# Practical Approach for Assessing Wetting-Induced Slope Failure
## 1 Introduction  
The existence and role of negative pore-water pressure above the ground-water table is commonly ignored. The reason for such ignorance is due to the assumption that there is no need for such science [1]. Hence, analysing slope stability is conventionally carried out by assuming zero pore water pressure above the ground-water table and positive pore-water pressure below the ground-water table. The position of the ground-water table is assumed based on instrumentation or borehole data and the effect of climates is taken care of by assuming a higher ground-water level. The role of suction is commonly viewed to increase soil shear strength [2] which then leads to less conservative analysis. However, a number of landslides occurred due rainfall infiltration [3; 4] which then raised a question whether the conventional approach which ignores the role of soil suction has been sufficiently conservative. [PERSON], [PERSON] [5] have shown that water-table location is secondary in rainfall-induced slope failure, especially when the water table is sufficiently deep. Hence, slightly raising the water table might not be sufficient as the effect of infiltration is located at a shallower depth. Rainfall infiltration causes an increase in pore-water pressure or a reduction in soil suction, which then leads to an increase in degree of saturation and the formation of a perched water table or increase in ground-water level [6], which leads to a reduction in soil shear strength which triggers slope failure [1; 7; 8; 9; 10]. Such a phenomenon has been observed all over the world, such as an increase in ground-water level which triggered a deep-seated landslide in Zhejian, China [11; 12], and formation of a perched water table which induced a landslide in Shilong, India [13]. The role of suction in altering soil permeability [14; 15] is very important in controlling the formation of perched water table and changes in ground-water level. However, rainfall is not the only triggering mechanism which causes slope failure. Soil properties and slope geometry also take an important role in determining whether the slope failure is susceptible to landslide [9; 16]. [PERSON] [17] shows that the effect of rainfall in Hong Kong is not significant due to high soil permeability. Hence, slopes geometry with different soil properties or geological formation and under different ground-water table positions will have different critical rainfall infiltration rates and durations [18]. The problems become especially severe when the slope is located on top of expansive soils as described in [PERSON], [PERSON] [19]. Unsaturated soil mechanics allow a more realistic determination of the perch water table or an increase in ground-water level which is transient by considering the interaction between climates, soil properties, and slope geometry, which can be more conservative than conventional approach. It also allowed us to determine critical rainfall infiltration and durations which can be important in assessing whether the slope of interest is prone to failure due to rainfall or not. In order to conduct unsaturated slope stability analysis, it is necessary to have a soil-water characteristic curve (SWCC), permeability function of unsaturated soils, and shear strength of unsaturated soils. SWCC describes the amount of water that can be retained by the soil pores at a certain mattic suction value and has a very important role in determining or estimating shear strength, compressibility, and permeability of unsaturated soils [20; 21]. However, directly measuring SWCC is difficult [22] as it is not part of routine geotechnical practice especially in Indonesia. Moreover, when the SWCC test is conducted, it is important to note that experimental and field SWCC are usually different [22; 23]. Hence, it is very difficult to directly conduct SWCC tests and obtain SWCC for each soil layer.