Field Testing for Monopile to Be Installed in Weak Carbonated Rock-Paper

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EDF Renouvelables conducted field tests to support monopile design for offshore wind farms where foundations will be installed in weak carbonated rock. Existing design methods were based on limited testing, often involving slender bored piles and loads applied at ground level, and did not adequately address driven monopiles, larger diameters or cyclic loading.<br /><br />In 2016, researchers installed 11 open-ended piles, 0.76 or 1.2 m in diameter, at a former quarry near Gouvieux, France. The calcarenite had a UCS of roughly 5–11 MPa. Tests were performed in dry conditions, with pile length-to-diameter ratios of about 2.6–4. Four installation approaches were investigated, including driving, drilling and grouting. Eight lateral load tests—four static and four cyclic—applied loads 5 m above ground level to represent wind-turbine overturning forces. Instrumentation measured pile movement and ground response.<br /><br />Installation caused shallow rock chipping, radial fractures and crushing around the pile wall. During lateral loading, a gap formed behind the pile; cyclic loading repeatedly opened and closed it, but it did not fully close because of irreversible crushing and rock degradation. Additional radial fractures developed, and some tests showed uplift and rapid displacement. Most tests were stopped before ultimate resistance to prevent pile yielding.<br /><br />The observations demonstrate that installation damage, crushed rock, fracture development and gap formation strongly affect lateral stiffness and resistance. The campaign provided data for calibrating design models and informed subsequent work on cyclic loading in weak rock. It also showed the value of early field testing for validating monopile design methods for offshore projects.

Keywords

monopile design

offshore wind farms

weak carbonated rock

lateral load tests

cyclic loading

pile installation damage

calcarenite

open-ended piles

driven monopiles

field testing