Calcareous 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 the particle size (along with other factors). With the aim to propose an improved soil constitutive model for capturing the behaviour of calcareous 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 strength and dilative or contractive response of calcareous soils is discussed.
Calcareous soils cover over 35% of the ocean floor - prevalent in the Gulf 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 of these soils are not yet entirely understood, and have been identified as problematic 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 aim of this paper is to investigate the effect of particle size on mechanical behaviour of reconstituted calcareous sediments.
Calcareous sediments generally contain two polymorphs of calcium carbonate (CaCO3) – calcite and aragonite, and originate either through biogenic primary production or authigenic precipitation (Watson et al., 2019). Essentially the particles are highly angular, weak, fragile, and porous; 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 distinctive features of calcareous soils contribute to their specific behaviours: (i) the presence of intra-particle voids, and (ii) irregular shape 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. The features result from the various chemical, physical, mechanical and biological deposition processes of skeletal remains of marine organisms in deep water. These distinctive features and 70~98% carbonate content of 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).
