Design and Construction of the Foundations for the Watauga Raw Water Intake Facility in Karstic Limestone near the City of Johnson City, TN

The City of Johnson City constructed a new raw water intake structure on the banks of Watauga River near Johnson City, TN. The structure consists of an above-grade electrical room and pump house connected to a T-shaped clearwell that extends out into the river approximately 15 ft. below normal water level. The site is in the Valley and Ridge Physiographic Province of Northeast Tennessee and is in a region of valleys and hills formed on the less-resistant limestones, dolomites and shales. The bedrock at the site is a karstic limestone rock from the Knox Group. The original foundation design for the intake structure was a structural mat with wall footings on bedrock. Uplift resistance was to be provided by a series of rock anchors. However, when the Contractor mobilized to the site and drilled a series of probe holes and excavated test pits around the perimeter of the clearwell to determine the top of rock, they did not indicate the presence of any competent rock. Instead, the Contractor extracted numerous large boulders of limestone rock found within a matrix of residual soil. The boulders were highly irregular in shape and exhibited signs of weathering on all sides. The Owner then contacted Mueser, Rutledge Consulting Engineers (MRCE) to evaluate the conditions and to recommend a foundation system suitable for the difficult site. Additional borings were made at the site and MRCE ultimately recommended using mini-piles to support the new structure, a slurry wall cofferdam around the perimeter to support the excavation and to limit lateral seepage, and an extensive grouting program to help limit the under-seepage into the excavation. The work was subsequently performed by a specialty sub-contractor, and MRCE provided oversight during construction. This paper describes MRCE's investigations, findings and construction methodologies that led to the successful completion of the project.