Case Study: Variations in Bedrock Elevation Affect Dam Foundation Excavation (Kentucky, 2020)
Project Overview
• Name: Kentucky River Dam Rehabilitation
• Location: Kentucky River, Kentucky
• Year: 2020
• Project Size: $320 million
• Scope: Dam foundation rehabilitation involving deep excavation and grouting
• Lead Agencies/Contractors: Kentucky Department of Fish and Wildlife Resources /
Category of the Issue, Problem, or Challenge
• Geotechnical
• Differing Site Conditions
Summary of the Issue, Problem, or Challenge
Substantial variability in bedrock elevation was discovered during foundation excavation, requiring additional excavation, redesign of grout curtains, and extended construction duration.
Root Cause Analysis
- Initial subsurface investigations underestimated bedrock surface irregularity.
- Limited number of exploratory boreholes leading to poor characterization.
- Insufficient geophysical survey coverage.
- Contract provisions did not fully address variability in bedrock conditions.
Impacts Due to the Issue, Problem, or Challenge
- Schedule delay of 6 months for excavation and redesign.
- Cost increases due to extended excavation and additional grouting.
- Increased risk of seepage if grout curtain was inadequately designed.
Corrective Actions Taken
- Conducted detailed geophysical surveys and additional boreholes.
- Redesigned grout curtain to address bedrock variability.
- Enhanced contract terms to allocate risk for subsurface variability.
- Improved geotechnical and construction coordination during excavation.
Lessons Learned
- Detailed subsurface mapping of bedrock is critical for dam projects.
- Early geophysical surveys complement borehole data.
- Contracts must consider bedrock variability risks explicitly.
Audit & Prevention: Project Control Questions to Ask on Future Projects to Help Control the Situation
- Are bedrock elevations thoroughly mapped pre-construction?
- Is grout curtain design adaptable to bedrock variability?
- Do contracts clearly assign risk for unexpected subsurface conditions?