Fraeylemaborg giving information on earthquake damage in Groningen

Cracks. Dozens of which disfigure the walls of the stately Fraeylemaborg in Slochteren. The cause: earthquakes. This is more certain than ever now that 'earthquake professor' Ihsan Bal of the Hanze University of Applied Sciences is putting his monitoring method into practice at the estate. Students and researchers are keeping a close eye on the data.

Fraeylemaborg. (photo: Pepijn van den Broeke)

You don’t need a carpenter’s eye to see that the centuries-old castle is suffering from the earthquakes. The walls bear small and large cracks while the real misery is hidden behind them. “When the plaster is removed, you can see that the stone is sometimes cracked in three places. All these stones need to be replaced or mended. The areas to be restored are sometimes up to 30 centimetres deep”, says the director of the castle Marjon Edzes.

She is pleased with the various monitoring instruments that have been placed in the Fraeylemaborg. Out of sight of course. These devices record exactly how the building moves when the earth shakes. This is a major step forwards, especially the immediate correlation with seismological data. The link between cause and effect is thus immediately clear.

“The data being retrieved is unbelievable. We discovered some cracks once and the meters showed that the building had skewed a little, days after an earthquake. You wouldn’t discover these things without this technology.”

The niggling’s stopped

The Fraeylemaborg case isn’t comparable with houses in the earthquake region. And yet again it is. Marjon Edzes: “You were always left with a niggling feeling: are those cracks down to old age, or is this earthquake damage? Now that we can prove it clearly, the niggling’s stopped. Ideally, I would wish this for all the residents in the area.”

The building’s dungeons and attics house various instruments. There are accelerometers to measure vibrations, crack sensors are hung on the walls, and the previously placed tilt meters also play their part. One room is set up as a nerve centre, where computers model everything in a 3D simulation of the castle. Students and researchers from the Hanze UAS come there every week to collect the data and use it for further research.

Marjon Edzes in Fraeylemaborg. (photo: Pepijn van den Broeke)

All kinds of things are made visible. Which parts of the building are moving, in which direction, the effects on connecting parts. The data is combined with data on the structural strength of the old-fashioned bricks, the influence of the soil under the castle, and that of the moat, etc. The ultimate goal is to be able to predict how the building will behave in the event of variously sized earthquakes and to conclude which precautionary measures need to be taken.

Damage returned

This is knowledge that will benefit the entire earthquake region and in particular the Fraeylemaborg. Over the past few years, a great deal has been invested in restoration and reinforcement. Marjon Edzes: “About five years ago it began to dawn on us, when our staff said: the building is suddenly aging very quickly. At the time, we thought that earthquake damage occurred mainly in Loppersum and its surroundings. Experts concluded, however, that it had also penetrated to this area.”

The large-scale repairs went well, but the damage returned with the next earthquakes. With the research conducted by Ihsan Bal’s team, predictions can be made as to where the damage can be expected. He is especially interested in the question of whether earthquakes caused by the extraction of natural gas result in substantially different damage than earthquakes on the fault lines of tectonic plates. What also surprises him is that in the Netherlands earthquakes are assessed in a different way than in the rest of the world, and wrongly so. “We are convinced that draft legislation must be drawn up at European level for this type of earthquake and that the Netherlands must take the lead in this.”

Marjon Edzes is aware of the Fraeylemaborg’s special position. “The research has created a lot of clarity with regards to damage. The evidence is visible and available to everyone. Any doubt has been eradicated. And the castle is perhaps a standard-bearer of the struggle here in the region against how earthquake damage is currently settled. We are happy to show it to the Minister and the House of Representatives, and on King’s Day Willem Alexander studied a model of the Fraeylemaborg which stood on a vibrating table on the Grote Markt in Groningen”.

Fraeylemaborg. (photo: Pepijn van den Broeke)

Earthquake professor

The Turkish professor Ihsan Bal was appointed a year and a half ago by the Hanze University of Applied Sciences to carry out practical research into the seismic events in the region and their relationship with buildings. He does this under the name Earthquake Resistant Structures. Professor Bal is also an independent expert for the State Supervision of Mines.