Evaluation of citric acid-treated natural fibres as sustainable additives for improving expansive soil performance in highway construction

Abstract

This study addresses the evaluation of citric acid-treated natural fibres as sustainable additives for improving soil performance in highway construction. A series of tests were conducted on the soil to assess its baseline properties. The natural soil exhibited a Plasticity Index (PI) of 30 and Linear Shrinkage of 12%, along with a Liquid Limit (LL) of 65 and Plastic Limit (PL) of 35. Additionally, the fines content (<0.075 mm) and sand content (0.075–4.7 mm) were determined to be 79.6% and 20.4%, respectively. The soil's mineral composition included Quartz (5%), Orthoclase (3–5%), Montmorillonite (2.6%), and Illite (18.2%), with key components identified through chemical analysis: SiO2 (52.721 wt%), Fe2O3 (16.132 wt%), and Al2O3 (15.202 wt%). Subsequently, a range of tests were performed with varying additives to assess their impact on the soil's engineering properties. The sample containing 6% Sawdust (SD) additive exhibited notable improvements in multiple aspects. This sample achieved a maximum dry density (MDD) of 1856 kg/m³, surpassing the control (1900 kg/m³) and the 10% Rice Husk Ash (RHF) sample (1698 kg/m³). Furthermore, the 6% SD sample demonstrated enhanced Unconfined Compressive Strength (UCS) of 320 kPa, as opposed to the control's 121 kPa. Triaxial shear strength followed a similar pattern, peaking at 41 kPa for the 6% SD sample. The California Bearing Ratio (CBR), the 6% SD sample showcased significant improvements, achieving 41% for Unsoaked CBR and 36% for Soaked CBR. These results outperformed the control (25%) and the 4% RHF sample (37%). Additionally, the hydraulic conductivity of the 6% SD sample was notably lower at 4.98 ×10–8 cm/s compared to the control (3.35 ×10–8 cm/s) and the 4% RHF sample (4.58 ×10–8 cm/s). The Coefficient of Volume Change (Mv) further supported the superior performance of the 6% SD sample with a value of 0.6, indicating reduced settlement potential compared to the 4% RHF sample (Mv: 0.7). Based on the results, the 6% Sawdust (SD) additive consistently demonstrated the most favourable engineering properties for the expansive clay soil. hence, it is recommended that the 6% SD sample be considered as an optimal choice for enhancing subgrade material, leading to improved performance and sustainability in highway construction.