Case Study: QC Deficiencies in Tunnel Lining Grouting on New York Subway Extension (New York, 2019)

Project Overview
• Name: Second Avenue Subway Extension
• Location: New York City, New York
• Year: 2019
• Project Size: $2.5 billion
• Scope: Tunnel boring machine (TBM) excavation and precast lining installation
• Lead Agencies/Contractors: Metropolitan Transportation Authority (MTA) /

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

Summary of the Issue, Problem, or Challenge
Poor grouting behind tunnel lining segments caused voids, leading to water infiltration and delayed construction due to necessary re-grouting and inspections.

Root Cause Analysis

• Inadequate grout mix design and lack of quality control during mixing.
• Insufficient monitoring of grout injection pressures and volumes.
• Poor coordination between grouting and lining installation crews.
• Lack of comprehensive training for grouting teams on quality standards.

Impacts Due to the Issue, Problem, or Challenge

• Delay of 5 months for re-grouting and quality assurance testing.
• Additional material and labor costs.
• Increased risk of long-term water damage and tunnel integrity issues.

Corrective Actions Taken

1. Revised grout mix designs with improved quality testing protocols.
2. Implemented real-time monitoring of grout injection parameters.
3. Enhanced communication protocols between installation and grouting crews.
4. Conducted targeted training on grout quality and injection techniques.

Lessons Learned

• Grout quality and injection control are critical to tunnel stability.
• Real-time monitoring prevents incomplete grout fills.
• Cross-team coordination improves construction quality.

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

• Are grout mixes tested and monitored consistently?
• Is grout injection pressure and volume tracked in real time?
• Are grouting crews trained and supervised effectively?