Case Study: Unexpected Fill Material Causes Retaining Wall Failure in New Jersey (2019)
Project Overview
• Name: Route 35 Coastal Reconstruction
• Location: Monmouth County, New Jersey
• Year: 2019
• Project Size: $250 million
• Scope: Coastal highway reconstruction including retaining walls and embankments
• Lead Agencies/Contractors: New Jersey DOT /
Category of the Issue, Problem, or Challenge
• Geotechnical
• Differing Site Conditions
Summary of the Issue, Problem, or Challenge
During excavation for retaining walls, previously undocumented uncontrolled fill was discovered, causing instability and partial wall collapse.
Root Cause Analysis
- Site investigations failed to detect undocumented fill layers.
- Lack of geotechnical sampling in key excavation zones.
- Design assumptions based on natural soil rather than fill properties.
Impacts Due to the Issue, Problem, or Challenge
- Partial collapse of retaining wall sections causing safety concerns.
- Project halt for 4 months to redesign and stabilize.
- Increased costs due to additional soil testing, excavation, and engineered backfill.
Corrective Actions Taken
- Performed additional subsurface investigations including test pits and borings.
- Redesigned retaining walls with reinforced foundations and drainage improvements.
- Removed unsuitable fill and replaced with engineered compacted backfill.
- Strengthened quality control during excavation and backfill operations.
Lessons Learned
- Thorough subsurface investigation must include testing for undocumented fills.
- Design assumptions should be validated against actual site materials.
- Continuous quality control critical in excavation/backfill stages.
Audit & Prevention: Project Control Questions to Ask on Future Projects to Help Control the Situation
- Were all excavation areas tested for fill presence and properties?
- Are retaining wall designs adaptable to differing soil and fill conditions?
- Is quality control enforced during excavation and backfilling?