Case Study: Unanticipated Rock Fracturing in Tunnel Excavation Causes Delay in Pennsylvania (2018)

Project Overview
• Name: Pennsylvania Turnpike Tunnel Rehabilitation
• Location: Allegheny Mountains, Pennsylvania
• Year: 2018
• Project Size: $200 million
• Scope: Rehabilitation of highway tunnel through fractured rock zones
• Lead Agencies/Contractors: Pennsylvania Turnpike Commission /

Category of the Issue, Problem, or Challenge
• Geotechnical
• Differing Site Conditions

Summary of the Issue, Problem, or Challenge
Highly fractured rock zones encountered led to increased excavation difficulty, requiring additional support and stabilization.

Root Cause Analysis

• Initial rock quality designation (RQD) and fracture mapping were incomplete.
• Underestimation of joint spacing and rock mass variability.
• Limited use of 3D geotechnical modeling during planning.

Impacts Due to the Issue, Problem, or Challenge

• Delays of 5 months due to installation of additional rock bolts and shotcrete.
• Increased costs for specialized ground support and slower excavation.
• Elevated safety concerns for workers in unstable zones.

Corrective Actions Taken

1. Enhanced geotechnical investigations including 3D rock mass modeling.
2. Implemented more comprehensive rock support design with flexibility.
3. Introduced real-time monitoring of tunnel face stability.
4. Updated contract terms to include provisions for variable rock conditions.

Lessons Learned

• Thorough rock mass characterization is essential for tunnel projects.
• Flexible ground support design improves safety and adaptability.
• Contracts should explicitly cover variability in rock conditions.

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

• Is rock mass variability sufficiently mapped and modeled?
• Are ground support systems designed for expected variability?
• Do contracts allocate risk for unforeseen rock conditions?