Maintenance of Tunnels: An Indispensable Part of Sustainability

The technical lifetime of underground structures does not only depend on the long-term-behavior of individual support elements and material characteristics, but also on various boundary conditions that influence the entire long-term behavior of the structure. In almost all technical areas, a prediction of materials and structural components towards lifetime exists. In contrast to this, system-wide investigations in tunnels are rarely performed.

A sustainability-oriented tunnel design must find answers to questions like, “How efficiently can the tunnel be operated? Which preventive or reactive maintenance and repair measures follow from the draft? For which percentage of its service life will the underground system be in operation? What are the most likely repair scenarios to be expected?”

Considering a life cycle-oriented design, the additional cost for a larger clearance or for a redundant drainage system, becomes economically interesting. Today, participation by the tunnel designer in these lifecycle management issues is more the exception than the rule. Anyhow, for the operator of this infrastructure, two cost figures are getting more focus: the first is the construction cost, and the second (no less important) is the forecast of the life cycle cost.

Regarding the forecast of the life cycle cost, two components have to be considered: one is the expected cost during the operating phase and the other is the cost of demolition of the structure, where the second has hardly become realized up to now.

Cost during the operating phase has to be subdivided into maintenance and energy costs, where the maintenance cost includes the cost for service, inspection, refurbishment and repair.

Maintenance is defined as a combination of all technical, administrative and management measures during the life cycle of the tunnel that serves to maintain or restore its functional condition so that it can fulfil the required function. In contrast to maintenance, service only covers operational measures, including minor construction measures that serve to maintain the target condition of the tunnel. For example, cleaning, flushing of drainage systems, removal of vegetation in the portal areas, etc. are included. Service measures also include the personnel cost incurred for this as well as the necessary operating equipment.

Inspection is understood to be all the measures for evaluating the actual condition of the tunnel through continuous monitoring. The necessary measures to be considered are according to the guidelines of the respective operators and include not only the operator's internal personnel costs but also costs for external parties. Repair is understood to be a large-scale measure in which an aged tunnel is restored to its original, functional target condition, where refurbishment covers all structural and administrative measures to increase the load-bearing capacity, and safety or performance of a structure that goes beyond the original target condition.

Energy costs include all expenses for electrical or thermal energy for heating, cooling, ventilation, lighting, pumps, etc. incurred for the safe operation of the tunnel infrastructure.

Maintenance strategies are mainly following damage mechanisms of ageing due to concrete corrosion, steel corrosion, geomechanical and hydrochemical processes, but fatigue, fire loads, and wear have to be considered also. So, the trigger for maintenance can be split into degradation processes based on ageing and wear, and on the other side to the so-called obsolescence.

Ageing is a physical phenomenon that results in a change in the physical and/or chemical properties of the material, like corrosion does. Wear is a physical phenomenon, which leads to an erosion or deformation of the material, which can be observed by fatigue failure. The reasons for obsolescence are manifold. On the one hand, safety standards are being raised; these require the supplementation or at least adaption of structural operating and safety equipment like escape routes, ventilation systems, structural fire protection, and also the enlargement of cross sections due to electrification and safety factors.

To fulfill the requirements of sustainable tunneling, all these maintenance strategies should be considered in both the cost estimation from the cradle to the grave as well as in early stage design steps of the tunnel structure, so that maintenance works will not be in conflict with the operation of the tunnel later on.