The former Franciscan monastery of Werthenstein was built on rock face that is eroding more and more by the day. Now stabilisation measures are being taken to stop this process. In order to implement them as quickly and precisely as possible, the planners are relying on a flying assistant, and finding new ways to use building information modeling.
Stefan Meile steps out of a narrow door at the back of the monastery. After just one step, he is on the brink of an abyss: a vertical drop of 50 metres. Secured by a steel cable, Stefan Meile, Director Geomatic at Basler & Hofmann, and his colleague Adrian Gill assess the condition of the edge of the cliff and the monastery walls.
The Franciscan monastery was built high above the Kleine Emme river in the town of Werthenstein, Lucerne, in 1636. The listed building on the cliff is as impressive as ever, but the ground beneath it is crumbling. Precipitation and water run-off have eaten away at the eastern side, leaving deep rifts in the soft layers of marl. The Kleine Emme also flows just along the base of the rock face, washing it away bit by bit. What is left are massive projections of harder rock that are now threatening to break off. As a result, rockfall and landslides have become more frequent in recent years. A piece of the monastery walls even caved in over a particularly brittle piece of rock a few years ago.
Now the responsible authorities in the canton of Lucerne want to prevent further damage. “Before we can do that, we need a precise image of the rock”, says Stefan Meile. That is why he brought along a flying assistant today: a drone to take pictures of the 150-metre-wide, 50-metre-high slope. First, the surveyors leave clearly visible ground control points up on the edge of the cliff. Later, they will use a tacheometer to measure the points the traditional way and fit the rock face into the national projection coordinates. Stefan Meile and Adrian Grill carefully make their way along the precipice and secure the ground control points at various places in the ground. After a short drive, they do the same thing at the base of the cliff. Then the drone takes over, whirring, rising and floating towards the rock face. In keeping with a pre-programmed pattern, it flies along the slope at a distance of about 20 metres until it has recorded it from the bottom up.
The entire cliff in a single picture
Using this high-tech flying device is unusual. “It’s normally enough to do a scan from the ground”, says Stefan Meile. But because the Werthenstein cliff is jagged and interspersed with overhanging rocks, conventional surveying would not be able to record all of the face. Because time is of the essence, the team needs to quickly get the most accurate image of the rock possible. The perfect job for a drone. At first, everything goes according to plan. Then, suddenly, the drone comes to a standstill in mid-air: an emergency stop. The drone pilot stopped it just in time to avoid a collision with a cable running over the Kleine Emme to the cliff. Then he has to reprogram the course. Afterwards, recording continues at a lively pace. The little flying object takes over 1’000 overlapping pictures. Later, Stefan Meile will use these pictures to construct a single image of the entire rock face on his computer. Based on this image, software will generate a 3-D model of the face, accurate down to two centimetres.
More precise planning
“This 3-D model is a unique tool”, says Mathias Amstad. The geotechnical engineer from Basler & Hofmann is responsible for stabilisation measures on the Werthenstein cliff. For example, the company plans to break off a rock spur and surround the remaining rock with a cover net and concrete discs. Anchored below the largest overhanging rocks, they are intended to protect the rock from further weathering, while shoring up the overhanging parts. Mathias Amstad has a few cross sections of the measures hanging on the walls of his office. “But they only show individual sections of the overhanging rocks. It’s not the full picture”, he remarks. “With the conventional method, there are often surprises waiting for you in the rock face”.
More cost-effective implementation
The 3-D model is a different story. It gives the engineers a much more accurate picture in advance. In order to make the most of this information, the team of engineers imported the model into a building information modelling system (BIM). This is an unusual approach because planners usually use BIM to plan an entire building in 3-D, but it also works for the rock model. It allows the engineers to plan their stabilisation measures in 3-D from the very beginning. Instead of individual drawings, the planners and building contractors can now base their work on a complete, precisely calculated image of the face and the stabilisation measures. “Our planning is custom-made”, explains Mathias Amstad, “and we can easily share our thoughts with our partners”. For example, the team knows in advance how much concrete they need for the stabilisation features, and how the structures need to fit onto the rock face. That offers the client a major advantage: more reliable planning and calculation of costs.
In Werthenstein, Stefan Meile and his colleagues have packed up their measuring instruments and drone again. This winter, excavators and concrete mixers will make their way up to the 400-year-old monastery to ensure that it will safely grace its cliff for many years to come.