Tunnels represent major investments for governments and other infrastructure owners. New tunnels are commonly required to remain functional for 100 years or more and must be designed accordingly, with provision made for inspection and maintenance. However, in many major cities of the world, existing transportation and utility tunnels are already approaching this age(1, 2), and yet remain critical to the ongoing operation and functioning of key infrastructure.
By their nature, tunnels are extremely difficult to decommission on a permanent basis. Even if taken out of service there is invariably an ongoing requirement to continue inspection and repair of the tunnel structure itself – if serious deterioration, collapse and potential impact on other property and infrastructure are to be avoided. Furthermore, tunnel infrastructure is unique in that demolition, removal and reinstatement to a pre-existing condition are rarely a feasible option. High replacement costs, and the difficulties of achieving acceptable new alignments through often congested underground space, place great demands on owners to maintain tunnels to continuing high standards, and to extend the life of the infrastructure almost indefinitely.
Asset management of tunnels is therefore an extremely important part of infrastructure management, with particular technical and safety considerations arising from the very nature of underground construction. The need for properly structured asset management programmes is becoming more widely recognised worldwide. For example, in 2001 the US Department of Transportation announced a major initiative to develop a tunnel management system for the nation’s highway and transit tunnels(2), and in the United Kingdom major programmes are being initiated to involve the private sector in the long-term maintenance and upgrading of London’s underground railway system.
The needs of each tunnel system are wide and varied, depending on the type, location and ownership arrangements, in addition to specific geological, technical and safety issues. Nevertheless, it is possible to identify some broad general requirements and processes for the effective management of tunnel infrastructure assets.
Asset management
The strategy for the management of a particular tunnel asset will usually be part of a wider strategy and policies covering the management of a complete system, such as a road, rail or drainage network, of which the tunnel may be a single component. For example, the UK Highways Agency provides a Standard for the inspection of its road tunnels(4) as part of its overall Design Manual for Roads and Bridges, thus ensuring consistency and completeness. Although the approach to the management of each tunnel asset will be different, there are a number of principles that are generally common to all major civil assets, including tunnels. A suggested framework for the management of tunnel assets is set out below. This has been developed largely from a UK Code of Good Practice for Highway Maintenance(5), but is considered to reflect international practice in its general principles.
Objectives, policies and standards
A systematic and properly documented approach is fundamental to all modern asset management programmes and must be adopted by all levels of management. The responsibility for setting objectives, policies and standards for a maintenance regime lies with elected politicians, government officers, or company main board directors, depending on particular asset ownership arrangements.
To establish an economic, efficient and effective approach to maintenance it is important that a clear statement of policies is prepared that properly reflects the short and long-term objectives of the maintenance regime. Policy statements set down the objectives: an objective is the result that a given level of service is designed to achieve. Objectives might include, as a simple example, restricting major refurbishment (involving closure of the tunnel) to not more than once in every 25 years. Worthwhile policies require a detailed knowledge of the asset through the preparation of comprehensive inventories and condition assessments and regular inspections. Policies themselves should be subject to regular review and revision as a result of performance monitoring, or possibly changed financial, political or other circumstances.
Having prepared a policy statement, the policies are translated into standards. Standards provide the framework of target levels of service to be achieved by the maintenance management team. Standards may cover a wide spectrum, ranging from the physical condition of a tunnel structural lining or secondary cladding system, to the requirements for operational response and the carrying out of emergency repairs. Standards also provide a basis for:
As a general rule, budgeting for maintenance should be “needs based” and not simply on the basis of historical budget allocations. A detailed knowledge of the asset allows often limited funds to be allocated effectively, i.e. based on a clear understanding of the needs, and controlled properly.
Management strategy
An outline management strategy for tunnels, which incorporates the above points, is shown in Figure 1. This framework can be developed as necessary to suit particular requirements. For example, tunnels with significant electrical and mechanical equipment, such as for roads and railways, may require more detailed consideration in respect of these component parts of the total asset. The emphasis is on a comprehensive knowledge of the asset, and information management systems are central to most modern asset management programmes. Proprietary software systems are available, but often owners of major assets will develop customised systems to suit their particular needs.
Pre-planning and risk assessment
Tunnel inspections should not be undertaken without adequate pre-planning and risk assessment. The form and extent of risk assessment will depend largely on the type and size of tunnel. Clearly in the case of water tunnels a key question is the risk of water re-entering the tunnel during the inspection. For road and rail tunnels, access arrangements and timing must be carefully planned and coordinated with tunnel operators; exhaustive procedures will normally exist in these cases. Some smaller tunnels, and parts of larger tunnels, may be designated as “confined spaces” for which special requirements and regulations usually apply. In all cases, gas testing and ventilation are fundamental considerations, including the risk of methane migration into tunnel voids(6, 7).
Safety during tunnel inspections should not be underestimated. All members of inspection teams should be properly trained, experienced and have a sound understanding of basic safety requirements(7, 8).
Desk study and planning
In order to maximise the value of inspections it is important that sufficient time be given to adequate planning and acquiring as much knowledge as possible relating to the tunnel and its contents before the inspection commences. Documentation that might be researched includes the following, all of which should be held in the management information system:
As a result of the planning stage, it should be possible to prepare an overall inspection plan (including reference to applicable standing procedures etc.) and programme. Where necessary, these should be submitted to the appropriate authority for approval.
Inspection, investigation and testing
The inspection should be carried out strictly in accordance with the approved inspection plan. The plan will contain all the necessary procedures to allow the inspection to be carried out efficiently and in safety. Tunnel inspections will normally involve a number of people, both engineers and operatives, and an experienced team leader should be nominated to direct and control all aspects of the team’s activities.
The inspection should be carried out systematically and be properly recorded. Proformas are usually considered to be the most appropriate and efficient means of recording the results of inspections. The UK Highway Agency provides an example that covers civil, electrical and mechanical engineering assets(4). Depending on particular circumstances it may be necessary to supplement the basic visual inspection with probing, geophysical investigations and/or in situ testing. In some cases, it may be necessary to sample and test concrete, groundwater and/or other materials. Penetrative investigations of this type should be planned in the same way as the initial inspection, with consideration given additionally to plant access, space and power requirements etc.
Diagnosis, study/analysis, design and specification
Following completion of the site inspection it is necessary to assess the results and determine the need for any repair work (other than for planned maintenance). This stage may require comprehensive diagnosis of the results of investigations and tests, particularly in the case of a major failure or accidental damage. Study and analysis of deteriorated or damaged elements may be required to assess current reserves of strength and factors of safety, and also to estimate the time period within which repairs should be completed. In some cases it may be sufficient to initiate a monitoring programme, or it may be necessary to monitor and observe for a further period of time in order to arrive at a reliable diagnosis.
When the diagnosis is complete it may be necessary to prepare designs and specifications for repair work or for replacement equipment. Structural designs should consider the stability of the tunnel, and the safety of operatives, at all stages of any proposed demolition and re-construction, and also allow for a slow and possibly piecemeal process. If the deterioration or damage is insured, the insurer should be advised at the earliest possible time.
Procurement
Tunnel repair work can involve specialist skills and techniques, and require a sound understanding of safety procedures and safe methods of working. Potential tenderers for repair contracts should therefore be carefully assessed before being allowed to tender for this type of work. A professional, flexible and co-operative approach should be demonstrated.
Method statements and risk assessments
Tunnel repairs often have to be carried out under strictly controlled access arrangements, with restricted working areas, and in difficult environmental conditions. It is therefore extremely important that the appointed contractor prepares and plans his work properly. Fundamental requirements are detailed method statements and risk assessments(9, 10) covering all stages of the work, with appropriate contingency and emergency measures included.
Repair and refurbishment work may often require demolition of existing tunnel linings. There is therefore the potential for operatives having to work below unsupported roofs. This should be avoided by planning the work using remotely operated equipment, such as hydraulic scaling and water jetting, and remotely controlled shotcrete and bolting machines. Temporary protective canopies may be appropriate in some cases. Safe working conditions must be ensured at all stages by careful and detailed planning, and rigorous control on site.
Risk assessment should similarly consider all stages of the process, including tunnel stability during demolition. Consideration should be given to the monitoring of surface and/or other underground infrastructure, and other measures to ensure public safety.
Implementation and Quality Assurance
The maintenance and/or repair work should be carried out in accordance with the approved method statements. Prior to commencement all necessary mitigation measures identified by the risk assessment should be completed. All work should be carefully supervised and completed in accordance with the quality assurance requirements for the project. It may be necessary to prepare specific quality control procedures to cover particular aspects of the work.
Records
It is important that all maintenance, repair and refurbishment work is accurately and comprehensively recorded, and the records entered into the management information system. Key information includes:
This information will provide valuable data for performance monitoring and the assessment of value for money.
Recommendations
The final duty of the inspection team is to review the overall process, and the work carried out, and to make recommendations to the tunnel management team on any matter relating to the future management of the asset. For example, recommendations can usefully be made relating to updating of documentation to reflect new national standards etc., inspection procedures and frequency, procurement procedures, and safety issues, as well as more general recommendations for amendment of the maintenance policies and standards.
Conclusion
There are a number of general principles of asset management, drawn from international practice, that are applicable to tunnels in general. These have been described and set out in the paper to provide a general framework from which particular requirements may be developed. The emphasis in all applications must be on proper planning and risk assessment, for both the inspection and implementation phases.
Related Files
Figure 1 – Suggested framework for the management of tunnel assets
