Investigation of Rate Effects for Monopile Foundations: Laboratory-Scale Model Tests-Paper

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The study investigates how loading rate and combined loading conditions affect monopile foundations used for offshore wind turbines. Existing design methods generally do not account for rate-dependent soil resistance, although research suggests pile capacity can increase with loading rate.<br /><br />The authors developed a laboratory-scale rig with two high-speed actuators, enabling variable loading rates and changing load eccentricity. It can apply complex, combined loads and measure pile movement and rotation. Initial tests in dry Leighton-Buzzard sand examined the effects of load eccentricity, vertical load, displacement rate and cyclic frequency.<br /><br />Preliminary results confirmed that the rig could reproduce prescribed loading paths. In very loose sand, lower load eccentricity increased lateral resistance, while added vertical load also increased the measured lateral reaction. These findings indicate that combined loading conditions influence pile response.<br /><br />Under monotonic lateral loading, changing displacement rate produced temporary capacity changes. However, these transients may be caused by inertia in the test equipment rather than by soil rate effects. Overall, the tests found no clear rate-dependent increase in pile capacity in dry sand. Cyclic tests at 0.5 Hz and 0.01 Hz likewise showed no visible difference in secant stiffness or accumulated ratcheting, though the limited test range means further investigation is needed.<br /><br />The authors conclude that equipment inertia must be carefully distinguished from genuine soil behaviour. Planned work includes testing in stiff clay and denser sand, broader cyclic-loading programmes, and further development of a constitutive model to represent rate effects. The results are intended to inform more accurate and potentially more economical monopile design.

Keywords

monopile foundations

offshore wind turbines

loading rate

combined loading

rate-dependent soil resistance

lateral resistance

load eccentricity

cyclic loading

pile capacity

soil behavior