Yangjia Shan instability has been evidenced by the occurrence of the July 16, 1997 landslide. The instability factor which leads to activating the landslide is the intense rainfall; lithology of Luoreiping Formation and the highly weathered slopes’ rocks have played a great role in starting and aggravation. Weathering at the landslide site consists largely of attack on the cement and removal of support of the sandy mudstone and sandstone and decompose of shale. The weathering degree of the rocks decreases vertically with increasing depth from high, medium to slightly weathered corresponding to grade IV, III and II, respectively. The slip surface consists of moderate to highly weathered intercalated layers of sandy mudstone (mudstone) and shale, while, the layer below the slip surface is characterized by dark gray, moderately weathered and thick layered mudstone. The moderate to highly weathered subsurface lithology is probably attributed to the accumulation of the infiltrated rainfall water through fractures and porosity, raising the ground water level and wetting of the sandy mudstone and shale rocks of Luoreping Formation. The wetting contributed more or less to the disintegration of the sandy mudstone and shale, lowered the shear strength and created cracks on the upper part of the slope leading thus to increase the landslide susceptibility. Therefore, the frequency and magnitude of landslide at the study area and its vicinity are expected to increase through the activation of old landslides and triggering of new ones under circumstances similar to those of the past.
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