Triad Engineering, provided Dam Safety and engineering services to the Ferndale Lake Landowner’s Association to complete a routine annual engineer’s inspection of the existing conditions of the earthen and rockfill dam structure, as well as the associated dam appurtenances. Shortly after this routine inspection of the impoundment, the owner observed the development of a solution feature approximately midway up the downstream dam slope along the alignment of the principal spillway pipe.
Based on visual inspection, the solution feature was connected to the development of erosion along the principal spillway system, a reinforced concrete pipe (RCP). The erosion at the outfall allowed the end of the RCP to settle, causing separation of the RCP pipe joints and a chimney effect creating subsidence of the overburden material that resulted in a sinkhole on the downstream slope.
Until permanent repairs could be implemented, Triad worked with the Owner to proactively report the Dam Safety issue to the West Virginia Department of Environmental Protection Dam Safety Section. Triad also assisted the Owner with the installation and operation of a temporary siphon system to maintain a pool level below the elevation of the observed failure in the downstream slope while permanent repair measures were scoped and designed. To provide a permanent repair solution to address the failure associated with the principal spillway conduit, Triad developed a scope of work that included a study of the failure and the determination of the likely failure modes, which was attributed to internal erosion along the spillway conduit. This resulted in the development of potential repair and/or modification alternatives and the implementation of the Owner’s preferred design alternative.
Triad developed a construction package and an opinion of the probable construction cost. Specific aspects of the design package included a repair of the primary spillway and subsurface seepage interceptor drains. The spillway repair included the removal and resetting of the misaligned sections of the discharge conduit pipe, installation of a filter diaphragm to allow seepage along the pipe to be collected while protecting soils from internal erosion and the repair of the discharge apron. A new riprap discharge apron was also installed.
Subsurface seepage interceptor drains were also designed and constructed along the toe of the dam to assist with the dewatering of chronically wet surface conditions that were observed along the downstream slopes. This toe drain was recommended to not only improve dam safety by providing filtered seepage collection, but to also improve surface conditions to facilitate the operation and maintenance of the dam.
The construction package was let by the Owner and a qualified contractor was selected for the repairs. Triad provided construction representation as well as quality control testing of construction materials. Upon the acceptance of final completion of these repairs, Triad provided the final engineer’s inspection of the contractors work and updated the emergency action plan and inspection report to bring the dam into compliance with the West Virginia Department of Environmental Protection Dam Safety Section rules and regulation. Triad continues to perform the routine engineer’s inspection for this owner.