Building on soft soil and clay
Many tunnels connect the roads and waterways of the city of Antwerp, supporting high volumes of traffic. Building deep foundations and underground structures is challenging there due to the presence of a subsoil layer called the “Boom Clay”. This clay is prone to slow, long-term swelling behavior that puts the stability of the infrastructure at risk. In order to evaluate the long-term effects of this swelling clay, the Belgian Department of Public Works tasked SkyGeo with assessing the exact actual deformation on three tunnels to prevent uncontrolled release of road segments at the entrance of the tunnel.
Three tunnels were investigated using InSAR data: Liefkenshoektunnel, Kennedytunnel and Beverentunnel. We combined image stacks from multiple satellites and aggregated uplift per segment along the highway. The data showed a clear correlation between uplift rate and the distance to the tunnel entrances. Local geotechnical inspection then confirmed the uplift is caused by swelling of the Boom Clay. The added insight is, that this uplift is occurring at a steady rate over decades and is still ongoing. Historical data Obtaining tunnel deformation data since 1992 allowed the asset manager to extract the subtle long-term deformation patterns. Uplift of only 1 – 1.5 millimeter per year affects the tunnel entrances. By measuring with a single, uniform method over decades, an accurate picture of stability trends emerges.
Visual interpretation of patterns: tunnel stability
In this project, plots of aggregated deformation rates versus the distance to the tunnel entrance proved useful and are configurable by the user. The numerical results of our satellite analysis are accessible via our online platform for direct reference or for download.
Next steps: risk audits on tunnels
Using satellite radar interferometry, bridges and tunnels located anywhere on earth can be monitored for millimeter-level displacements with a revisit time of a few days. Displacements can be decomposed into horizontal and vertical deformation. After interpretation and correlation with performance – or even incidents – the tunnels can be ranked based on deformation’s impact on functional health. The asset manager gets city-wide insights into the necessity for inspection and remediation.
Aggregate time-series of a single segment between 1993 and 2018 at the entrance to the Kennedytunnel.
Aggregated data of 120,000 measurement points over 20 years shows distribution of uplift rates along the highway. This indicates stress accumulating slowly, getting worse towards tunnel entrances for the Kennedytunnel.
“It was an essential discovery for us that there is steady uplift for decades at the entrances to the Kennedytunnel. We could confirm the InSAR trends by on-site follow-on measurements. All in all, we are very pleased with the InSAR results and we will use the technique to monitor other tunnels as well.”
Dr. ir. Leen de Vos
Geotechnical Engineer Department of MOW Belgian Government