# Performance Evaluation of Laetitic Subgrade Soil Treated with Lime and Coir Fibre-Activated Carbon
## 1 Introduction  
Ever increasing population and scarcity of suitable land lead to passing the transportation alignment through regions with marginal soils. However, the marginal soils possess low engineering and mechanical properties. Moreover, the rainfall infiltration further decreases the stability of transportation infrastructures [1]. Therefore, it needs to be stabilised before being used for infrastructure construction [2]. To date, various stabilisation such as chemical stabilisation [3, 4, 5, 6], reinforcement [7, 8, 9], deep mixing [10, 11], and prefabricated vertical drain [12, 13] have been utilised to improve the engineering properties of unsuitable soils, and this issue leads to stable constructions. Soil stabilisation by chemical admixtures has recently gained popularity in geotechnical engineering. Increased modulus of elasticity and resilient modulus, strength properties, reduced plasticity index, permeability, decreased swelling potential and volume instability, deformation and settlement, and improved durability are only a few of the benefits of lime as stabilisation materials [14, 15]. In places with wetting and drying cycles, utilising low lime concentration to enhance the mechanical properties of soil is not an effective strategy since the swelling potential of soil is not greatly diminished [16]. On the otherhand, adding lime to expansive clay soils, such as black cotton soil, is beneficial because the flexibility of the soil is reduced when lime is added. The application of lime as a stabiliser agent is not helpful for some cases where soil bearing capacity, density, and hydraulic conductivity are insufficient [17]. Durability, compressibility, and soil strength are increased by lime stabilisers while having varying effects on permeability. Moreover, sulfate attack, carbonation, and environmental effect are disadvantages of using lime as soil stabilisation. Adding fibre to lime-treated soil can reduce the brittleness of treated soil [18]. As a reinforcement, synthetic and natural fibre increases tensile strength, improves stability, and reduces soil's lateral deformation [19]. Furthermore, fibre minimises the risk of lime stabilisation causing brittle failure. In soil stabilisation with cement, including polypropylene fibres, increased tensile strength, density, and initial elastic modulus. Fibre added to a cement-stabilised clay soil enhanced tensile strength, and reduced swelling potential, shrinkage, and crack width [20]. Moreover, fibre positively influenced the flexural behaviour of cement-based soil stabilisation [21]. In sandy soil, discarded type textile fibres were used to improve damping ratio, resilient modulus, and permanent strain [22]. Fibre content and aspect ratio reduced critical confinement stress and enhanced shear strength in sandy soil [23]. Moreover, using fibre in embankments improved slope safety factor, strength, and stability [24]. Furthermore, when fibre-reinforced soil was exposed to freeze-thaw cycles, Scanning Electron Microscopy (SEM) pictures revealed that the fibres remained intact despite the repeated freeze-thaw [25]. Several studies have assessed the impact of including natural fibres in stabilised soil mass with additives. This previous research has revealed that adding natural fibre reinforcing to the soil led to a significant enhancement in strength and a decline in soil stiffness. Table 1 provides a summary of the components explored in prior investigations. Coir fibre is a natural fibre and one of the productions of coconut waste. Coconut waste is one of the most abundant waste products in agriculture, with a global production of 62.5 million tonnes annually in over 90 countries worldwide [34; 35]. Coir fibre has complex properties; each layer structure includes two layers; the first is called the thin primary wall, and the second is called the secondary wall. The mechanical properties of the fibre can be determined in the thick middle layer of this secondary wall, which consists of three layers [36].