saw-cutting in conjunction with anchoring is not warranted at this time. Furthermore, the analyses supported recommendations to continue the current monitoring program to obtain additional data, and to revisit the finite element analyses in 3-5 years to further refine the estimates of the future impacts of ASR on the Roanoke Rapids Dam. 13 pp., 11 figures, 10 references.