Case Study: Variable Soil Conditions Cause Bridge Abutment Settlement in North Carolina (2020)

Project Overview
• Name: US-70 Bridge Replacement
• Location: Wake County, North Carolina
• Year: 2020
• Project Size: $130 million
• Scope: Bridge replacement with new abutments and approaches
• Lead Agencies/Contractors: North Carolina DOT /

Category of the Issue, Problem, or Challenge
• Geotechnical
• Differing Site Conditions

Summary of the Issue, Problem, or Challenge
Unexpectedly soft soil zones beneath bridge abutments caused differential settlement during construction, requiring remediation and foundation strengthening.

Root Cause Analysis

• Soil borings underestimated variability in compressible soil layers.
• Lack of advanced in-situ testing for soil strength variability.
• Insufficient pre-construction ground improvement planning.

Impacts Due to the Issue, Problem, or Challenge

• Delay of 4 months due to foundation underpinning and ground stabilization.
• Increased costs from additional piling and soil stabilization.
• Potential safety risks if settlement not controlled.

Corrective Actions Taken

1. Performed additional soil testing with cone penetrometer tests.
2. Installed deep foundation elements such as micropiles under affected abutments.
3. Applied ground improvement techniques including controlled densification.
4. Increased monitoring of settlement during construction.

Lessons Learned

• Soil variability requires comprehensive testing beyond standard borings.
• Ground improvement should be planned for variable soil zones.
• Continuous settlement monitoring is essential during foundation works.

Audit & Prevention: Project Control Questions to Ask on Future Projects to Help Control the Situation

• Are advanced soil tests like CPT included in site investigations?
• Is ground improvement incorporated into designs where soil variability is high?
• Are monitoring systems in place to detect settlement during construction?