All over the world, infrastructure and asset owners are struggling to keep up with the maintenance needs of critical civil infrastructure like roads, railways, bridges and tunnels. Dealing with restricted budgets, the needs on renovation and replacement of these assets are insufficient or not addressed. To prioritise the available budget, asset owners typically try to classify the assets based on parameters like risks, renovation or replacement cost, usage, remaining lifetime and others. In this article we’ll cover a couple of ways to optimise the availability and remaining lifetime of new and existing tunnels
In today’s tunnel maintenance management, we see the following methods applied:
In today’s operation and maintenance processes for tunnels, manual and visual inspections are an important method to gather required information. During these periodic checks, inspectors look for visual indications and damage. Based on the quantity and severity of these damages, the tunnel is categorised or evaluated in a risk-based analysis. This categorisation is then used to define new or modified inspection programs, remediation actions and future maintenance works.
Inspections can be a cost-effective way to gather information on your tunnel’s status, but there are of course some drawbacks:
- What’s not visible? As it’s often a visual check, there are plenty of phenomena that are not or barely visible during these inspection rounds. Water ingress, concrete integrity and rebar corrosion, early deformation or settlement of tunnel segments, wear & tear on ventilation motors and pumps, tunnel segment joints, increased energy usage and many others are important factors that can influence future availability or lifetime of the tunnel.
- Periodic check As inspections typically occur once in 3 or 4 years, the inspection or check doesn’t reveal any trend or historical events. The impact of nearby construction activity, low groundwater levels, traffic peaks, tunnel accidents or seasonal influences are hard to expose with periodic inspections
- Human factors As we rely on humans to perform inspections, there is subjectiveness and variability to be expected in our inspection reports. Based on experience, training, briefing and inspection conditions, inspectors can only report what they observed.
When visual inspections reveal problems, the maintenance manager can decide to execute specific measurement campaigns to provide more insight. With the use of Non-Destructive Testing (NDT) methods (laser scanning or total stations for tunnel geometry, ultrasound for motor bearing status,…) and others like core sampling for concrete or asphalt analysis , the asset owner can obtain specific details for those phenomena. While providing the required details, this approach still comes with a couple of challenges:
- Lagging indicator These campaigns are often triggered by visual indications, but they only reveal the impact or damage of phenomena that are sometimes already taking place since years.
- Budget Ad-hoc measurement campaigns often require experts and measurement technology that come with a cost on the yearly maintenance budget.
- Effect, not a cause While the ad-hoc measurement should bring clarity of what’s really going on, it’s less straightforward to determine the cause of the damage. When faced with high relative displacements of tunnel segments, are you sure it’s a permanent situation or rather a seasonal effect?
To overcome some of the above challenges, new technology allows us to improve tunnel maintenance management.
In existing or older tunnels, the use of continuous monitoring becomes a cost effective way to gather real-time information on specific issues or risks. With the use of battery powered sensors and cloud-based monitoring software, the barrier to get continuous data out of the tunnel has been lowered significantly in the last years. With a focus on specific risks (joint or crack monitoring, tunnel segment inclination or wall convergence, vibration, settlement, ground water levels and many others), there are plenty of options to make the risk objective and quantified. Sensor data can be used to trigger alarms in specific conditions, but can also be used to track longer term trends. Using other relevant and available data (environmental or meteorological, traffic or mobility,…), these datasets are be transformed in valuable prediction models.
(The graph above shows the displacement of tunnel segments over time. The straight lines indicate no movements during a certain time span. Imagine what it means if there is e.g. a runway above the tunnel...)
In new tunnel projects, it’s crucial to consider an integrated data approach over all disciplines and phases of the project. From an early design stage to the long term maintenance phase, the potential for smart and effective asset management is huge. As all disciplines and systems of a tunnel are linked to each other, it’s important to break down the data silos that still exist today. Data can be combined from:
- geotechnical sensors (ground pressure, vertical or horizontal ground displacements, ground water levels,…)
- structural health sensors (strains gauges, FBG fibre optics, corrosion, acceleration, relative displacements,…)
- SCADA or electro-mechanical systems (ventilation systems, pumps, lighting systems,…)
- Traffic or mobility
Determining the scope of required data and sensors is done by assessing the risks identified during design phase, working closely together with the tunnel design team.
The outcome of combining these datasets should not only be an early-warning system for operations or maintenance, but it should also allow the tunnel owner to determine an accurate remaining lifetime.
When you’re dealing with tunnel maintenance or management, feel free to get in touch and we’ll be happy to see how these new methods could help you.