Proceedings of the Thirty-second (2022) International Ocean and Polar Engineering Conference
Shanghai, China, June 5–10, 2022
Copyright © 2022 by the International Society of Offshore and Polar Engineers (ISOPE)
ISBN 978-1-880653-81-4; ISSN 1098-6189

www.isope.org

Changing soil responses during episodic cyclic loading in DSS tests
Noor Laham1, Katherine Kwa1, Andrew Deeks2, Yusuke Suzuki2, David White1 & Susan Gourvenec1
1University of Southampton, Southampton, UK2Norwegian Geotechnical Institute, Oslo, Norway

ABSTRACTINTRODUCTION

Whole life geotechnical design is an emerging philosophy in offshoregeotechnical engineering to improve design outcomes by considering thewhole life of imposed actions coupled with geotechnical properties thatevolve with each action. Softening of normally consolidated clays fromundrained cyclic loading intervened with consolidation has been shownto lead to hardening, manifested by evolving strength, stiffness, andcoefficient of consolidation. This paper presents results from a set ofstress-controlled direct simple shear (DSS) tests. The tests follow prefailure episodic cyclic loading stress paths where each episode comprisesa packet of undrained cycles of loads followed by full consolidation. The
soil response under different numbers of cycles per loading packet andnumber of loading packets with intervening consolidation isinvestigated. The results from this study quantify the effect of theundrained cyclic loading history, for the same final number of cycles, on
the evolution of the soil properties, to support the application of whole
life geotechnical design in practice. Outcomes allow calibration ofdesign curves that are traditionally used to capture softening, byintroducing consolidation effects. This enables capturing the whole life
softening and hardening processes, by extending the traditional contour
diagram representation for undrained cyclic loading without
consolidation to allow for consolidation periods.

Renewable Energy Future
The global offshore energy industry is going through a significant periodof transition aiming to net zero greenhouse gas emissions in the coming
few decades, where renewable energy is expected to be one of the fastestgrowing energy sources globally in this domain (IEA, 2021). In line with
the Paris agreement for climate neutral energy or energy with nogreenhouse gas emissions in the coming 30 years, investments andtechnologies are needed to decarbonize the energy system. East Asia isresponsible for 34% of the global energy-related emissions, where 53%
reduction in carbon emissions is planned by 2050 through the transition
towards renewable energies (IRENA, 2019). Asia’s share of the global
offshore market is expected to grow from 24 % in 2019 to 42% in 2025
(Reglobal, 2020). In addition, Europe is aiming to increase its offshore
capacity from wind and other ocean energy (e.g., wave and tidal) to atleast 340 GW by 2050, i.e., multiplying the capacity for offshore
renewable energy by nearly 30 times (EC, 2020). In parallel, the UK isaiming to quadruple the amount of offshore wind generated to 40 GWby 2030 and to 100 GW or more by 2050 (CCC, 2020). Fig. 1 presents
the global growth of the offshore wind market over the next decade
where Asia is expected to double its share by 2030 although Europe isstill expected to remain as the largest regional offshore wind market. The
transition towards a clean decarbonized energy future is underway. To
cope with the rapid development the world is witnessing, offshore
industries are aiming for advancement of efficient offshore renewableenergy structures through increasing the capacity and efficiency ofdesigns.

KEY WORDS
Offshore engineering; Geotechnics; Direct simple shear; Clay.

NOTATIONS
𝑒𝑒 void ratio, 𝑒𝑒0 initial void ratio

𝐺𝐺𝑠𝑠 specific gravity of solid fraction
𝑠𝑠𝑢𝑢 undrained strength
𝑠𝑠𝑢𝑢0 monotonic undrained strengthΔ𝑢𝑢 pore-water pressure generation during undrained shear stage
𝛾𝛾 shear strain
𝜅𝜅0 slope of one-dimensional swelling line
𝜆𝜆0 slope of one-dimensional normal compression line𝜙𝜙 ′ angle of internal friction
𝜎𝜎𝑣𝑣′ vertical effective stress′
𝜎𝜎𝑣𝑣0
equilibrium vertical effective stress′
𝜎𝜎𝑣𝑣𝑣𝑣consolidation vertical effective stress𝜏𝜏 shear stress
D damage ratio between the pore- water pressure generation and theconsolidation effective stress𝑐𝑐𝑣𝑣 coefficient of consolidation

Fig. 1 Global offshore wind growth to 2030 (GWEC, 2020).

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