Exploring the soil-water retention characteristics for unsaturated soil using coconut shell waste for subgrade improvement

Introduction: Suction distribution has a major impact on the stability of unsaturated soil because it affects seepage and shear strength. The Air Entry Value (AEV) and Residual Water Content (RWC) variations determine the Soil-Water Characteristic Curve (SWCC), which is a crucial component of suction distribution. The subgrade is submerged in water during a flood event, which has an impact on the AEV and RWC, and lowers their performance. Crushed Coconut Shell (CCS), a waste product, has been recycled in this study and used as an addition to increase subgrade strength by enhancing AEV and RWC. Currently, researchers abroad confirm that CCS improves subgrade performance. Nevertheless, the optimum content of the soil-CCS mixture varies based on the soil type of the respective regions. Moreover, there is a lack of scholarly research examining the enhancement of AEV and RWC affected by CCS following a flood occurrence.
Aim: This research study sought to determine the deviations of AEV and RWC for the optimum CCS content of unsaturated soil in Malaysia.
Methods: In this research, control samples and soil mixed with 0.5, 1.0, 1.5, 2.0, and 2.5% of CCS were used. The pressure plate extractor equipment and analysis generate SWCC, which characterises the AEV and RWC parameters and quantifies the performance of submerged subgrade because of a flood event.
Results: The outcome revealed a significantly higher AEV for mixed soil as compared to the control sample. The AEV for the control sample was 10 kPa, whereas that for the optimum mixture of soil-CCS of 0.5% was 25 kPa. Therefore, it has been confirmed that the use of CCS has a positive impact on AEV and RWC. This, in turn, is expected to improve the overall performance of the subgrade when subjected to submersion in water.
Conclusion: The objective of this study was to assess the characteristics and potential applications of hazardous waste by transforming it into a soil improvement agent. This research supports Chapter 8 of the 12th Malaysia Plan 2021-2025: Environmental Sustainability: Improving Resilience to Climate Change and Disasters, and Chapter 6 of the Green Technology Master Plan Malaysia 2017-2030: Waste.