Flexural toppling as a causal factor of the rain-induced slope failure in 2016, Hsinchu Prefecture, Taiwan

A slope failure at Taigang, Taiwan, triggered by rainfall on 1 October 2016, was followed by episodic rockfall events over several months. This study reports on the basic geological causes of the slope failure, to clarify its mechanisms of failure to inform landslide mitigation measures and risk management. Cumulative rainfall in the region reached 488.5 mm in about five days, and the slope failure occurred four days after the peak in rainfall intensity. The slope failure occurred in an old landslide scar at the lower part of a gravitationally deformed slope. The bed rock of this slope consists of argillite and alternating beds of sandstone and shale with a steeply dipping foliation. The beds are 300 m thick and show flexural toppling down-slope, which has led to the formation of many open fractures. The flexural toppling is expressed as a depression at the ridge top and a convex slope at lower levels, which indicate that the slope deformation must have already taken place long before. This deformation was the fundamental cause of the slope failure. The occurrence of new scarplets and tension cracks behind the landslide scar suggests the landslide is active and should be monitored.