Case Study: Unforeseen Rock Layers Disrupt Foundation Excavation on Denver Light Rail Extension (Colorado, 2019)
Project Overview
• Name: Denver RTD Light Rail Expansion
• Location: Denver, Colorado
• Year: 2019
• Project Size: $450 million
• Scope: Light rail track and station foundations
• Lead Agencies/Contractors: Regional Transportation District (RTD) /
Category of the Issue, Problem, or Challenge
• Geotechnical
• Differing Site Conditions
Summary of the Issue, Problem, or Challenge
Unexpected hard rock layers encountered during foundation excavation required specialized drilling and blasting, causing schedule disruption and cost overruns.
Root Cause Analysis
- Limited geophysical surveys and borehole sampling in certain areas.
- Underestimation of rock layer depth and hardness.
- Lack of flexible construction planning for unexpected subsurface conditions.
- Contractual ambiguities regarding handling of unforeseen rock excavation.
Impacts Due to the Issue, Problem, or Challenge
- Delay of 5 months due to change in excavation methods and procurement of specialized equipment.
- Additional costs from blasting permits, labor, and extended schedule.
- Safety risks associated with blasting operations in urban area.
Corrective Actions Taken
- Expanded geotechnical and geophysical investigations for remaining areas.
- Revised excavation plans to incorporate blasting and specialized drilling.
- Enhanced contractual clauses addressing unexpected rock conditions.
- Implemented strict safety protocols for blasting in populated zones.
Lessons Learned
- Early and detailed geotechnical surveys reduce surprises.
- Construction planning must remain flexible to adapt to site conditions.
- Clear contracts help manage risks and responsibilities.
Audit & Prevention: Project Control Questions to Ask on Future Projects to Help Control the Situation
- Are geotechnical investigations sufficiently detailed and comprehensive?
- Is construction planning adaptable for unexpected conditions?
- Are contract provisions clear about unforeseen site conditions?