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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

Behaviour of Calcareous Soils as a Function of Particle Size
Benjamin S. Turner1, Muhammad S. Hossain1, Shah N. Ullah2, Youngho Kim1 and Usha Mani1
1Centre for Offshore Foundation Systems, Oceans Graduate School

2School of Engineering and Technology
Central Queensland University, Gladstone, Australia

The University of Western Australia, Perth, Australia

distinctive features of calcareous soils contribute to their specificbehaviours: (i) the presence of intra-particle voids, and (ii) irregularshape of particles from microfossils such as coccoliths (Hyodo et al.,
1996, 1998; Sharma & Ismail, 2006; Lim et al., 2018) (see Fig. 1). A clay
from the Gulf of Mexico (Fig. 1c) is also included for comparison. Thefeatures result from the various chemical, physical, mechanical andbiological deposition processes of skeletal remains of marine organisms
in deep water. These distinctive features and 70~98% carbonate contentof calcareous sediments have led to significantly higher liquidity index,sensitivity, in-situ void ratio, friction angles, compressibility, and strain
rate dependency. The effect of mean particle size (d50) on undrained
shearing behaviour was investigated by Mao & Fahey (2003) for two
calcareous silts. The muddy silt (d50 = 0.001 mm) showed contractive
behaviour, and the silt (d50 = 0.025 mm) showed dilative behaviour after
a phase transition (see Fig. 2).

ABSTRACTCalcareous soils have been identified as problematic due to the special
characteristics such as higher compressibility and softening, higher strain
rate dependency as well as contractancy and dilatancy depending on theparticle size (along with other factors). With the aim to propose an
improved soil constitutive model for capturing the behaviour ofcalcareous silt, an experimental program on a range of reconstituted
calcareous soils is undertaken for developing and calibrating the model.
This paper investigates the undrained shearing response (as measured
under triaxial compression conditions) of two calcareous soils of varying
silt contents. The effect of particle size on the undrained shear strengthand dilative or contractive response of calcareous soils is discussed.

KEY WORDS: calcareous soils; clay; one-dimensional consolidation
tests; triaxial compression and extension tests; critical state parameters.
INTRODUCTION
Calcareous soils cover over 35% of the ocean floor - prevalent in theGulf of Mexico along the Florida coastline and in the Bay of Campeche,
Arabo-Persian Gulf and the Red Sea, Southern Mediterranean Sea,
offshore India and North West Shelf of Australia (ISO, 2016). Despite
such widespread occurrence, the fundamental mechanical behaviours ofthese soils are not yet entirely understood, and have been identified asproblematic due to the special characteristics such as high carbonate
content, high compressibility, strong rate dependency and dilation after
a phase transition (Mao & Fahey, 2003; Coop et al., 2004; Sharma &Ismail, 2006; Boukpeti & White, 2011; Miao & Airey, 2013). The aimof this paper is to investigate the effect of particle size on mechanicalbehaviour of reconstituted calcareous sediments.

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Calcareous sediments generally contain two polymorphs of calcium
carbonate (CaCO3) – calcite and aragonite, and originate either throughbiogenic primary production or authigenic precipitation (Watson et al.,2019). Essentially the particles are highly angular, weak, fragile, andporous; and any deposit matrix consists of both intra and interparticle
voids (Hyodo et al., 1996, 1998; Coop et al., 2004; Sharma & Ismail,2006; Lehane et al., 2014; Lim et al., 2018). Microscopically, two main(a) Calcareous silty sand (after Beemer et al., 2018)

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