Seeing Tunnel Movements in a New Light
For the first time, distributed optical fiber sensors (DOFS) have been used to continuously track how every joint along an immersed tunnel moves through the seasons. The results show—clearly and quantitatively—how temperature cycles drive joint opening and closing. For engineers responsible for aging tunnel infrastructure, this is a major step toward smarter monitoring and maintenance.
The Challenge: Aging Tunnels and Moving Joints
Immersed tunnels are critical links beneath rivers, estuaries, and harbors. They are built from prefabricated concrete elements that are floated into position and joined underwater. Two types of joints are key:
- Immersion joints between elements
- Dilation joints between segments within each element
Globally, more than 150 immersed tunnels are in service, and many are over 50 years old. As these structures age, engineers increasingly encounter:
- Uneven settlement
- Joint leakage
- Local cracking
Even tunnels only 20–30 years old can show such issues.
A major driver behind these problems is temperature. Concrete expands in warm weather and contracts in cold weather. Over hundreds of meters, even small thermal strains add up. The result is cyclic joint movement:
- Joints close in summer
- Joints open in winter
These movements can reduce compression in rubber gaskets (such as GINA and OMEGA gaskets), lower shear resistance, and ultimately threaten watertightness.
The First Heinenoordtunnel in the Netherlands, opened in 1969, is a good example. After decades of service, some joints have shown leakage and cracking, and certain immersion joints have experienced uneven settlement.
The real difficulty? Measuring all this in detail.
Traditional sensors are expensive to deploy at many locations, provide only local data, and can interfere with operations. As a result, engineers have had limited field data on how entire tunnels behave seasonally.
The Innovation: Turning Optical Fibers into Continuous Sensors
Researchers Xuehui Zhang and Wout Broere (Delft University of Technology) addressed this gap with a distributed optical fiber sensing system designed specifically for immersed tunnels.
The principle is elegant. Using Brillouin scattering, the system measures tiny frequency shifts in light traveling through an optical fiber. These shifts relate directly to strain and temperature. In effect, the fiber becomes a continuous sensor along its entire length.
Field Implementation
The First Heinenoordtunnel’s 574 m immersed section contains:
- 5 concrete elements
- 6 segments per element
- 31 joints in total (6 immersion, 25 dilation)
A 1.4 km loop of polyurethane-sheathed optical fiber was installed along the tunnel wall. At each joint, the team created a compact “sensor block” with:
- One horizontal fiber segment
- One diagonal fiber segment
- Three fixed anchoring points
By combining these strain readings—and compensating for temperature—the researchers calculated:
- Joint opening (Δy)
- Uneven settlement (Δz)
Laboratory validation showed sub-millimeter accuracy.
A BOFDA (Brillouin Optical Frequency Domain Analyzer) unit in a service building allowed remote measurements with minimal disruption to traffic. Protective steel covers shielded fibers crossing joints.
Monitoring was carried out in two phases:
- 13 joints for one full year
- 17 joints for half a year
System reliability was confirmed by strong agreement between fiber-based temperature readings and meteorological data.
What the Tunnel Revealed
The data painted a remarkably clear picture of seasonal behavior.
Joint Opening: Strongly Temperature-Driven
Joint movement showed a strong negative correlation with temperature:
- Warmer → joints close
- Colder → joints open
- Typical delay: 1–2 days after temperature change
Immersion joints showed the largest ranges, up to about 6 mm. Most dilation joints moved 1–2 mm, though some near the tunnel approaches moved much more. One known leakage location showed over 5 mm of closure.
Across the full immersed length, seasonal thermal deformation summed to roughly 41.5 mm from summer to winter—well aligned with theory.
Uneven Settlement: Smaller but Detectable
At most joints, uneven settlement stayed below 1 mm. Some locations exceeded this and showed seasonal trends, but the correlation with temperature was weaker than for joint opening.
Engineering Takeaways
Several practical insights emerge:
- DOFS works. It delivers reliable, high-resolution, long-term data on both opening and settlement.
- Seasonal movement is real and measurable. It is not just theoretical—it shows up clearly in field data.
- Immersion joints move more than most dilation joints.
- Approach zones deserve attention. Larger deformations near portals matched known leakage sites.
Overall, seasonal joint opening generally did not threaten structural integrity or watertightness under normal conditions. However, some approach-area joints showed movements large enough to raise concern.
Why This Matters for Practice
For tunnel owners and asset managers, this work shows that continuous, distributed monitoring is no longer impractical or experimental. It is a viable tool for:
- Early detection of abnormal behavior
- Better maintenance planning
- Extending service life of aging tunnels
- Reducing the risk of costly failures
As immersed tunnels worldwide continue to age, data-driven management will become increasingly important. Technologies like DOFS give engineers the ability to see how structures truly behave—not just at a few points, but along their entire length.
That shift from sparse measurements to continuous insight could redefine how we manage underwater infrastructure.
Reference: Zhang, X., & Broere, W. (2023). Monitoring seasonal deformation behavior of an immersed tunnel with distributed optical fiber sensors. Measurement: Journal of the International Measurement Confederation, 219, 113268. https://doi.org/10.1016/j.measurement.2023.113268