To address the issues of high porosity and low strength in calcium sand of artificial islands, this study focuses on improving the calcium sand's mechanical properties. The effects of WER curing methods and coconut fiber modification on the UCS and microscopic mechanisms of calcium sand are investigated. The results indicate that both fiber incorporation and the increase in WER ratio can enhance the unconfined compressive strength of calcareous sand, with the addition of a certain amount of coconut coir fiber showing a more significant strength increase. The optimal recommended dosage of WER is 15%, which results in an UCS of 1218 kPa, an increase of nearly 4.27 times compared to 9% WER dosage. Coconut coir fiber has good tensile strength that can improve the compressive strength of calcareous sand after curing. The UCS of calcareous sand cured with a fiber content of 0.3% to 0.5% is increased by 1247 kPa to 1792 kPa compared to cured soil with no fiber. The strong binding nature of WER addresses the issue of large porosity in calcareous sand. Together with the penetrating coconut coir fibers, it forms a three-dimensional reticular framework structure, thereby enhancing the compressive performance of the calcareous sand-cured soil mass.

Two common waste by-products in Thailand, rice husk ash (RHA) and coir fiber (CF), were used alongside lime (L) to stabilize laterite soil and create a sustainable subbase material for pavements. The mechanical properties of the laterite soil mixed with RHA, lime, and CF were evaluated through compaction characteristics, unconfined compressive strength (UCS), California bearing ratio (CBR), scanning electron microscopy (SEM), and energy-dispersive X-ray (EDX) analyses. The dry soil mass was replaced with 10% and 20% RHA, and CF was added at 0.5%, 1%, and 1.5%. Additionally, based on the initial consumption of lime (ICL) test, 8% lime was incorporated into the mixture. The samples were cured for 7 days (short-term) and 56 days (long-term) before undergoing various tests. In terms of compaction, results showed that increasing the content of RHA, CF, and lime led to a decrease in maximum dry unit weight and an increase in optimum moisture content. The 10RHA8L and 20RHA8L mix designs demonstrated the highest UCS and CBR values after 56 and 7 days of curing, respectively. EDX analysis revealed the formation of calcium silicate hydrate (C-S-H) and calcium aluminate hydrate (C-A-H) gels on the particle surfaces, leading to a denser and more cohesive structure. Based on these findings, the mixture containing 20% RHA and 8% lime exhibited the most favorable properties for use as a subbase material in road and embankment construction.

Thailand is situated in the heart of Southeast Asia and is classified as having a tropical climate with high rainfall frequency and occurrence of floods. The weakening effect of water on laterite soil has led to different road damages such as potholes. Under these adverse environmental conditions, heavy traffic could also result in the formation of cracks and poor performance of roads. This study investigates the effects of Rice Husk Ash (RHA), Lime (L), and Coir Fiber (CF) as soil reinforcement material on the engineering properties of laterite soil. Several tests were conducted including the Unconfined Compressive Strength (UCS) test, three-point bending flexural strength, direct shear test, completely soaked durability test (to mimic flood conditions), X-ray Fluorescence (XRF), and Scanning Electron Microscopy (SEM) to observe the micro-structural changes of stabilized soil. The laterite soil was replaced by 10%, and 20% of RHA, 1% of CF, and 8% of L. The samples were cured for 7, 28, and 56 days before conducting the tests. The 20RHA8L mix designs showed the highest UCS value after 56 days curing period. In terms of the durability test results, the 20RHA8L mix design also exhibited the lowest reduction in compressive strength (3.8% drop) after undergoing 6 wetting-drying cycles. According to flexural strength, the 20RHA1CF8L (20%RHA, 1%CF, 8%L) mix design indicated a tenfold increase in flexural strength compared to the natural laterite soil after 28 days of curing. Based on the findings of this research, CF and RHA are beneficial for earth structures such as embankments and road layers that are subjected to significant tensile stresses. These waste materials can also reduce the brittleness of lime-stabilized soil.

In tropical regions, heavy rainfall induces erosion and shallow landslides on road embankments. Cement-based stabilization methods, common in these regions, contribute to climate change due to their high carbon footprint. This study explored the potential application of coir fiber-reinforced laterite soil-bottom ash mixtures as embankment materials in the tropics. The objective is to enhance engineered embankment slopes' erosion resistance and stability while offering reuse options for industrial byproducts. This study examined various mix designs for unconfined compressive strength (UCS) and permeability, utilizing 30% bottom ash (BA) and 1% coir fiber (CF) with varying sizes ranging from 10 to 40 mm, 6% lime, and laterite soil (LS), followed by microstructural analyses. The results demonstrate that the compressive strength increases as the CF length increases to 25 mm. In contrast, permeability increases continuously with increasing CF length. Lime-treated mixtures exhibit superior short- and long-term strength and reduce permeability owing to the formation of cementitious materials, as confirmed by microstructural analyses. A lab-scale slope box was constructed to evaluate the surface erosion of the stabilized laterite soil embankment. Based on the rainfall simulation results, the LS-BA-CF mixtures show better resistance to erosion and deformation compared to untreated LS, especially when lime is added to the top layer. This study provides insights into a sustainable and cost-effective approach for slope stabilization using BA and CF, offering a promising solution for tropical regions susceptible to surface erosion and landslides.

Micaceous weathered granitic soil (MWGS) is prevalent in the tropical regions of southern China, characterized by high compressibility, low compactability, and inadequate strength properties due to the presence of mica. Stabilization is crucial for transforming MWGS into a sustainable construction material for geotechnical engineering. This study focuses on enhancing the mechanical properties of MWGS by using coir fibers and fly ash, both locally available agricultural and industrial byproducts. Unconfined compression tests and consolidated drained triaxial compression tests were conducted on fiber-reinforced and fly-ash-treated MWGS under different stabilization conditions, considering the effects of fiber content, fly-ash content, and curing age. Scanning electron microscopy and energy-dispersive spectroscopy were also used to trace the microstructural evolution of the soil fabric in response to fiber reinforcement and fly-ash treatment. The experimental results indicate that higher fly ash content and longer curing give significantly improved soil strength and stiffness but poorer ductility. Incorporating coir fibers into the cemented soil matrix not only enhances the composite's strength, stiffness, and toughness but also shifts the shear response from brittle to ductile. For example, the compressive strength of MWGS could be improved by 49.1% with the inclusion of 1% content of coir fiber. Under the optimal dosage of fiber and fly ash, the soil compressive strength increased significantly from 114 kPa to 725 kPa. Microstructural analysis reveals that the bonding, friction, and interlocking among fibers, hydration products, and soil particles are the main contributors to the stable and strong microstructure and consequently the enhanced mechanical behavior of MWGS. This study provides an innovative and effective method for utilizing waste byproducts in stabilizing MWGS for practical geotechnical engineering applications.