Breaking New Ground
Eolien Maritime France (EMF), an offshore wind development company, is developing two offshore windfarms founded on monopiles driven into calcarenite weak rock. Turbines around 150m high with more than 50m diameter blades will be placed on seven 7m diameter piles in the calcarenite weak rock, and in an industry first, Fugro has conducted the onshore tests to understand the behaviour of the monopiles in this type of rock.
To learn how monopile foundations will penetrate weak calcarenite rock and their resistance to environmental forces, large diameter lateral pile tests were performed in an old quarry in Gouvieux in the North of France, using technology specially designed and built for the project by Fugro. Fugro Pile Monitoring Specialist Michel Maron explains: “Pile tests of such significant diameter are very rare and these types of loads applied to the piles as part of the tests have never been done before in this type of rock. It is not clear how the rock will react to the monopiles. Our onshore pile load tests were completed to address these concerns, as well as optimise the monopile design and installation.”
For the tests, which were conducted over a three-month period at the end of last year, Fugro developed a purpose-built load frame and monitoring system using the latest sensors and data acquisition technologies. The loading system, which can pull a pile up to 250t horizontally until failure at 5m above ground level, is the only one of its kind. It uses a hydraulic power pack and electronic controls which allow maximum lateral load, speed of loading, number of cycles and loading steps to be modified on site as needed. The high flow hydraulic assembly is adapted to mimic the cyclic loading of the environmental conditions, such as the waves and wind, at the offshore windfarms.
Preliminary interpretation of the results were supervised by internationally recognised experts in geotechnics and pile driving – Dr Alain Puech and Dr Mark Randolph of the University of Western Australia, both employed by Fugro. The test results are currently being used as high quality data for benchmarking the adopted design methodology for static and cyclic, laterally loaded testing of monopiles.