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Transit Cooperative Research Program
Synthesis of Transit Pratice 23
Transportation Research Board - National Research Concil
A transit agency's inspection program for rail transit tunnels and underground structures is a formally adopted system of institutional objectives, standards, and procedures that collectively describe the tunnel inspection practice.
Rail tunnel inspection has received relatively little attention as a professional practice from the transit industry, its professional organizations, or from the federal government. Little information has been compiled, and few professional organizations focus on the development, evaluation, and enhancement of the practice of rail tunnel inspection. Given the importance of rail tunnels to metropolitan transportation, this synthesis was undertaken to address the absence of information in this area.
A survey of tunnel inspection practice was conducted to review the status of the rail tunnel inspection practice in North America and overseas. The survey was sent to 47 transit agencies, 24 domestic and 23 foreign. In addition, five case studies were conducted to look more closely at the tunnel inspection practices of four domestic and one foreign transit agency. The results of this review of the practice are mixed, in the sense that less was learned about the practice than might have been expected, but what was learned confirmed the need for further research and development of universal, standardized tunnel inspection procedures. Transit agency tunnel inspection practices reflect different histories, underground systems, problems, and challenges. Therefore, it would be expected that their inspection procedures would be as individualized as the differences in their experience or underground systems.
For example, results from a review of tunnel structure inspection frequency demonstrated many different inspection cycles, sometimes even within the same transit system. Frequency of inspection does seem, however, to substantially depend on agency history. That is, if an underground tunnel section has a history of leaks, for example, that section would probably tend to be more frequently inspected.
Similar variability is found in other aspects of the tunnel inspection practice. Inspection protocols such as scheduling, inspection depth (visual vs. destructive testing methods), inspection documentation (photos, sketches, narratives of the inspection) and management focus (inspection planning and accountability) differ from agency to agency. Likewise, staffing of tunnel inspections varies. Number of crew members, their training or accreditation requirements are different from one transit agency to the next.
There is one aspect of the tunnel inspection practice on which transit agencies, their consultants and professional peers are agreed: the number one problem affecting tunnels and underground structures is groundwater intrusion and the subsequent damage caused by the presence of tunnel leaks. This groundwater intrusion is responsible for more problems affecting a tunnel's concrete liners and steel reinforced concrete than all other tunnel structural problems combined.
It is not at all clear that the variability in the practice of transit rail tunnel inspections described demonstrates a condition of substandard transit rail tunnel inspections. However, since respondent transit systems have tunnels and underground structures that are between 50 and 100 years old, the absence of similar inspection standards raises useful questions about the tunnel inspection practice. In the context of the agency and public safety issues inherent in the rationale for the tunnel inspection practice, and given the resource challenges currently confronted by transit agencies, the absence of universal standards of adequacy for tunnel inspection procedures is of concern.
Management of infrastructure systems is typically more efficient and productive, individual issues of work performance aside, if management operates from a set of standards that have been thoroughly reviewed and proven and that are repeatable. However, since there have not been demonstrable failures in rail tunnel structures as a result of inadequate inspection procedures, the argument for universal procedural or performance standards in the practice is less vigorous than it might otherwise be.
Rail tunnel environments adversely impact structural materials. Changes in tunnel loadings and human error do occur. Loss of life or property and the inconveniences that could result from structural failure are costs whose probability can be significantly reduced through adequate tunnel inspection and needed repairs.
Further questions remain to be more specifically addressed by subsequent research. Some suggestions are offered for the direction of that additional research. Clearly, questions regarding-tunnel integrity and safety are too important to leave unaddressed until potential tunnel structural failure prompts the inevitable investigations and emphasis on tunnel infrastructure maintenance.
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