Sometimes Bigger is Better: VE Drilled Shaft Design at Chickamauga Lock and Dam-PPT Presentation

PPT Presentation

Pdf Summary

The Chickamauga Lock replacement project includes a new 110-by-600-foot lock, replacing a deteriorating 60-by-360-foot structure. The $954 million project’s approach-wall contract used drilled shafts in challenging subsurface conditions that included limestone, shale, clay seams, voids, cobbles, and boulders.<br /><br />The baseline design called for 28 shafts, each 6.5 feet in diameter, with 6-foot rock sockets 25 to 45 feet long. Closely spaced shafts and congested reinforcement created construction challenges. A value-engineering (VE) redesign, developed by the geotechnical and structural engineers in collaboration with the contractor, instead used 14 larger, 8.5-foot-diameter shafts. Most had 8-foot-diameter sockets about 16 feet long; three received longer sockets to address karst conditions.<br /><br />Because the original plans lacked axial design criteria and shaft capacity information, a bidirectional load test was performed. Its results supported the design’s use of measured unit resistance and an axial resistance factor of 0.5. During construction, some socket lengths were adjusted, several sockets required stabilization and redrilling, and two experienced significant concrete overbreak.<br /><br />The VE design reduced reinforcement congestion and improved constructability in difficult ground. It saved an estimated $3.4 million—about 5.6% of the approach-wall contract—and may have prevented larger cost overruns. The presentation emphasizes that drilled-shaft design and construction should inform one another, and that close coordination among designers and contractors can reduce risk while delivering savings.

Keywords

Chickamauga Lock

lock replacement project

drilled shafts

approach wall

value engineering

bidirectional load test

geotechnical engineering

karst conditions

constructability

cost savings