Forensic Review of Rail Tunnel Collapse During Planned Repair Outage Using Fore-Spilling Ground Support Methods

Fore-spiling is a well-known and common method that has been used for decades in the tunnel industry, specifically in the tunnel repairing process. In this method, steel channels supported by equally spaced steel frames are often used to temporarily support the earth loading imposed on the tunnel from above and from the sides after the removal of the concrete liner. The process requires a good understanding of the geotechnical and geological conditions of the site; it also requires excellent collaboration between the field engineer who is monitoring the dynamic conditions on site and the structural engineer who is designing the steel temporary steel supports and providing guidance on the fore-spiling process. In this paper, we are presenting a case study of a tunnel collapse in a mountainous area in the northwest section of the United States. 

This case study will show the importance of the ongoing coordination and collaboration between the field engineer and the design engineer through the construction process. Appropriate documentations and communications are essential to properly report the existing conditions in order to provide an accurate assessment of soil stability and provide a clear picture on the behavior of the temporary steel structure to the imposed loading such that the design engineer can make appropriate assumptions and change design plans to accommodate the new conditions that could be revealed (previously unknown) after liner removal and exposure of soil inside the tunnel. We will explain the wrong assumptions leading to collapse, in addition, the poor communication and inaction, after experiencing field conditions different from key design assumptions which resulted in design elements being overloaded. It is worth noting that currently there are no clear industry guidelines that are specific for fore-spiling. This paper will highlight the specific checks required to reduce the potential of tunnel collapse.

• Communications between the field engineer and the design/structural engineer during fore-spiling are key for a safe removal of tunnel liner without tunnel collapse.
• Design assumptions determined prior to liner removal should be checked, verified, and any changes in assumptions should be reported by the field engineer.
• Specific guidelines and checks to reduce the risk of tunnel collapse and consequent loss of lives or injuries during fore-spiling will be discussed.
• The signs of tunnel failure during fore-spiling should not be left unreported without appropriate re-evaluation of the design.
• Appropriate actions that must be taken to prevent tunnel collapse after experiencing field conditions suggesting tunnel failure will be highlighted and explained.

1.5 PDHs