Case Study: QC Breakdown in Shotcrete Application on Denver Light Rail Project (Colorado, 2018)

Project Overview
• Name: Denver Light Rail Expansion
• Location: Denver, Colorado
• Year: 2018
• Project Size: $550 million
• Scope: Tunnel construction and shotcrete lining application
• Lead Agencies/Contractors: Regional Transportation District (RTD) /

Category of the Issue, Problem, or Challenge
• Quality Control
• Materials
• Structural

Summary of the Issue, Problem, or Challenge
Uneven thickness and poor adhesion of shotcrete layers led to premature spalling and reapplication needs during tunnel lining construction.

Root Cause Analysis

• Inconsistent shotcrete mix batching and application techniques.
• Insufficient QC inspection during layering processes.
• Lack of training for shotcrete operators and supervisors.
• Poor documentation of quality checks and acceptance criteria.

Impacts Due to the Issue, Problem, or Challenge

• Delay of 4 months due to removal and reapplication of shotcrete.
• Additional costs in materials and labor.
• Increased risk of long-term structural problems if not corrected.

Corrective Actions Taken

1. Standardized shotcrete mix design and batching quality controls.
2. Increased inspection frequency during application phases.
3. Implemented training programs for shotcrete crews and QC staff.
4. Improved QC documentation and reporting systems.

Lessons Learned

• Consistent material batching and application ensure shotcrete durability.
• Frequent QC inspections prevent defects.
• Training enhances operator and inspector effectiveness.

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

• Are shotcrete mixes controlled and tested consistently?
• Is application quality inspected regularly?
• Are operators and inspectors properly trained?