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Highway and Rail Transit Tunnel Inspection Manual
Chapter 1: Introduction
The National Bridge Inspection Standards (NBIS) were established in the early 1970s to ensure highway structures received proper inspection using uniform procedures and techniques. The NBIS address a number of issues including personnel qualification, inspection frequency, and reporting of inspection findings. Following the issuance of the NBIS, the Federal Highway Administration (FHWA) developed a comprehensive training course, including an inspector's manual, designed for those individuals in the highway community responsible for bridge inspection. The training course and manual covered the typical types of highway structures in the nation, providing information on inspection procedures for the various components of those structures. Missing from the material was appropriate procedures to employ for preparing and conducting inspections on the various features of highway tunnels. Tunnels were considered unique structures and special applications would be needed for them.
Recently, the FHWA created an office specifically to focus on management of highway assets. This office has a major function - to work with the highway community to design, develop, and implement state-of-the-art systems for managing highway assets, including bridges and pavements. One area of the highway needing emphasis was a management system for tunnels. Similarly, the Federal Transit Administration (FTA) is responsible for providing transit tunnel owners with a wide range of assistance, including guidance on appropriate management techniques. Because of the common interest on tunnel management procedures from both agencies, the FHWA and FTA have joined to sponsor the development of a system for highway and rail transit tunnels. A project to develop the system was initiated in March of 2001 to include preparing an inventory of highway and rail transit tunnels in the U.S., an inspection manual, a manual for maintenance and repair, and a computer software program for data management. All of these products will be furnished to each highway and transit tunnel owner across the nation, and will be available as public domain.
The purpose of this manual is to provide highway and rail transit tunnel owners guidance for establishing procedures and practices for the inspection, documentation, and priority classification of deficiencies for various elements that comprise an existing tunnel. It is also the intent that this manual be used as part of a comprehensive inspection and maintenance program. The preliminary research performed indicates that a majority of tunnel owners believe there is a need to develop guidance for procedures for managing tunnel activities that could be readily implemented.
This manual addresses inspection procedures for the functional aspect of the tunnel, focusing on the civil/structural, mechanical, and electrical components. The manual does, however, provide brief guidance on other systems/appurtenances, such as track, traction power, signals, and communications, which comprise the operational aspects of a rail transit tunnel. This brief guidance is only meant to provide general knowledge and not in-depth inspection criteria for such systems/appurtenances.
To ensure consistency of definition of particular elements, this manual contains several chapters that explain the various types of elements that exist within the tunnel. For example, the description of tunnel components such as tunnel configuration, liner types, invert types, ventilation systems, lighting systems, tunnel finishes and other systems/appurtenances (track, traction power, signals, and communications) are each provided in separate sections to assist tunnel owners in educating their inspectors as to the particular system used for the tunnel to be inspected. Furthermore, the manual provides suggested guidelines for inspection personnel qualifications and equipment to be used for performing the inspection. Since most tunnels are constructed of concrete, steel, masonry, and timber (to a very limited degree), this manual provides extensive definitions of the types of common defects that occur within these major structural elements so that the inspection documentation is consistent according to the guidelines provided.
The manual contains procedures for documenting the inspection findings. These range from identifying a particular defect (cracking, scaling, spalling, corrosion, etc.) and its severity (minor, moderate, or severe) to assessing the overall condition of an element within a particular region of the tunnel. The manual is based upon a condition assessment scale that varies from "0" to "9," with 0 being the worst condition and 9 being the best condition. This is similar to the scale used for the National Bridge Inventory that is familiar to most highway/transit tunnel owners. The length of a tunnel segment for which these ratings will be applied will vary with each tunnel and tunnel owner. Some tunnels have panels that are numbered between particular joints, which make it easy for determining the segment length over which condition assessments are to be evaluated. Other tunnel owners may choose to have the evaluation performed for a segment of a tunnel, say 30 m (100 ft) or 60 m (200 ft). Regardless, the entire tunnel is to be inspected and condition assessments applied for all tunnel segments.
The manual will also provide guidance for the inspector to prioritize defects for repair and rehabilitation. Although this manual proposes the use of three prioritizations for conducting repairs, namely critical, priority, and routine, tunnel owners can adopt other prioritizations as appropriate.
This manual is developed for a hands-on, up-close inspection of the tunnel structure. The procedures developed herein are for visual and non-destructive methods of evaluating the tunnel elements. This does not preclude the lead inspector from requesting that certain destructive means (e.g., extracting cores for determination of freeze/thaw resistance or concrete strength) be requested to aid in determining soundness/adequacy of the tunnel elements.
Although this manual is produced for a hands-on, non-destructive evaluation of the inside face of the tunnel structure, other state-of-the-art, non-destructive testing methods may be used in areas that require a more in-depth structural evaluation. These methods may include mechanical oscillation techniques such as sonic or ultrasonic measurements (more commonly referred to as Impact-Echo), electronic techniques such as georadar, and optical techniques such as infrared thermography and multispectral analysis1. Each of the above methods has been used successfully in tunnels; however, a full understanding of the applications and limitations of each method is necessary to maximize their benefits.
It is felt, however, that these state-of-the-art methods are probably only cost effective in long, rail transit tunnels in metropolitan areas. It is assumed that these methods will mostly supplement and not replace the hands-on, non-destructive testing methods described in this manual for many tunnel owners in the United States.