Monitored transition from sagging to sliding during deep-seated gravitational slope deformation, Akatani-nishi, Japan

To clarify the mechanism of deep-seated gravitational slope deformation (DSGSD), we investigated crushed rocks and subsurface deformation in a slope southwest of Akatani landslide, in Gojo city, Japan, on which a deep-seated catastrophic rapid landslide occurred in 2011. We analyzed subsurface deformation from 2017 to 2022 using monthly inclinometer data from three 85-90 m deep cored boreholes. Crushed rocks were investigated based on visual observations of the core. We examined the fractures of rock in the borehole wall based on borehole camera images. The inclinometer measurements within the DSGSD slope indicated that sliding occurred in the upper slope, and sagging occurred in the mid-slope, with two sliding surfaces identified at depths of 47.5 and 54.5 m. At a depth of 47.5 m, the average annual displacement increased by one order of magnitude when the groundwater level rose remarkably due to rainfall. This suggests that the movement of slope transitioned from sagging to sliding under the action of rising groundwater in the mid-slope. There was more crushed rock in the cores from the upper slope, where sliding occurs, than in the cores from the mid-slope, where sagging occurs. The fractures of rock by the number and width of the cracks were also greater in the upper slope cores than in the mid-slope cores. Our results support that the sliding progressed from the head to the toe of the slope.