Renovation – upgrading of existing tunnels
The upgrading (in particular for safety improvement) and refurbishment of existing tunnels in operation gives rise to specific problems of analysis and method. The degree of freedom is less than for new tunnels, because it is necessary to take into account the existing space and constraints. The technologies peculiar to each type of equipment and their integration are however identical.
The renovation and upgrading of a tunnel under operation quite often result in an increase of the construction schedule and costs, in much lower safety conditions during the works, and with badly controlled impacts on the traffic volume and conditions. These disadvantages are often the result of an incomplete analysis of the existing situation, the real condition of the tunnel, its facilities and its environment, as well as of a lack of strategy and procedures that would mitigate the effects on the traffic.
The page on Assessing and improving safety in existing tunnels proposes a methodology for the safety diagnosis of existing tunnels and the development of an upgrading programme. In addition, the page on Operation during maintenance and refurbishment works presents specific issues related to works carried out on tunnels in operation. Their dispositions help mitigate the problems mentioned above.
It is appropriate to draw the reader's attention to the key points of the following sections.
Detailed and rigorous diagnosis of a tunnel is an essential stage in the process of its upgrading or renovation. Unfortunately this stage is often neglected.
The physical diagnosis of a tunnel is required in order to:
- establish in detail and to describe in a precise manner the functions and the geometry of the structure,
- establish a detailed condition statement of the structure. To evaluate in particular fire resistance, uncertainties and potential risks, and to list the tests that would be needed in order to provide a solid basis for the detailed design,
- list all existing equipment, their functions, their condition, their technology, their actual features (tests or measurements will be required) and the stock of spare parts that might be available,
- evaluate the remaining life span of the aforementioned equipment before their replacement, and to identify the availability or not of spare parts on the market (notably because of the technological obsolescence),
- identify maintenance and inspection reports, equipment malfunctions and the rate of breakdowns.
This physical diagnosis must be supplemented by a diagnosis concerning the organisation, maintenance and operation procedures, as well as by a specific diagnosis concerning all documents relating to the organisation of safety and rescue interventions. This stage of diagnosis may eventually lead to the setting up of actions for the training of the various intervention parties in order to improve the global conditions of safety of the tunnel in its initial state prior to renovation.
The diagnosis must be followed by a risk analysis of the tunnel based on its actual state. This analysis has a double objective:
- to assess if the tunnel can continue to be operated in its present state prior to renovation, or if it is necessary to take temporary transitional arrangements: restriction of access to some vehicles only - strengthening of the arrangements for surveillance and intervention - additional equipment - etc.,
- to constitute a referential of the existing state from the point of view of safety in order to refine the definition of the renovation programme.
The diagnosis has to identify (without running the risk of late discoveries during the works period) if the existing facilities, supposedly in working condition, can be modified, be added to or integrated in the future updated facilities (technological compatibility - performance in particular for data collection and transmission, automatic functioning devices and SCADA).
The renovation or upgrading programme proceeds from two stages.
2.1. First stage: programme development
The development of the programme results from:
- the detailed diagnosis as described above,
- the risk analysis developed considering the initial state of the tunnel,
- the gaps noted concerning safety,
- the analysis of what it is possible to achieve in the existing spaces and their potential enlargements in order to enable the upgrading of the tunnel.
Depending on the physical environment of the tunnel and the spaces available, the optimum upgrading programme for the infrastructure or equipment may not be feasible under acceptable conditions, and that it is necessary to define a more restricted programme. This restricted programme may require the implementation of mitigating measures ensuring that the required level of safety is achieved in a global sense, after completion of the works.
2.2. Second stage: validation of the programme
The validation of the programme requires:
- development of a risk analysis based on the final state of the tunnel after upgrading in order to test the new arrangements introduced by the programme. This analysis has to be established with the same methodology used for the prior analysis based on the initial state. It also enables a search for optimisations,
- detailed examination of the feasibility of the works to be carried out for the improvement or the renovation under the requisite conditions of operation: for example, banning of tunnel closure or of temporary traffic restrictions. In case of incompatibility between the objectives of the programme and the works required for its application, iteration is necessary. This iteration may concern :
- the programme itself, insofar as adaptation of the programme is compatible on the one hand with the safety objectives, and on the other hand with its implementation in the required conditions of operation,
- the required conditions of operation that may be necessary to modify in order to be physically able to carry out the works resulting from the upgrading programme.
The upgrading or improvement programme does not necessarily require physical works. It may only require modification of the functions of the tunnel, or of the operating arrangements, for example:
- modification of the category of the vehicles authorised to access the tunnel: no access to trucks – no access to vehicles carrying dangerous goods,
- setting up of specific procedures for traffic restriction: in a permanent way or only during peak traffic,
- tunnel operated initially in bi-directional traffic, transformed for the implementation of unidirectional traffic,
- modification of the means for supervision or intervention.
The stage of design implementation and construction involves translating the renovation or upgrading programme into technical and contractual specifications and implementing it.
This stage requires a very detailed analysis of:
- the successive stages of construction, the content of each of these stages, the logical and priority sequences of the works,
- safety conditions inside the tunnel at each construction stage. This requires partial risk analyses and the implementation if necessary of mitigation arrangements: traffic regulation – traffic restrictions - patrol – strengthening of the intervention means - etc.
- traffic conditions inside the tunnel and on its approaches, with partial and temporary restrictions according to the various stages of works (different arrangements for daytime and night-time, for normal periods and vacation periods), of the potential diversions, of the global impact on the traffic and safety conditions in the areas concerned by the works,
- the constraints and subjections, the partial and global contractual deadlines for the works, in order to be able on the one hand to define the contractual specifications for the contactor, and on the other hand to implement all necessary temporary arrangements, and to proceed to an information campaign for the users and residents.