Sensors and sustainability

In the mid-20th century, Europe began rebuilding its infrastructure after the Second World War, leading to a significant expansion of networks like roads, railways, and waterways. A large portion of the infrastructure used today dates back to this period of economic growth in the 1960s, 70s, and 80s. These structures, however, were built to withstand traffic loads much lower than those of today. Modern traffic, particularly the weight of heavy trucks, places a significantly greater load on this infrastructure than was originally anticipated.

While 20th-century structures were designed with a specific lifespan, this does not automatically render them unsafe once that period ends. It simply means the original engineers could not guarantee their safety beyond that point. This raises the question of whether these assets should be demolished. While this may be necessary in some cases, it’s also possible to extend a structure’s usable life for decades beyond its design lifetime. The Netherlands, despite its long history of civil engineering expertise, faces challenges with its aging infrastructure. 

A smart approach to infrastructure management

Infrastructure owners need to determine not only that renovation work will be necessary, but also when it will be needed. Renovating too early wastes money, resources, and materials, and causes unnecessary environmental impacts. It can also be a waste of skilled technical personnel. Fugro has partnered with the Dutch infrastructure agency Rijkswaterstaat (RWS) to address this issue by using a new approach to infrastructure asset management that aims to reduce costs and protect the environment..

One example of this approach is the A9 motorway, a vital part of the Dutch highway system that connects Amsterdam with Schiphol international airport. As part of a 2014 programme to expand the A9 from four to six lanes, RWS asked Fugro to assess how much of the existing structures could be reused. Fugro’s project leader, Diederick Bouwmeester, explained that they evaluated the A9’s 11 viaducts using a combination of structural and geotechnical engineering knowledge, archival information, and a review of the viaducts’ current condition. They then projected future traffic loads over the next 30 years to inform their recommendations.

Fugro focused on the viaducts’ foundations, which consist of concrete abutments supported by steel piles^. While the viaducts’ girders and abutments could not be reused, the piles could be. Fugro used Cone Penetration Testing (CPT) vehicles, which push a sensor-filled cone into the ground, to assess the condition and placement of the piles. They checked the concrete and reinforcement, and used a magnetometer CPT to detect the presence of steel in the soil. The CPT data, combined with drawings from the 1970s and 80s, allowed Fugro to verify the piles’ depth, inclination, and overall condition.

Additional CPTs were conducted and showed that the soil condition was the same or even better than anticipated. Detailed calculations for the reuse of the pile foundations concluded that only a small number of new piles would be needed, in addition to those required for the road widening. To protect the existing foundations, Fugro recommended using drilled piles instead of traditional hammered piles. Drilled piles, such as the steel-column-and-concrete Tubex piles, are preferred because they don’t cause vibrations or compaction, which could damage the older, existing piles. By reusing the 208 piles from one of the viaducts on the A9, Fugro’s engineers estimated a saving of over 1,200 tonnes of carbon emissions.

The challenge of open tunnels

The Netherlands also uses open tunnels, which are U-shaped concrete sections lowered into cuttings, to pass under roads and canals. During construction, the groundwater table was lowered, an environmentally damaging practice that is no longer permitted. Once construction was complete, the water table was allowed to rise again, and steel-reinforced anchors were used to prevent the tunnel floor from lifting due to water pressure. These anchors were built with a fixed design life.

In 2010, the anchors on the Vlaketunnel failed, causing a section of the entrance ramp to lift up by about 10 centimetres. The tunnel had to be closed and all the anchors replaced. This failure indicated a widespread problem, as RWS has five other tunnels with the same type of piles. A second failure of the same type occurred in 2022 at the Prinses Margriettunnel. This led RWS to investigate all tunnels built in this manner. Investigations revealed that the anchors were failing due to a type of corrosion, causing them to break and the tunnel section to rise.

While the first two failures occurred in less populated, less busy areas, four other tunnels with the same anchors are located in the highly populated western part of the Netherlands with very high traffic loads, posing a significant risk. Since there is no way to monitor the anchors’ condition directly, RWS can only monitor the overall deformation of the structures.

Continuous monitoring and new technology

RWS commissioned Fugro to provide continuous monitoring for these tunnels. In January of the previous year, this monitoring proved its value at the Vollenhoventunnel, located on the A27 motorway near Utrecht, when one of the entrance ramp sections was detected moving upwards. The original plan was to use total stations, which are high-end surveying instruments that use a laser to precisely measure the position of small mirrors installed on the structure. However, total stations are expensive, costing around €30,000–40,000 each, and are vulnerable to theft. 

As an alternative, Fugro proposed and installed “joint meters” on the Vollenhoventunnel. These are simple mechanical sensors that measure the movement between two elements at a joint. While a single total station can measure many points, more joint meters are needed, but they are much cheaper, easier to install, and less attractive to thieves. The data they provide is also easier to interpret, making them an overall cheaper solution.

Based on Fugro’s warning, RWS was able to take emergency action by placing large concrete blocks on the hard shoulder to act as a counterweight against the buoyant force from the groundwater. This temporary solution successfully stopped the movement and even allowed the section to return to its original position. The continuous monitoring system allows RWS to guarantee the safety of the tunnel, enabling its continued use while a long-term solution is found.

RWS also uses satellite-based remote monitoring, which provides historical data but is not continuous. This is considered the lowest level of monitoring, and if it detects unusual movement, RWS switches to more detailed monitoring with joint meters or total stations.

The future of monitoring

In addition to joint meters and total stations, Fugro also trialled a new, more cost-effective system called TotaLite on the Vollenhoventunnel. TotaLite has similarities to a total station and uses the same mirrors. However, instead of using a laser, it takes a photograph to achieve the same level of precision. TotaLite is a camera-based solution that is less complex, cheaper to produce, requires less maintenance, and is smaller and lighter than a total station. It is also less visible and less valuable, making it a less attractive target for thieves.

By using solutions like joint meters and TotaLite, which are more scalable and cost-effective than traditional methods, infrastructure owners can monitor tens of thousands of structures across Europe. This ubiquitous sensing allows for a more sophisticated approach to asset management, where decisions are based on a structure’s actual condition and performance rather than just its age. This approach allows owners to save money, prioritize repairs on hot spots where they are most needed, and safely extend the lifetime of their assets. It also helps flatten the curve of required renovations, spreading the work out over a longer period.

This move towards continuous monitoring and away from traditional inspection-based maintenance also has benefits for safety and efficiency. Sensors are becoming more affordable, and they eliminate the need to put inspectors at risk in dangerous environments like under bridges with traffic. Additionally, the data collected from monitoring can be shared and analysed, potentially using new tools like AI and algorithms, to gain further insights. This could allow engineers to apply data from a few monitored bridges to make informed decisions about many similar structures that have no sensors installed.