1 g Model Tests on the Displacement Accumulation of Large-Diameter Piles Under Cyclic Lateral Loading

Abstract

Large-diameter monopiles are an established foundation type for offshore wind energy converters. A crucial aspect in the design of such piles is the bearing behavior under cyclically acting wind and wave loads, namely the accumulation of deflections and rotations under these loads. Regarding the prediction of the behavior under cyclic one-way load, some empirical approaches and also numerical simulation methods exist. In contrast, the behavior under cyclic two-way loading is widely unclear and must be investigated by pile tests. This paper presents the results of 1 g model tests with large-diameter piles in sand under arbitrary cyclic loading types. It was found that for almost rigid large-diameter piles, the effect of the loading type on the rate of displacement accumulation could be accounted for by a function which is almost independent on the system parameters, e.g., pile stiffness or relative density of the sand, but dependent on load eccentricity. The maximum accumulation rate occurs for asymmetric two-way loading. Using the functions found in the tests, a prediction of displacement accumulation for load spectra consisting of packages of arbitrary loads becomes possible. For flexible piles, a different effect of the loading type on the rate of displacement accumulation was observed. Here, asymmetric two-way loading was obviously not more unfavorable than one-way load.

Effect of Stress Level on Response of Model Monopile to Cyclic Lateral Loading in Sand

“…Nine piles were installed per strongbox, with a minimum center-to-center spacing of 7.4D. This layout was chosen to avoid significant boundary effects while maximizing the number of tests per strongbox sample; the center-to-center spacing is in line with previous studies (e.g., Leblanc et al 2010;Abadie 2015;Albiker et al 2017).…”

“…The power-law coefficient A has been found to vary with ζ b and ζ c (Leblanc et al 2010;Nicolai and Ibsen 2014), and the power-law exponent α has often been reported as a constant within individual studies (Leblanc et al 2010;Nicolai and Ibsen 2014;Albiker et al 2017). However, Truong et al (2019) suggested that α varies with ζ c and D R .…”

“…This trend line suggests that for a full-size monopile in dense saturated sand (D ¼ 8 m, γ 0 ¼ 10 kN=m 3 , and the ratcheting exponent may be as low as 0.11, approximately half of the value in the 1g model tests. The value of α appears to vary with the chosen strain variable, and it is therefore difficult to compare α values from independent tests campaigns to build confidence in this conclusion on stress dependency; for example, Albiker et al (2017) found α ¼ 0.23 with pile rotation as the strain variable and α ¼ 0.135 with pile displacement (some distance above mudline) as the strain variable. However, the results presented here appear to be consistent with the increase in normalized pile displacement for 1g test compared with centrifuge tests reported by Rudolph et al (2014a, b).…”

Empirical approach based on centrifuge testing for cyclic deformations of laterally loaded piles in sand

“…The subsequent systematic experimental investigations of Rosquoet et al (2007), and Truong & Lehane (2015) show that the accumulation coefficient (α y ) depends primarily on the cyclic load ratio ( c ) and to a lesser extent the cyclic magnitude ratio ( b ), and varies from a negative value for two-way loading ( c = -1) to between about 0.05 and 0.2 at  c values in the range of -0.5 to 0.75 (noting  c = 0 corresponds to one-way loading with H min = 0). LeBlanc et al (2010) and Albiker et al (2017) present formulations based on 1-g small scale tests in which the accumulation coefficient is considered independent of sand relative density, although scale issues associated with the very low stress level prevalent in these tests need to be acknowledged. It is also of note that Albiker et al (2017) observed a dependency of the accumulation coefficient on the lateral load eccentricity as well as a difference between rigid and flexible piles.…”

“…LeBlanc et al (2010) and Albiker et al (2017) present formulations based on 1-g small scale tests in which the accumulation coefficient is considered independent of sand relative density, although scale issues associated with the very low stress level prevalent in these tests need to be acknowledged. It is also of note that Albiker et al (2017) observed a dependency of the accumulation coefficient on the lateral load eccentricity as well as a difference between rigid and flexible piles. Cyclic loading applied to monopiles is not uniform and a number of studies have examined the ability of the method of superposition presented by Lin & Liao (1999) to predict pile response under various packages of (uni-directional) cycles.…”