New guide specifications on Load and Resistance Factor Design for road tunnels have been published by the American Association of State Highway Transportation Officials (AASHTO). The LRFD Tunnel Design and Construction Guide Specifications, available on the AASHTO website, is the result of a five-year research effort by WSP USA performed under a task order from the National Cooperative Highway Research Program (NCHRP).
The specifications are intended for the design, evaluation and rehabilitation of road tunnels constructed using cut-and-cover, bored, mined and immersed tunnel construction methodologies. In developing these specifications, consideration was given to safety and operations, maintenance and inspection of tunnel systems. The publication, the first national design and construction specifications for road tunnels, represents groundbreaking work to advance and support the professional practice in tunnel and underground engineering in the United States.
The recommended LRFD-based tunnel design methodology used to develop the Guide is presented in the final NCHRP research report. A literature search was performed to review existing design codes and standards, project-specific design criteria, reports and technical publications. Limited calibration of the load factors was performed based on results from the analysis of a circular bored tunnel. A summary of the calibration procedure is presented in the report.