Existing highway and rail transit tunnels are gradually becoming functionally obsolete and structurally deficient. These structures cause safety concerns. Older tunnels need frequent inspections, higher levels of maintenance, and rehabilitation. There is limited experience in the State Departments of Transportation and other tunnel authorities with respect to rehabilitation of existing tunnels. For these issues, most agencies rely on assistance from bridge engineers or consultants specializing in tunnels. Rock bolting, grouting, Shotcrete and other techniques have often been used in tunnel rehabilitation. Other techniques have been used to improve tunnel safety by increasing vertical and horizontal clearances, drainage, extending tunnel portals for rock fall mitigation, tunnel illumination and ventilation.
Section 12 of the AASHTO LRFD Bridge Design Specifications addresses Buried Structures, but very little guidance is provided for tunnel rehabilitation. Additional guidance must be developed in a separate stand-alone document on rehabilitation of existing tunnels.
Our nation’s tunnels are aging, and the increasing traffic volumes and environmental deterioration they experience results in a reduction in their service lives. Guidance should be provided to the engineer to provide cost-effective preventive rehabilitation strategies to preserve existing tunnel structures. Advancements in our knowledge of materials, details, components, structures, and an increased array of construction materials and methods, make it an opportune time to develop solutions to extend the service life and preventing premature deterioration of existing tunnel structures. This problem statement supports “Extending Service Life” and “Optimizing Structural Systems”, as noted in the 2005 version of the AASHTO Grand Challenges for strategic planning.
The objective of this research was to develop guidelines for rehabilitation of existing highway and rail transit tunnels. The research focuses on structural aspects of tunnel rehabilitation, drainage, leaking, and safety. This report provides draft guidelines for rehabilitation of existing tunnels in the format of other AASHTO guidelines.