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Mechanical Behavior of Micaceous Clays

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Mechanical Behavior of Micaceous Clays Journal of Rock Mechanics and Geotechnical Engineering 11 (2019) 1044e1054 Contents lists available at ScienceDirect Journal of Rock Mechanics and Geotechnical Engineering journal homepage: www.rockgeotech.org Full Length Article Mechanical behavior of micaceous clays Jiahe Zhang a,*, Amin Soltani b, An Deng a, Mark B. Jaksa a a School of Civil, Environmental and Mining Engineering, The University of Adelaide, Adelaide, SA 5005, Australia b Department of Infrastructure Engineering, The University of Melbourne, Parkville, VIC 3010, Australia article info abstract Article history: This study aims to investigate the effect of mica content on the mechanical properties of clays. Received 6 October 2018 Commercially available ground mica was blended with a locally available clayey soil, at varying mica Received in revised form contents by mass of 5%, 10%, 15%, 20%, 25% and 30%, to artificially prepare various micaceous clay blends. 28 February 2019 The preliminary testing phase included consistency limits and standard Proctor compaction tests. The Accepted 26 April 2019 primary testing program consisted of unconfined compression (UC), direct shear (DS) and scanning Available online 30 May 2019 electron microscopy (SEM) tests. The test results showed that the liquid and plastic limits exhibited a linear, monotonically increasing trend with increase in mica content. The rate of increase in the plastic Keywords: Micaceous clay limit, however, was found to be greater than that of the liquid limit, thereby leading to a gradual Mica content transition towards a non-plastic, cohesionless character. The soft, spongy fabric and high water demand Consistency limits of the mica mineral led to higher optimum water contents and lower maximum dry unit weights with Compaction increasing mica content. Under low confinement conditions, i.e. the UC test and the DS test at low normal Shear strength stresses, the shear strength was adversely affected by mica. However, the closer packing of the clay and Confinement mica components in the matrix under high confinement conditions offsets the adverse effects of mica by Frictional resistance inducing frictional resistance at the shearing interface, thus leading to improved strength resistance. Ó 2019 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/). 1. Introduction minerals, although rather resilient, may gradually recover their initial shape due to the elastic rebound (or springy action), thereby The mica group of sheet silicates is among the most widely reducing the efficiency of compactive effort and hence potentially distributed minerals around the world; they generally occur in compromising the performance of various facilities constructed on igneous, sedimentary and certain metamorphic rocks (Harvey, micaceous clays (Weinert, 1980). When such soils are unloaded, 1982; Galán and Ferrell, 2013). Although the compositions and elastic rebound is likely to occur, resulting in undesirable volu- properties of mica minerals vary depending on their geological metric expansion in the matrix. During compression, tension or formation and climatic conditions, the unique platy structure, high shearing, the mica particles tend to rotate and orient themselves in elasticity and nearly-perfect basal cleavage (owing to the hexagonal a somewhat parallel fashion (attributed to mica’s platy shape), sheet-like arrangement of mica atoms) are the common features thereby resulting in low strength resistance in micaceous soils which demand further attention (Zhang et al., 2019). Where mica (Harris et al., 1984). In this instance, micaceous soils are basically minerals are separated from their host rocks, these features may characterized by high compressibility, poor compactibility and low affect naturally weathered soils, thus leading to some adverse shear strength; such attributes present significant challenges for changes in the mechanical behavior of such soils. road construction, building foundations, earth dams and other Due to the extremely elastic properties of mica minerals, geotechnical engineering systems, as reported in several countries attributed to mica’s soft, spongy fabric, the micaceous soils in around the world (e.g. Mitchell et al., 1975; Gogo, 1984; Northmore particular micaceous clays may deform remarkably under applied et al., 1996; Paige-Green and Semmelink, 2002; Netterberg et al., load and hence affect the bulk compressibility of such soils. Mica 2011). The majority of documented studies have addressed the me- chanical responses of coarse-grained micaceous soils (e.g. Tubey, * Corresponding author. 1961; Tubey and Bulman, 1964; Moore, 1971; Tubey and Webster, E-mail address: [email protected] (J. Zhang). 1978; Harris et al., 1984; Ballantine and Rossouw, 1989; Clayton Peer review under responsibility of Institute of Rock and Soil Mechanics, Chi- et al., 2004; Mshali and Visser, 2012, 2014; Zhang et al., 2019). To nese Academy of Sciences. https://doi.org/10.1016/j.jrmge.2019.04.001 1674-7755 Ó 2019 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. This is an open access article under theCCBY- NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). J. Zhang et al. / Journal of Rock Mechanics and Geotechnical Engineering 11 (2019) 1044e1054 1045 the authors’ knowledge, however, there are still limited studies Table 2 involving the mechanical behaviors of fine-grained micaceous soils. Physical and chemical properties of ground mica. Of those examining fine-grained micaceous soils, no relationship Properties Value was developed between the mica content and the mechanical Physical Appearance Fine white powder M behavior of these soils. Meanwhile, the ever-increasing need to Specific gravity of solids, Gs 2.8 expand urban areas to satisfy population growth and industriali- Fines (<75 mm) (%) 93 zation has required additional land, and in some cases, land with Sand (0.075e4.75 mm) (%) 7 Particle diameter, D90 (mm) 53.6 suboptimal soil properties. The utilization of local materials, one 2 Specific surface area, Sa (m /g) 5.3 being micaceous clays, may eliminate the costs associated with Natural water content, wN (%) 0.41 transporting new materials from other locations. Therefore, the Hardness (Mohs) 2.5 potential reuse of micaceous soils, and micaceous clays in partic- Chemical SiO2 (%) 49.5 ular, can lead to improved efficiencies and enhanced infrastructure Al2O3 (%) 29.2 K O (%) 8.9 performance, if an in-depth understanding of their geotechnical 2 Fe2O3 (%) 4.6 properties can be obtained. TiO2 (%) 0.8 The present study seeks to investigate the effect of mica content MgO (%) 0.7 on the mechanical properties of clays. A test program was designed Na2O (%) 0.5 and conducted, which consisted of two phases, namely preliminary CaO (%) 0.4 Acidity, pH value (20% slurry) 7.8 and primary tests. The preliminary testing phase included consis- Oil absorption (mL/100 g) 36 tency (Atterberg) limits and standard Proctor compaction tests, and Loss on ignition, LoI (at 1000 C) (%) <6 the primary tests consisted of unconfined compression (UC) and direct shear (DS) tests. Moreover, scanning electron microscopy (SEM) studies were carried out to observe the evolution of fabric in water content of wopt ¼ 22.04% corresponding to a maximum dry response to the mica inclusions, and thus perceive clayemica 3 unit weight of gdmax ¼ 16.21 kN/m . interactions. 2.2. Ground mica 2. Materials Commercially available ground mica, sourced from a local fi 2.1. Clayey soil distributor, was used to arti cially prepare various micaceous clay blends. The physical and chemical properties of the ground mica, as Locally available reddish-brown clay was used for this study; it supplied by the manufacturer, are summarized in Table 2. In terms fi was sourced from a landfill site located near Adelaide, South of grain size distribution, the ground mica consisted of a nes < m e Australia. The physical properties of the clay soil, hereafter simply fraction ( 75 m) of 93%, along with 7% sand (0.075 4.75 mm). fi M ¼ referred to as the natural soil, were determined as per relevant The speci c gravity of the mica particles was found to be Gs 2.8, fi American Society for Testing Materials (ASTM) and Australian which is quite similar to that of natural ne-grained soils including S ¼ Standards (AS), and the results are summarized in Table 1. The the one used in the present study, i.e. Gs 2.74. Other physical fi ¼ 2 conventional grain size analysis (ASTM D422e63(2007)e2, 2007) properties included a speci c surface area of Sa 5.3 m /g. The indicated a clay fraction (<2 mm) of 37%, along with 32% silt (2e chemical composition of the ground mica was found to be domi- 75 mm) and 32% sand (0.075e4.75 mm). In terms of consistency, the nated by silicon dioxide (SiO2) and aluminum trioxide (Al2O3) with liquid limit and plasticity index were, respectively, measured as mass fractions of 49.5% and 29.2%, respectively. In terms of acidity, the ground mica slurry was classified as a neutral substance cor- wL ¼ 46.21% and IP ¼ 28.1%; the soil was hence classified as clay with intermediate plasticity (CI) in accordance with the Unified Soil responding to a pH value of 7.8. Classification System (USCS). The standard Proctor compaction test, carried out as per ASTM D698e12e2 (2012), indicated an optimum 3. Experimental work In this study, a total of seven soilemica mix designs consisting of one control, the natural soil, and six micaceous clay blends were Table 1 examined (see Table 3). Hereafter, the coding system Mx, where x is Physical properties of the natural soil. the mica content or Mc and x ¼ {0, 5, 10, 15, 20, 25, 30}, is used to Properties Value Standard designation designate the various mix designs; the mica content was defined as S the ground mica to the natural soil mass ratio.
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