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

1. Performed additional subsurface investigations including test pits and borings.
2. Redesigned retaining walls with reinforced foundations and drainage improvements.
3. Removed unsuitable fill and replaced with engineered compacted backfill.
4. 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?