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

1. Installed advanced dewatering and grouting systems ahead of excavation.
2. Adapted tunnel boring machine (TBM) operations for wet ground conditions.
3. Conducted continuous groundwater monitoring during tunneling.
4. 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?