With offshore wind turbines becoming larger and moving to greater water depths, monopile design is now also driven by fatigue and serviceability limit states and soil-structure analyses at very small strains become increasingly important. New design methods, such as the PISA framework, directly apply the small strain shear modulus (G/sub max/) as input parameter, and therefore a reliable assessment is critical for design. Bender elements (BEs) are widely used in the laboratory to determine G/sub max/. However, practical design cases have shown that BE G/sub max/ measurements may sometimes be quite scattered in sands. Therefore, a rigorous laboratory program was performed to consistently 1) investigate the repeatability of BE G/sub max/ measurements in clean sand, 2) investigate potential sources of uncertainty in the BE measurements and 3) compare BE and resonant column (RC) measurements. BE measurements were taken in triaxial and RC apparatuses on nominally identical medium dense to very dense sand specimens at three different stress levels. The variation in G/sub max/ among different specimens tested under nominally identical conditions is found to be generally small. The variation in peak strength is however significantly less compared to the corresponding variation in G/sub max/. Trace quality appears to be a key factor for understanding the source of uncertainty in the measurements. RC G/sub max/ values compare well with BE G/sub max/ values.
Pasquale Carotenuto, Yusuke Suzuki, Rune Dyvik, Anders H. Augustesen and Lone Krogh
Keywords: Small strain shear modulus, shear wave velocity, bender element, resonant column, repeatability
Publisher: DFI