Diverse shear behaviors of clayey materials: Implications for differing landsliding behaviors within the same area in Niigata, Japan

Abstract

In Niigata, Japan, many landslides have occurred in Tertiary rocks. Although most of these landslides were slow-moving, some underwent catastrophic failure, and different types of landslides sometimes occurred nearby under almost identical geological and hydrological conditions, but their mechanisms remain unclear. This study introduces a landslide triggered by Typhoon Hagibis in 2019 in an area in Niigata prone to landslide disasters. The landslide was small in scale, but the displaced materials were fluidized and deposited on a flat field at the toe of the slope after a long runout. To unravel this landslide's initiation and movement mechanisms, we conducted ring shear tests on the samples (clayey soils) taken from the landslide source area to examine their shear behaviors under both undrained and naturally drained conditions. Our results of undrained shear tests showed that shearing of the saturated landslide material can develop high pore-water pressure, lowering the shear resistance and elevating the mobility of the landslide. The results of shear tests under naturally drained conditions at different shear displacement rates (V) revealed that landslide materials behave rate-weakening at low and rate-strengthening at high rates, respectively. Further, it was found that during continuous shearing, a sudden change in V results in a sharp but temporal change in the mobilized shear strength. Different accelerations through which the shear rate was increased to a given V also result in different mobilized peak shear strengths. Our results indicate that the different landsliding phenomena occurring in the same area might be regulated by the diverse shear behaviors of the landslide materials involved in the sliding: contractive behavior may result in catastrophic failure, while the shear rate- and state-dependent shear behavior of the landslide material regulates the slow landsliding phenomena. Our results provide new insights for better understanding and predicting different landsliding phenomena within the same geological settings.