Lithological effects in soil formation and soil slips on weathering-limited slopes underlain by granitic bedrocks in Japan

Soil slips occur every few years due to heavy rains on biotite granite (Gb) and hornblende biotite granite (Ghb) slopes in the Taga Mountains, Ibaraki Prefecture, Japan. The occurrence density of soil slips per unit area is 2.7 times greater in the Gb slopes than that in Ghb slopes. We examined the chemical, mineral, physical, and mechanical properties of two soil profiles on soil-slip scars in these slopes to study the effect of bedrock mineral composition on the density of soil slips. For a given size of mineral grain, the amount of biotite (which is susceptible to chemical weathering) in granitic bedrocks strongly influenced the density of soil slips. In the Gb slope the soil layer becomes thinner because of the smaller proportion of biotite in the bedrock; this restricts the downward movement of soil water into the material below the slip plane. During heavy rainfall the water surface then rises toward the slope surface above the slip plane. This promotes soil slips, increasing the density of soil-slip scars. In contrast, in the Ghb slope, with more biotite in the bedrock, the soil layer becomes thicker, so that soil water moves easily into the material beneath the slip plane. This limits the rise of water surface toward the slope surface above the slip plane even during heavy rainfalls. There are consequently fewer soil slips and density of soil-slip scars.