Case Study: Unexpected Cobble and Boulder Layer Slows Foundation Work in Oregon (2018)

Project Overview
• Name: I-5 Bridge Widening
• Location: Portland, Oregon
• Year: 2018
• Project Size: $190 million
• Scope: Bridge widening with pile foundation construction
• Lead Agencies/Contractors: Oregon DOT /

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

Summary of the Issue, Problem, or Challenge
During pile driving, thick layers of cobbles and boulders were encountered, complicating pile installation and requiring specialized equipment.

Root Cause Analysis

• Limited borings failed to identify extent of cobble/boulder layers.
• Underestimated soil resistance led to inadequate piling equipment selection.
• Contract specifications did not anticipate difficult ground conditions.

Impacts Due to the Issue, Problem, or Challenge

• Delay of 3 months due to equipment changeover and slow pile installation.
• Increased costs from equipment rentals and longer labor hours.
• Potential structural risks if pile refusal zones were not addressed.

Corrective Actions Taken

1. Supplemented borings with geophysical surveys to map cobble zones.
2. Rented heavy-duty pile driving rigs capable of handling obstructions.
3. Reassessed pile design to ensure adequate embedment despite obstructions.
4. Updated contract clauses to handle unforeseen subsurface obstacles.

Lessons Learned

• Geophysical surveys useful to supplement limited borings in complex ground.
• Equipment selection must consider potential obstructions in subsurface.
• Contract terms should clearly allocate risks for subsurface difficulties.

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

• Are geophysical surveys included in subsurface investigations?
• Is piling equipment matched to the expected ground conditions?
• Are contracts clear on risk and responsibility for unexpected obstructions?