• The technology developed by the Catalan company Worldsensing consists of a network of autonomous GNSS sensors and will be tested in Sallent, an area near Barcelona, monitored by the ICGC.
• A Sallent neighborhood was built on land above an old deep salt mine, which causes continuous subsidence, prompting its evacuation some years ago. 
• The Catalan institute maintains a continuous monitoring network of ground movement at the surface and depth, producing a monthly bulletin of results.

The Cartographic and Geological Institute of Catalonia (ICGC) has contributed to the installation of an innovative sensor currently being tested in a Sallent neighborhood that has been monitored for over two decades due to ongoing land subsidence. With this initiative, Sallent serves as a testing ground for validating advanced monitoring systems that could offer new solutions for similar challenges elsewhere.

Sallent, a municipality in the province of Barcelona, Spain, includes a neighborhood known as “Barri de l’Estació,” built on land above a decommissioned deep salt mine. This type of soil is particularly prone to collapse, exacerbated by its saline composition, which makes it highly erosive. Furthermore, the presence of a natural cavity in the area has added to the latent instability of the ground.

The effects of subsidence became apparent in 1997, marked by the emergence of cracks in the houses. The situation steadily worsened, prompting the initiation of ground movement monitoring, both at the surface and at depth. This monitoring made it possible to identify and characterize the phenomenon, predict the structural damage the buildings could sustain, and ultimately declare the area unsuitable for residential use.

Monitoring was the basis for creating a local alert system while the neighborhood was still inhabited, ensuring the safety of its residents. The local Civil Protection authorities implemented a specialized management plan corresponding to the various alert levels that arose.

In 2009, the Catalan Government ultimately ordered the evacuation of the neighborhood and the relocation of its approximately 150 residents. Today, the area is uninhabited, with nearly all structures demolished. Only a single road remains open to traffic, awaiting rerouting through safer terrain.

More than 20 years of monitoring

Since the neighborhood’s evacuation, the ICGC has scaled back its monitoring efforts due to the reduced risk but continues to maintain a basic network as long as certain infrastructures remain in use and pose some level of risk. Subsurface movements are monitored using extensometers in boreholes, while surface-level changes are tracked automatically via a robotic station. Each month, the ICGC publishes a bulletin summarizing and evaluating the subsidence levels, which is shared with the City Council and Civil Protection.

Over more than two decades of observation and measurement, it has been confirmed that the ground continues to move relentlessly and is expected to do so indefinitely. Since the ICGC began monitoring, the Barri de l’Estació has subsided by approximately two meters in its most affected area. This extensive dataset has made the site a reference case for understanding subsidence phenomena and for testing and validating ground monitoring systems.

Testing new technology

Years of experience in monitoring the Sallent case have allowed the ICGC to offer it as a testing and validation site for innovative monitoring systems that could provide solutions in similar scenarios. In line with this objective, a new GNSS (Global Navigation Satellite System) sensor, developed by the company Worldsensing, has been installed. This sensor operates autonomously, wirelessly, and within a network, enabling enhanced precision.

The new sensor being tested complements the existing conventional monitoring system, allowing its measurements to be cross-validated.

Geological risks in Catalonia

Geological phenomena that pose risks to society are highly varied, differing in both their timescales and spatial impact. In Catalonia, the most significant geological risk is river and coastal flooding, followed by land movements such as landslides and subsidence. 

Ground monitoring is a crucial tool for managing geological risks, as it provides insights into the dynamics and contributing factors of these phenomena. This knowledge enables the optimization of risk mitigation measures, whether through designing protective structures or planning land use and activities based on predictions of terrain behavior over time.

The various places in which the ICGC intervenes to some extent in risk management make up the geotechnical auscultation network (XAG) that integrates different sensors depending on the variable to be measured. Mainly the movement of the ground is measured, both in depth and on the surface and its consequences on built structures, as well as the factors that influence the behavior of the ground (the action of water and environmental agents).

The various sites where the ICGC is involved in risk management form the geotechnical auscultation network (XAG), which integrates different sensors based on the specific variables being measured. Primarily, the network monitors ground movement, both subsurface and surface, as well as the impacts on built structures. Other factors that influence ground behavior, such as water activity and environmental agents, are also monitored.

The ICGC monitors a range of geological risks, including subsidence and collapse (Sallent, Súria), land swelling (Barberà de la Conca, Sant Vicenç dels Horts), landslides (Sant Esteve Sesrovires), rockfalls (Montserrat, Sant Miquel del Fai), and escarpment retreat (Castellfollit de la Roca), among others.

All images courtesy of ICGC.

This article was originally published at the Catalan Government press room on 24 November 2024. The original article can be accessed here.