Numerical modelling of coupled seepage-deformation subjected to rainfall infiltration in unsaturated slope Edgar Jr Joe, Nazri Ali and Siti Norafida Jusoh School of Civil Engineering, Faculty of Engineering, Universiti Teknologi Malaysia, Johor Bahru, Malaysia Abstract Purpose – This study aims to propose another alternative numerical modelling tool, i.e. ABAQUS, to simulate a fully coupled seepage-deformation analysis in unsaturated slope subjected to rainfall infiltration. Design/methodology/approach – The effect of rainfall infiltration on the response of pore water pressure and factor of safety has been analysed and discussed. The results of this study have also been validated based on the results of field monitoring and previous numerical modelling. Findings – The results from ABAQUS show a better agreement with those from field monitoring compared to other numerical modelling tools. Originality/value – The procedures adopted in this study can be adopted and used as a guide to model similar slope problems in ABAQUS. Keywords Numerical modelling, Finite element, ABAQUS, Unsaturated, Seepage-deformation, Slope Paper type Technical paper 1. Introduction The occurrence of rainfall-induced landslide has been one of the main concerns in the geotechnical engineering field. The landslides in residual soil commonly occur in the unsaturated soil zone, as the groundwater table is relatively situated at a deeper soil, beneath the ground surface. For instance, Lee et al. (2014) found that the groundwater table was situated at a depth of 15m below the slope crest at a landslide in Hulu Kelang, Malaysia. For landslide cases in Jiangxie Province, China, the groundwater table is typically located at 5–20 m below the ground surface (Xue et al., 2016). The study of the rainfall-induced landslide in unsaturated slope is a complex process, as it involves infiltration of rainwater, seepage flow in the transient state and variation in the effective stress (Oh and Lu, 2015). To comprehend the landslide process, modelling of the unsaturated slope by using the numerical method in a two- dimensional model has been widely adopted. For example, Tiwari and Sneha (2014) modelled a two-dimensional slope to simulate a landslide occurrence in Kathmandu, Nepal, due to the coupling effect of rainfall and unplanned construction activities. Li et al. (2019) investigate a rainfall-induced landslide in Yunnan, China, based on a two-dimensional slope. Comparison with the result from field monitoring shows that numerical modelling using the concept of unsaturated soil mechanics has successfully explained the actual site condition. Based on a case study in Hubei, China, Hamdhan and Schweiger (2013) and Taib et al. (2018) found that the simulated pore water pressure distributions from numerical modelling can well represent those captured by the field monitoring instruments. The increasing use of numerical modelling is envisaged to endure in many years to come, as the method provides a more cost-effective solution for slope stability simulation, which can be performed promptly as compared to the physical modelling. However, most of the numerical modelling in this sense has been limited to uncoupled seepage and deformation analyses in unsaturated soil. In fact, the seepage is strongly related to the deformation process in the unsaturated soil subjected to rainfall infiltration (Zhang et al., 2005). In a fully coupled seepage-deformation analysis, changes in pore water pressure affected by the seepage process will influence the stress, as well as the soil deformation. In turn, the changes in stress will modify the seepage process as the hydraulic properties are subjected to soil stress. In a comparative study between uncoupled and coupled seepage- deformation analysis, Ouria et al. (2007) found that the uncoupled analysis overestimated the calculated effective stress. Thus, the factor of safety in the uncoupled analysis is lower compared to that in the coupled model. Khanzaei et al. (2017) also proposed that the coupling analysis of seepage and deformation is essential to capture a more realistic response of pore water pressure and displacement based on a study of the interaction between seepage and stress of concrete dam in Kinta, Malaysia. The well-known software being used for the uncoupled model pertain to the GeoStudio package, which The current issue and full text archive of this journal is available on Emerald Insight at: https://www.emerald.com/insight/1708-5284.htm World Journal of Engineering © Emerald Publishing Limited [ISSN 1708-5284] [DOI 10.1108/WJE-01-2020-0039] The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. The authors would like to acknowledge the financial support of the Research University Grant of Universiti Teknologi Malaysia (UTM Fundamental Research 20H95). Received 6 February 2020 Revised 21 August 2020 Accepted 1 September 2020