We quantified the effect of tree root systems on slope stability in a natural cool]temperate deciduous broad]leaved forest. We established three study plots on a mountain slope in Tomamae Town, Hokkaido, and measured tree diameter and position and identified the trees to species. We dug out the root systems and measured the root distribution for three to four trees per plot. We measured the root diameter and the force required to pull the roots from the soil after removing the stumps for two to four trees per plot. We measured cohesion, angle of internal friction, and bulk density of soil. We estimated the shear plane using core boring. We estimated the pullout resistance of the roots on the shear plane for all trees in each plot using root distribution data, the relationship between root diameter and the pullout resistance of the root, and the ratio between the diameter of each tree and that of dug out trees. We calculated the safety factor using a slice method (the Janbu Method). We added the pullout resistance of the roots to the soil cohesion in the formula used to calculate the safety factor. The pullout resistance of the roots was 2.29, 1.16, and 0.50kN/m2 in the three plots. The safety factor increased by 0.28, 0.1, and 0.04 in the respective plots by adding the pullout resistance of the roots to the formula for the safety factor.

Key wordsFroot, safety factor, shallow landslide, slope stability