Correlation of Geotechnical and Mineralogical Properties of Lithomargic Clays in Uttara Kannada Region of South India

The Uttara Kannada region of south India is renowned for its unique lithomargic clays (locally known as shedi soil), attracting significant attention from geotechnical and geological researchers due to their distinctive mineralogical and geotechnical characteristics. Lithomargic clays with dispersive properties are prone to erosion, and they are mostly found along south India’s western coast. Our forebears dismissed lithomargic clay due to its perceived issues, including high silt content and diminished shear strength upon contact with water. This soil type is prone to erosion and landslides. Presently, the coastal region of the Uttara Kannada district is experiencing substantial growth, encompassing industrial expansion, infrastructure initiatives and numerous other undertakings. Transforming lithomargic clay into a suitable foundation for diverse engineering structures, such as buildings, pavements, railways and dams, presents a considerable challenge, necessitating a thorough examination. Lithomargic clays, formed through the weathering of mafic rocks, have gained prominence in recent years as potential materials for various engineering applications. Lithomargic clays are deposited below lateritic formations at shallow depths, packed in between the parent granitic gneiss beneath and the hard lateritic crust [1]. This article aims to provide a comprehensive review of the correlation between geotechnical and mineralogical properties of lithomargic clays in the Uttara Kannada region, shedding light on their potential utilization in construction, geotechnical engineering and other relevant fields. The primary minerals found in the lithomargic clays of the Udupi regions were quartz and feldspar, including orthoclase and muscovite. The secondary minerals, which resulted from the breakdown and chemical modification of the primary minerals, included high concentrations of sheet minerals, such as kaolinite, halloysite, dickite, gibbsite and illite. The investigation also revealed the existence of iron compounds, including siderite, goethite and fayalite. Oxygen, aluminum, silicon, potassium and iron comprised most of the elements found, indicating the presence of the compounds found via XRD analysis [2]. A correlation study conducted by Bhagyashree et al. [3] on lateritic soil overlying lithomargic clay layer showed that the liquid limit decreases with quartz, magnetite and hematite, whereas the plastic limit increases with corundum and anatase, remains constant with quartz and decreases with magnetite and hematite. The coefficient of permeability decreases with anatase and magnetite content. Lakkimsetti and Nayak [4] investigated the performance of lithomargic clay through mineralogical analysis and observed that chemical stabilization using calcium chloride (CaCl₂) and an industrial by-product obtained from the iron industry, i.e., granulated blast furnace slag (GBFS), significantly increased the unconfined compressive strength of lithomargic clay through the development of compounds such as calcium-silicate-hydrate (CSH), calcium-alumina-hydrate (CAH), calcium-aluminate-silicate-hydrate (CASH) in the stabilized soil mixtures and helped in bonding the soil grains to form a stiff and strong soil matrix. Such revelations provide a nuanced understanding of the intricate connection between mineralogy and engineering properties. Amulya et al. [5] found that the unconfined compressive strength (UCS) and the California bearing ratio (CBR) increased when 30% of GGBS replaced by weight of soil and 30% of fly ash replaced by dry weight of soil were added, respectively. As we synthesize these diverse findings, our aim is to contribute meaningfully to the understanding of the correlation between geotechnical and mineralogical properties of lithomargic clays in the Uttara Kannada region. This comprehensive exploration not only deepens our scientific understanding but also provides valuable insights for future research endeavors and engineering applications in areas rich in lithomargic clays.

In the western coastal regions of India, stretching from Trivandrum to Mumbai, lithomargic clay is commonly found at depths from one to three meters beneath the uppermost lateritic soil. This type of clay comprises a significant portion of the silt deposits [6]. Lithomargic clay presents challenges, as it allows water penetration, significantly reducing its strength in saturated conditions. The high silt content contributes to issues such as slope failures, differential settlements, foundation failures and embankment failures. The feldspar undergoes significant mineral changes during the weathering of granite or granitic gneiss, changing into montmorillonite, illite, kaolinite or halloysite, and finally bauxite [7]. Stabilizers, such as ground granulated blast furnace slag, can improve the strength characteristics of clays, which are lithomargic [8]. Thomas et al. [9] found that soils with lower plasticity indices are more prone to erosion than lithomargic clays with higher indices. Highway and foundation engineers face a number of difficulties when working with shedi soils, such as the loss of shear strength when wet, the elimination of confinement, erosion issues, landslides and problems with slope stability. Soft soils present foundation problems, such as low shear strength and increased settlement, making it challenging for civil engineers to design and construct structures on such problematic soils. The addition of coir fibers by 0.5% of dry weight of soil significantly improved the soil’s strength properties. Reinforced with randomly distributed untreated coir fibers, the cohesion of the soil rose roughly fivefold [10]. The addition of coir mats positioned at one-third and two-thirds height of a UCS sample along with coir mats coated in liquid cashew nut oil strengthened the clay [11]. Narloch et al. [12] studied the influence of soil mineral composition on the compressive strength of cement stabilized rammed earth (CSRE), discovering that both beidellite and montmorillonite significantly decreased the compressive strength of CSRE. Slightly higher compressive strength was achieved with kaolinite. Montmorillonite—one of the clay minerals with the highest adsorptive ability—may chemically adsorb potassium ions, which can result in the creation of illite; commonly, igneous and metamorphic rocks include them. The potash feldspar typically occurs as a microcline rather than an orthoclase [13]. The soaked CBR values for lithomargic clays along the coastal area of the Uttara Kannada district were found to be less than 5% [14]. The XRD analysis of lithomargic clay showed the presence of minerals such as gibbsite, kaolinite, biotite and muscovite [15]. Dickite and other kaolin minerals are formed as a result of feldspar and muscovite weathering. The differences in layer stacking atop one another yield different members of the kaolinite subgroup [16]. Like sodium (Na), potassium (K) can also cause clay soil to inflate and disperse. Exchangeable potassium (K) has been shown to have an impact on soil structural stability, which is either comparable to or smaller than that of sodium (Na) [17,18].