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Landslide problem due to heavy rainfall is one of the major natural disasters in Thailand. Most of landslides often occurred on unsaturated slope in the mountainous area. In general, main causal factor of landslide is the changing of moisture content in the soil mass while rainfall. Whenever rainfall occurring, soil moisture content increased, soil shear strength decreased affect to slope instability and possible failure. This research investigated the effect of rainstorms on moisture content and slope stability with 3-D numerical seepage model and Infinite slope stability model in unsaturated soil. There are 90 cases by considering return period, duration and pattern of rainfall in Mae Phrong – Mae Phun watershed, Uttaradit province, northern Thailand, This is the area where severe landslides occurred in May 2006. The result shown that, the average value of moisture content of whole area before rainfall is 75% (Degree of Saturation) and the highest value from all cases is 86%. Moreover, when return period and duration of rainfall are higher, moisture content is higher. For effect of rainfall pattern, the pattern has more intensity on last day cause moisture content to highest. Average factor of safety (F.S.) of the whole area before rainfall is 3.55. The highest F.S. from all cases is 1.85, which is related to moisture content change. Finally, the comparison of areas with F.S. less than 2.0 is correspond to the landslide area for 73.81%
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