The expansion and combination of the two lakes was accomplished by enlarging the Lake Fort Smith Dam from the original top of dam elevation 845 ft. to elevation 946 ft. A portion of Lake Shepherd Springs Dam was removed and used in the construction of the enlarged Lake Fort Smith Dam. Major structures for the project included Lake Fort Smith Dam, outlet works tower, outlet works tunnel, principal spillway, and the auxiliary spillway.

Geotechnical challenges included: 

• The new dam was built on top of an existing dam
• Geotechnical work had to be done within constraints of established design and location
• 80 percent water supply was maintained throughout construction requiring that four of the new intake tunnels to the outlet works be wet taps
• Several inactive faults passing through the auxiliary spillway
• Construction phasing under five separate contracts
• Relocation of state park requiring new roads and facilities be constructed in areas prone to landslides

Shannon & Wilson provided design services, including geologic analysis, geotechnical analysis, embankment design, construction phasing/sequencing, borrow source identification and evaluation, tunnel design, instrumentation design, and cost estimating.  Our geologic analysis included a review of aerial photographs; detailed mapping of the existing rock outcrops, faults, and joints; detailed examination of the project area for problem areas, including earth and rock slides; and a regional seismic evaluation. We explored the subsurface conditions for the project with a series of borings and test pits.  Geotechnical analysis included embankment stability, seepage, and settlement; rock cut slope stability for the auxiliary spillway and the outlet works excavation; tunnel support; principal spillway foundation; misc. retaining walls; and landslide remediation during construction.

Shannon & Wilson provided construction observation on this project from when construction began through completion, and we continue to perform the annual inspection of the embankment, principal spillway, auxiliary spillway, outlet works, and misc. project features.