Case Study: Unforeseen Glacial Till Layers Impact Highway Foundation in Minnesota (2017)

Project Overview
• Name: I-94 Expansion Project
• Location: Minneapolis, Minnesota
• Year: 2017
• Project Size: $350 million
• Scope: Highway widening with deep foundation systems
• Lead Agencies/Contractors: Minnesota DOT /

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

Summary of the Issue, Problem, or Challenge
Unexpected thick layers of dense glacial till with high shear strength were encountered, complicating excavation and pile driving efforts.

Root Cause Analysis

• Preliminary borings underestimated thickness and density of glacial till.
• Limited use of cone penetration tests (CPT) for soil profiling.
• Contract lacked provisions for dealing with hard, abrasive soils.

Impacts Due to the Issue, Problem, or Challenge

• Schedule delay of 4 months due to slower excavation and pile installation.
• Increased equipment wear and replacement costs.
• Additional costs for specialized drilling techniques and tooling.

Corrective Actions Taken

1. Conducted detailed CPT and geophysical surveys for better soil characterization.
2. Utilized specialized heavy-duty drilling rigs and wear-resistant drill bits.
3. Revised project schedule and contract terms to accommodate subsurface challenges.
4. Implemented enhanced contractor communication on soil variability risks.

Lessons Learned

• Accurate characterization of glacial till requires advanced geotechnical tools like CPT.
• Contracts must address hard soil conditions to allocate risks fairly.
• Equipment selection should consider abrasive soil impact.

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

• Have CPT or similar profiling tests been performed?
• Are drilling equipment and tooling suitable for hard soils?
• Do contracts address differing site conditions such as glacial till?