Roanoke Rapids Dam

FIELD INSPECTIONS AND ANALYSIS OF THE ROANOKE RAPIDS DAM UNCOVER STRUCTURAL DISTRESS

The Roanoke Rapids Dam is a concrete gravity dam owned and operated by Dominion Power for the purpose of generating electricity. The dam is approximately 80 feet high and 3,000 feet long and includes 24, 24.5-foot-wide by 38-foot-high radial gates and four 26 megawatt power-generating units. The dam has a drainage area of more than 8,000 square miles and is located approximately eight miles downstream of the Gaston Dam, another hydroelectric facility where Gannett Fleming performed Federal Energy Regulatory Commission (FERC) Part 12 inspections and Probable Failure Modes Analyses.

Gannett Fleming was retained by Dominion Power to update the stability analysis for Roanoke Rapids Dam. We performed an onsite inspection because of concerns with structural movements and increased seepage patterns. Following a phased approach, we worked with Dominion to thoroughly investigate what turned out to be a problem with ongoing concrete distress resulting from alkali-silica reaction (ASR). In addition, during this time period, we performed FERC Part 12 inspections and the first Probable Failure Modes Analysis for Roanoke Rapids Dam.

What We Did

The ASR investigations and analyses led to resolving immediate stability concerns and a long-term plan to extend the service life of the structure. Working with a team of subconsultants, we:

Performed onsite inspections.
Reviewed the history of the structure.
Performed exploratory borings.
Installed additional instrumentation.
Tested concrete cores for current hardened properties.
Performed petrographic testing of the concretePerformed overcoring to determine in-situ stresses within the dam.
Performed underwater (diver) inspections of the upstream face of the dam.
Performed downhole video imaging.
Performed finite element analyses to assess potential future stresses because of the ongoing concrete growth resulting from the ASR and to evaluate remediation options to address. stability and seepage concerns.
Developed a long-term plan consisting of monitoring and future re-evaluations to safely extend the service life of the structure.

Key Features

At the end of a suite of inspections, investigations, and analyses, the owner and dam safety regulatory agencies were provided with:

An understanding of the root cause and extent of the stability and seepage issues, which was ASR–an incurable reaction that results in concrete deterioration–along with anticipated future concerns.
A conceptual design layout, which included post-tensioned anchoring and grouting, to address immediate dam stability concerns.
A plan for long-term management of the ASR issue to safely extend the service life of the structure.

Outcomes

Improved public safety of the dams and surrounding environment for swimming, fishing, boating and other water sports.
Fish and wildlife preservation.
Addressed dam safety stability concerns.
Implemented a plan to extend the service life of the structure.
Critical infrastructure remains in service.
Meets energy needs of the region in an environmentally responsible manner.
Provides recreational opportunities on and around the lake formed by the dam.

CLIENT

Dominion Virginia Power

LOCATION

Roanoke Rapids, N.C.

ROLE

Structural Movement and Seepage Investigations, Exploratory Drilling, Instrumentation, Structural Stability Analyses, Concrete Materials Evaluations, Petrographic Analyses, FERC Part 12 Inspections, Probable Failure Modes Analyses