CS10C - Concurrent Session 10C: Navigating Design & Constructability Risks During Construction of Secord Dam

Abstract Description: Secord Dam, built in the 1920s, has long served both hydroelectric power generation and recreation. Following the 2020 Edenville and Sanford Dam failures, its hydraulic capacity was found insufficient to handle larger storm events, requiring upgrades. Minimal maintenance over time led to structural deficiencies and outdated design standards. These factors, combined with challenges such as lack of as-builts, uncertain groundwater levels, interim stability, limited site access, demolition constraints, complex structure geometry, and the need to maintain base flows required flexible design and construction solutions.

Rehabilitation alternatives were evaluated, identifying the right training wall as the primary challenge. Due to its varied rebar, wall thickness, and lateral supports, simply adding a sister wall was insufficient to meet the new loading demands. Instead, a solution incorporating tiebacks, micropiles, and pile caps was developed to enhance the wall’s stability, with external bracing providing construction flexibility while maintaining operation of the Low-Level Outlet and two spillway bays.

A phased construction approach was implemented to minimize disruption to the environment and local communities. This approach included rerouting the river through the auxiliary spillway, integrating temporary dewatering, staged embankment restoration, spillway weir construction, and demolition, all while maintaining real-time monitoring of site conditions. Since the prestressing of tie rods directly impacted wall stability and demolition feasibility, precise staging and continuous coordination between engineers and contractors were critical for maintaining safety and efficiency.

However, limited site access, water control requirements, existing dam conditions, and phased construction further complicated the project’s execution. Adding to this was the complex 3D geometry of the barrel arches and rollway, along with the original design for a 9’ high mass concrete pour as part of the permanent weir structure. During construction, spatial constraints under the rollway and challenges in meeting maximum concrete temperature specifications made this approach impractical. After on-site workshops between the designer, contractor, and owner, the design was modified to use multiple concrete lifts connected by dowels, providing lateral support for the training wall and enabling the staged construction sequence.

This project highlights the importance of continuous communication and design flexibility in navigating design and constructability risks during construction. Collaboration between the design engineer, contractor, and owner facilitated cost-effective adjustments, optimized the schedule, and ensured the safe prestressing of a 100-year-old wall, contributing to the dam’s long-term safety.