The study of the effects of vegetation on slope stabilization for landslide prevention in Thailand

Landslide causes major disturbance to natural ecosystem in tropical montane forest. The main factor that causes slope failure or landslide is climate triggered which is intense or prolonged rainfall and seepage. During the past few decades, rain-induced landslides in Thailand occur more frequent causing mortality and economic loss. The consequences of shallow landslide at depth 0.5-3.0 m are the most destructive type of landslide in Thailand. Existing countermeasures for landslide prevention and mitigation are mostly geotechnical engineering structure, and they are considered to be costly and might not be compatible with the soft ground of eroding soil. Therefore, the use of bioengineering method to prevent shallow landslide has been alternatively used for providing long-term stability of slope. One of the factors controlling landslide is vegetation cover. Most commonly used soil bio-engineering technique in Thailand and other Southeast Asian countries is the use of Vetiver) Chrysopogon zizanioides) due to its cost-effectiveness, environmental-friendliness, complex root structure, and high mechanical strength. However, the propagation rate of Vetiver is too low which produces insufficient amount of Vetiver for the use in landslide-prone area. The objective of this study is to survey indigenous plant species which have high mechanical strength and similar root characteristics to Vetiver in order to further promote to use as soil bioengineering structure. Three candidate species: Thysanolaena maxima, Calamus viminalis, and Sida acuta were selected for studying root morphology and mechanical strength. Thysanolaena maxima performed best in root tensile strength, while Sida acuta has the deepest rooting distance. Due to its long life span and economic value, Thysanolaena maxima was selected for the testing on the improvement of slope stability. It was found that there was 19.23 kN/m2 increase in soil shear strength in root-permeated soil compared to bare soil in non-slide site, while there was 2.21 kN/m2 decrease in slide site. The study also found that there was no significant different in soil permeability of three soil samples. The other engineering method should be employed together with soil bioengineering in order to improve drainage in landslide-prone area.滑坡会对热带山地森林的自然生态系统造成严重干扰。引发边坡失稳或滑坡的主要诱因是气候因素，即强降雨、持续性降雨及渗流作用。近数十年来，泰国境内降雨诱发型滑坡的发生频次持续升高，已造成人员伤亡与经济损失。深度介于0.5至3.0米的浅层滑坡，是泰国境内破坏性最强的滑坡类型。当前滑坡防控的主流措施多为岩土工程结构，但这类措施成本高昂，且难以适配侵蚀性土壤的软质地基。因此，采用生物工程手段防控浅层滑坡，作为提升边坡长期稳定性的替代方案得到了应用。植被覆盖是影响滑坡发生的关键因素之一。香根草（Vetiver，*Chrysopogon zizanioides*）是泰国及其他东南亚国家最常用的土壤生物工程材料，其具备成本效益高、环境友好、根系结构复杂且机械强度高等优势。但香根草的繁殖速率过低，无法满足滑坡易发区的大规模应用需求。本研究旨在调研具备高机械强度、且根系特征与香根草相近的本土植物物种，以推广其作为土壤生物工程结构材料。研究选取了3种候选植物：大粽叶芦（*Thysanolaena maxima*）、省藤（*Calamus viminalis*）以及黄花虱麻头（*Sida acuta*），对其根系形态与机械强度展开研究。结果显示，大粽叶芦的根系抗拉性能最优，而黄花虱麻头的根系入土深度最大。鉴于大粽叶芦寿命长且具备经济价值，研究选取其开展边坡稳定性提升试验。研究发现，在非滑坡区域，根系渗透土壤的抗剪强度较裸地提升了19.23 kN/m²；而在滑坡区域，该数值反而下降了2.21 kN/m²。此外，研究还发现三种土壤样本的渗透性无显著差异。为改善滑坡易发区的排水性能，需将土壤生物工程与其他工程措施结合使用。