Exploring rate effects for monopiles installed in silty sand

Large diameter monopiles supporting offshore wind turbines may experience loading at a range of rates during their lifetime. The preliminary study reported in this paper investigates the effect of rate on monopile monotonic and cyclic response in a silty sand, which is considered equivalent to the response in sand at full scale. Tests were performed in which the monopile was subject to lateral loading at height above the soil surface, with the results suggesting no significant difference in load-displacement behaviour due to loading rate. Some changes in the failure mechanism were observed, in terms of depth to the pivot point, while the results also highlight potential density changes around the pile as a result of (slow) loading. A slow cyclic test demonstrated mostly expected behaviour, e.g. log-linear cyclic ratcheting and greater post-cyclic lateral capacity, although a non-monotonic change in stiffness with numbers of cycles was unexpected and requires further investigation.