The 2017 Northern Kyushu meteorological disasters consequently resulted in several landslides in areas of strongly weathered granite sand. Therefore, this study aimed to examine the relationship between water conductivity changes and the shear displacement of strongly weathered granite sand collected from the aforementioned landslides using a ring-shear experiment. We identified a notable influence of effective normal stress on the reduction of the hydraulic conductivity and total porosity of the sand. Plotting water conductivity as a function of shear displacement generated three patterns-no change, linear reduction, and rapid-gradual reduction in hydraulic conductivity-at each effective normal stress level. When the effective normal stress was 200 kN/m², the hydraulic conductivity at approximately 1,000 mm and 6,000 mm post-shear was decreased by two and three orders of magnitude compared to that before shear, respectively. Contrarily, there was no change in hydraulic conductivity when the effective normal stress was 50 kN/m². Particle size distribution post-shearing varied from coarse sand― and coble-sized particles to fine sand― and silt-sized particles, with particle size subsequently decreasing with effective normal stress. This particle size reduction is in accordance with post-shear porosity reduction. These results indicate that the minimum strength along the slip surface is fully softened strength in landslide occurrence. Fully softened strength is defined as no effect of previous stress histories due to weathering. The higher residual strength is applicable in the slip surface after small shear displacement. The furtherance of landslide movement in strongly weathered granite sand are induced in large and deep sliding surface zones because of low hydraulic conductivity following heavy rainfall.

  The Shimajiri Group is widely distributed in the central and southern regions of Okinawa Island, an area prone to frequent landslides. To elucidate the rainfall infiltration processes that trigger rainfall-induced landslides, this study examined the relationship between hydraulic conductivity and grain-size distribution. The target strata primarily comprise alternating layers of sandstone, originating from sediment gravity flow deposits, and mudstone, derived from hemipelagic mud. X-ray CT imaging was performed on specimens used for hydraulic conductivity measurements to observe the presence of cracks. The hydraulic conductivity of specimens without cracks demonstrated a power-law relationship with the median grain size. Conversely, specimens with cracks exhibited increased hydraulic conductivity. The hydraulic conductivity of remolded specimens without cracks closely followed the power-law approximation curve, underscoring the impact of cracks on increasing hydraulic conductivity. These findings suggest that the presence of micro-cracks within the Shimajiri Group may significantly influence the groundwater supply mechanism, thereby contributing to the occurrence of rainfall-induced landslides.