Fraeylemaborg is a noble house in an earthquake-stricken area of the Netherlands due to the induced seismicity events in the region. The structure is located in the middle of the town of Slochteren which gave its name to the largest gas field in the world upon its discovery in 1959. The gas extraction has caused small-magnitude shallow earthquakes during the last decade, damaging not only the residential inventory but also the historical structures in the area. The main building of Fraeylemaborg sits on an artificial island surrounded by water channels, rendering the problem of earthquake response even more complicated. A small part of the main structure on the island was built in the 14th century, while the construction of additional parts and morphological alterations had taken place until the 18th century. The structure has been subjected to several small magnitude earthquakes causing damages on the load bearing system. An extensive renovation and repair of damages took place in recent years, however the latest seismic events imposed again damage to the structure. This paper presents a project of monitoring, assessment and diagnosis of problems for the Fraeylemaborg, the most important “borg” of the region, underlining the particularities of the induced seismicity problem. The FE model has been calibrated by using ambient vibration tests. Combination of earthquake and soil settlement loads have been applied on the calibrated model. The paper develops scenarios that help in explaining the reasons behind the damages on this structure during the recent shallow and low-magnitude induced seismicity earthquakes.
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Post-earthquake structural damage shows that wall collapse is one of the most common failure mechanisms in unreinforced masonry buildings. It is expected to be a critical issue also in Groningen, located in the northern part of the Netherlands, where human-induced seismicity has become an uprising problem in recent years. The majority of the existing buildings in that area are composed of unreinforced masonry; they were not designed to withstand earthquakes since the area has never been affected by tectonic earthquakes. They are characterised by vulnerable structural elements such as slender walls, large openings and cavity walls. Hence, the assessment of unreinforced masonry buildings in the Groningen province has become of high relevance. The abovementioned issue motivates engineering companies in the region to research seismic assessments of the existing structures. One of the biggest challenges is to be able to monitor structures during events in order to provide a quick post-earthquake assessment hence to obtain progressive damage on structures. The research published in the literature shows that crack detection can be a very powerful tool as an assessment technique. In order to ensure an adequate measurement, state-of-art technologies can be used for crack detection, such as special sensors or deep learning techniques for pixel-level crack segmentation on masonry surfaces. In this project, a new experiment will be run on an in-plane test setup to systematically propagate cracks to be able to detect cracks by new crack detection tools, namely digital crack sensor and vision-based crack detection. The validated product of the experiment will be tested on the monument of Fraeylemaborg.
The structure will be monitored real-time and reasons behind the damages will be found. Proposals for protecting the structure against earthquakes will be made. - Damage scenario of the building, in relation to the induced seismicity effects on structures in the region- Establishment of a real-time structural monitoring toolThe building will be instrumented with accelerometers and displacement crack sensors. Additionally to the monitoring efforts, the structure will also be modelled in FE computer simulations in an effort trying to find out possible future response of the monument to strong earthquakes. The monitoring data will be combined with FE simulations in concluding the response of the structure to recursive induced seismic events.
This top-up project is related to the on-going RAAK MKB-project SafeGo (Seismic Monitoring, Design And Strengthening For thE GrOningen Region) . SafeGo combines knowledge of SMEs in the earthquake region of Groningen with innovative solutions and demonstration of technologies, to improve the process of seismic strengthening of houses. Innovative methods and approaches for monitoring and strengthening of structures are tested and further developed in SafeGo In the monitoring part of the project, SafeGo combines soil data, structural data and the sensor data to reach conclusions for the reasons behind observed damages in buildings. Fraeylemaborg, a castle-museum in Slochteren dating back to the 14th century, is used as a testbed. Various sensors are used for monitoring accelerations, tilt and water pressure. In the strengthening part of the project, masonry walls were built and strengthened by the participating SMEs. These walls are placed on the shake table and tested with real earthquake vibrations. A shake table is an accurate laboratory equipment which simulates earthquakes. Majority of the tasks in SafeGo are related either to the site or to the laboratory, which are environments outside of the school. Although an intensive student participation was initially planned, this was not achieved due to COVID19 crisis and the series of mobility restrictions, neither in the monitoring nor in the shake table testing parts of the project. This top-up project aims to transfer the knowledge and create interaction with the students for the SafeGo project. Visitation to the monitored building and presentations to the students on the monitoring system, visitations to the shake table laboratory and interactive events are planned within this project.
