The slope of interest, locates in southern Taiwan, has its dip direction approximately parallel with the cleavage of Mesozoic slate formation. The Ailiao River and three highway traverses over the slope at various elevations, leading to river bank erosion inevitably and artificial cutting with engineering stability measures. Typhoon Morakot in 2009 brought heavy rainfall and caused hazardous landslides. After that, sliding and deformation of the slope with various degrees were observed at different locations. This study investigates the influence of river bank erosion and artificial cutting on the sliding and deformation of the slope by several methods. Results of multi-temporal aerial image interpretation indicate that the case slope is stable with limited deformation while daylight is unobserved. Gully developed along the dip direction with some instable movement masses adjacently falling down occasionally. Deformations and propagation of rock discontinuities in different scopes of the slope that are sub zoned by daylights caused by artificial cutting and gullies differ from each other. Typhoon Morakot lead to slide of residual soil with shallow rock stratum of the study slope, and silt up of the Ailiao River. The river water may weaken the rock stratum and cause bank erosion at higher elevation than before, reducing stability of the study slope. Site investigation and monitoring results also reveal various evidences consistent with those mentioned above.

1. Introduction

Deformation and associate stability of a slope are critical information for geohazard mitigation. Some slopes failure occur with limited monitored deformation before movement, while the other exhibits time-dependent deformation that is significant in magnitude, but unobvious sliding surface, for instance, gravitational induced deep-seated deformation. The understanding for deformation patterns and associated temporal variation for a slope is then paid high attentions recently.

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