Challenges in Active Dam Rehabilitation
The City of Winchester owns and operates the Percy D. Miller Water Treatment Plant in Warren County, Virginia which pulls source raw water from the North Fork of the Shenandoah River. The Plant, and the associated low head sill dam, raw water intake and pumping station, were constructed in the 1970s and are the sole source of drinking water supply to the City. The dam consist of a 170-foot long concrete ogee weir across the river with end walls constructed on stone masonry on each end.
In 2017, the City observed deteriorated of the end walls characterized by migration of soils through the wall and shifting of the stone masonry. The City also expressed concern about the existing low level outlet which was damaged by floating debris and was blocked with steel plate held in place solely by hydrostatic pressure from the river. Based on those primary concerns, in 2018 Hazen was contracted to perform a condition assessment of the dam and to develop potential emergency measures for the City to enact immediately in the event of dam or sidewall failure.
Preliminary analysis completed through 2018 and into early 2019 provided design recommendations which were subsequently accepted by the City and detailed design commenced. However, subsequent coordination with the property owner on the right of the dam led to delays that ultimately required the project to move forward with the intent for a future contract to address the right end wall. Documents were issued for bidding in July 2020.
Construction commenced in January 2021 and was completed by end of 2021. During the course of construction coffer dams were established to permit access and construction and, at that time, differing conditions in the geometry of the existing low level outlet were noted which required quick collaboration between all team members to help define the required work on the fly during construction. Due to the ongoing critical in-water construction activity and the potential cost of a lengthy delay, quick design collaboration kept construction moving without demobilization. Materials were tracked by the on-site team and swift communication and collaboration by the team were paramount to successful outcome. A diver visual inspection of the dam was also completed as a part of construction to confirm acceptable conditions in other areas of the dam.
At present, the City intends to move forward with the right end wall of the dam as a future standalone construction project.