Case Study: Unforeseen Underground Water Infiltration Delays Tunnel Excavation in New York (2018)
Project Overview
• Name: Second Avenue Subway Phase 2
• Location: New York City, New York
• Year: 2018
• Project Size: $5.5 billion (Phase 2)
• Scope: Urban subway tunnel excavation and station construction
• Lead Agencies/Contractors: MTA /
Category of the Issue, Problem, or Challenge
• Geotechnical
• Differing Site Conditions
Summary of the Issue, Problem, or Challenge
Unexpected groundwater inflows during tunnel excavation caused work stoppages and extensive dewatering challenges.
Root Cause Analysis
- Pre-construction hydrogeological studies underestimated water table fluctuations.
- Limited probe drilling ahead of tunnel face to detect water-bearing zones.
- Tunnel design and TBM specifications not fully adapted to variable water inflows.
Impacts Due to the Issue, Problem, or Challenge
- Excavation halted for 6 months for additional waterproofing and pumping systems.
- Significant cost increase for dewatering equipment and schedule extensions.
- Risk of soil instability and safety hazards for workers.
Corrective Actions Taken
- Installed advanced dewatering and grouting systems ahead of excavation.
- Adapted tunnel boring machine (TBM) operations for wet ground conditions.
- Conducted continuous groundwater monitoring during tunneling.
- Updated design with additional waterproofing linings.
Lessons Learned
- Hydrogeological investigations must be thorough and include seasonal water table variations.
- TBM and tunnel design must accommodate worst-case water inflow scenarios.
- Real-time groundwater monitoring critical to safe excavation.
Audit & Prevention: Project Control Questions to Ask on Future Projects to Help Control the Situation
- Has hydrogeological variability been fully investigated?
- Are tunneling methods appropriate for anticipated water conditions?
- Is continuous groundwater monitoring in place?