Abstract
Sand–clay mixtures can be encountered in both natural soils (e.g., residual soils, clay deposits and clinosols) and artificial fills. The method of utilising biopolymers in ground improvement for sand–clay mixtures has emerged recently. However, a full understanding of the strengthening effect of biopolymer-treated sand–clay mixtures has not yet been achieved due to a limited number of relevant studies. In this study, xanthan gum (XG), as one of the eco-friendly biopolymers, was used to treat reconstituted sand–clay mixtures that had various compositions in related to clay (or sand) content and clay type (kaolin and bentonite). A series of laboratory unconfined compression strength (UCS) tests were conducted to probe the performances of XG-treated sand–clay mixtures from two aspects, i.e., optimum treatment conditions (e.g., XG content and initial moisture content) to achieve the maximum strengthening effect and strengthening efficiency for the sand–clay mixtures with different compositions. The experimental results indicated that the optimum initial moisture content decreased as the sand content increased. The optimum XG content, which also decreased with the increasing sand content, remained approximately 3.75% for all sand–kaolin mixtures and 5.75% for all sand–bentonite mixtures if calculated based on clay fraction. While untreated sand–kaolin mixtures and sand–bentonite mixtures had comparable UCS values, XG-treated sand–kaolin mixtures seemed to have better improved mechanical strength due to higher ionic (or hydrogen) bonds with XG and low-swelling properties compared with bentonite. The deformation modulus of XG-treated sand–clay mixtures were positively related with UCS. The variation in UCS and stiffness for each treatment condition increased as the sand content was elevated for both sand-kaolin and sand-bentonite mixtures. An increment in the proportion of the heterogeneous composite formed by irregular sand particles conglomerated with the XG–clay matrix in total soil might be responsible for this phenomenon.
Highlights
Introduction iationsMechanical properties are key engineering properties of soils in geotechnical engineering practices
Sieving test was conducted to obtain the sand grading property Dmax = 0.6 mm, D50 = 0.46 mm, coefficient of uniformity Cu = 1.44 and coefficient of curvature Cc = 0.96, based on which the sand can be described as poorly graded with relatively uniform particle size
Taking the sand–kaolin mixture (SK13) as an example, the uniaxial compression behaviour was gradually improved as the initial moisture content increased compression behaviour was gradually improved as the initial moisture content increased from 16 to 22%
Summary
Mechanical properties are key engineering properties of soils in geotechnical engineering practices. For those natural soils that have insufficient mechanical strength, soil treatment is often employed [1,2,3,4,5,6,7]. The incorporation of biopolymers into soil stabilisation has gained increasing credence in sustainable geotechnical engineering for their environmental benefits [8,9,10,11], high strengthening efficiency [12,13,14], abundance in nature [15,16,17], suitable functional properties such as pH stability and ionic salt compatibility [18,19,20] and reasonable prices [8,21,22]. Selected polysaccharide biopolymers (e.g., xanthan gum, agar gum, gellan gum, chitosan, beta-glucan, starch, guar gum and carrageenan) have proved their potential in improving the soil performances under external loads in terms of unconfined compression, triaxial compression, direct shear, interface shear, tension, three-point bending and split [20,23,24,25,26,27,28,29,30,31,32,33,34].
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