Geotechnical investigation and stability analysis of bio-engineered slope at Surat Thani Province in Southern Thailand

Soil bioengineering in which pioneer plants are incorporated with simple structures such as flapped soil bags and erosion-control mats was implemented at Ban Na Tum, Surat Thani Province, Southern Thailand, a slope that suffered a major rainfall-induced landslide in 2011; these improvement methods were meant to rectify the landslide damages and restore the ecosystem of the area. This study investigates the performance of the bioengineering techniques employed at Ban Na Tum by means of 3D stability-seepage modelling. The analysis considers the influence of different land uses based on digital terrain model using unmanned aerial vehicle photogrammetry. A detailed site investigation was conducted which included soil boring, standard penetration tests, light-weight penetration tests, field density tests, resistivity surveying, and ground aeration sound tests to develop 3D soil stratigraphy and hydro-geological models. The height and perimeter of trees were selectively measured at the site to make corrections of the surface model. The analysis results revealed that the critical locations on slopes (i.e., the lowest factor of safety) were not necessarily stationary but could change with on-going water infiltration and seepage. In summary, the bioengineered slope covered with early-stage pioneer plants and rubber trees had a lower stability factor of safety than the natural forest, meaning that the surface slope covered with the early-stage pioneer plants is more likely to fail during heavy rainfall. These findings highlight the importance of incorporating land use in 3D stability modelling, as land coverage can have notable influences on slope stability.