# Performance Evaluation of Laetitic Subgrade Soil Treated with Lime and Coir Fibre-Activated Carbon
## 1 Introduction
Coir fibre has a high lignin content but a low cellulose content, making them extremely strong, resilient, and durable. It has a high resistance to abrasion, fungal and bacterial decay, and pilling. Furthermore, coir fibres can tolerate months of soaking without deterioration [37]. Activated carbon (AC) is another material that improves soil's compression strength, shear strength [38] and California bearing ratio (CBR) value [39]. AC contains carbonaceous material and has been derived from a wide variety of carbon-rich raw materials, including \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Test** & **Soil** & \begin{tabular}{c} **Stabilising** \\ **Admixture** \\ \end{tabular} & **Fibre Reinforcing** & \begin{tabular}{c} **Days** \\ **Curing** \\ \end{tabular} & **Ref.** \\ \hline UCS and DST & \begin{tabular}{c} SC \\ (Marginal soil) \\ \end{tabular} & Cement and Fly Ash & randomly distributing 2\% coir fibre & 3 & [26] \\ USC & \begin{tabular}{c} Clay \\ (A kaolin type) \\ \end{tabular} & \begin{tabular}{c} Lime \\ CL \\ \end{tabular} & 0.75\% basalt & 90 & [27] \\ UCS & \begin{tabular}{c} CL \\ \end{tabular} & \begin{tabular}{c} Lime and fly ash \\ \end{tabular} & 0.5\% sisal fibre & 7 & [28] \\ TPB, TST & marine clay soil & \begin{tabular}{c} Lime and cement \\ \end{tabular} & 1\% coconut fibre & 28 & [29] \\ UUT & Hefei clayey soil & \begin{tabular}{c} Lime \\ \end{tabular} & 0.4 wheat straw & 28 & [30] \\ UCS & \begin{tabular}{c} CH \\ \end{tabular} & \begin{tabular}{c} Lime \\ \end{tabular} & 0.5\% coir fibre & 7 & [31] \\ TPB, cyclic loadings, & \
\begin{tabular}{c} A-2-6 \\ A-7-5 \\ SP \\ \end{tabular} & Cement & 0.15\% hemp fibre & 7 & [32] \\ UCS, STS, &
\begin{tabular}{c} Direct shear test (DST), indirect tensile strength (ITS), unconsolidated–undrained triaxial (UUT), unconfined compressive strength (UCS), three-point bending (TPB), splitting tensile strength (STS). \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Natural fibre and additives investigated in some previous research. discarded apple pulp, coconut shells, straw, sugarcane bagasse, apricot stone shells, coir with, sawdust, peanut husk, and olive stones [40]. It is a widely used method in the environmental sector for removing pollutants from air or water streams in industrial and municipal processes such as groundwater remediation, wastewater treatment plants, oil removal, and air purification by adsorption [41]. AC has a large surface area to absorb CO\({}_{2}\) and contaminants in submicroscopic pores. Moreover, it is stable in acidic and basic situations [42,43]. This study investigates the effect of lime as a traditional calcium-based stabiliser and coir fibre with activated carbon obtained from waste agriculture materials on the mechanical properties of lateritic soil. First, pH tests are conducted to determine the initial lime consumption. After that, UCS tests are conducted to find the influence of various lime percentages and activated carbon with and without coir fibre on the mechanical properties of stabilised specimens. Then, the consolidation test is performed for optimum content to determine the void ratio and permeability. Finally, energy dispersive X-ray analysis (EDX), surface area analysis (BET) and field emission scanning electron microscopy (FESEM) tests are performed to show the microstructure of untreated and treated soil to better understand the stabilised soil reinforcement mechanism.