Regional seepage and slope stability analyses in Singapore

• Main Author: Lai, Han Ming
• Other Authors: Harianto Rahardjo
• Format: Final Year Project
• Language: English
• Published: Nanyang Technological University 2022
• Subjects: Engineering::Civil engineering::Geotechnical
• Online Access: https://hdl.handle.net/10356/157514
• Institution: Nanyang Technological University

Climate change has prompted more unpredictable and intense rainfall patterns, which pose threats and induce higher frequency and prevalence of rainfall-induced slope failures. This is especially critical for tropical climate countries subjected to abundant rainfalls like Singapore. In Singapore, water tables in residual soil slopes are generally deep and hence slip surfaces attributed to rainfall-induced slope failures often take place within the unsaturated zone. Therefore, it is vital to incorporate unsaturated soil mechanics into the slope stability analyses to ensure a more accurate identification of critical slope areas. While two-dimensional (2D) numerical analyses are often employed in assessing slope stability, it is often used for assessment on a particular slope. Regional analyses using three-dimensional (3D) numerical models on the other hand can provide a more holistic and accurate assessment of slope stability within a studied region, encompassing multiple slope areas. This study aims to evaluate the stability of residual soil slopes within a zone of Bukit Timah Granite in Singapore. Unsaturated soil properties incorporated into numerical models were obtained from laboratory tests such as Soil-Water Characteristic Curve (SWCC) tests and triaxial permeability tests. Thereafter, 3D regional analyses utilizing GEOtop and Scoops3D were used to assess the slope stability under two different rainfall events: maximum daily and 5-day antecedent rainfall. The results from 3D regional analyses were compared with the results from 2D numerical analyses in Geostudios using SEEP/W- SLOPE/W and GEOtop-SLOPE/W method. Comparison was done for both pore-water pressure profiles and variation of Factor of Safety (FOS) values. The outcome from this study shows that pore-water pressure distributions from 3D models were consistently higher than those from 2D models while the FOS results from 3D models were consistently lower than those from 2D models. It could be concluded that the 3D regional analyses resulted in more conservative results and are therefore appropriate as practical means in determining the potential critical slope areas that are vulnerable to rainfall-induced slope failures within a studied region. In addition, due to the low permeability of the soil, further 3D analyses of slope stability under different rainfall events revealed that the slopes are more critical under a low intensity and prolonged duration of rainfall as compared to a short and intense rainfall.