Effect of Tree Roots on Shallow-Seated Landslides

Forest vegetation, especially tree roots, helps stabilize hillslopes by reinforcing soil shear strength. To evaluate the effect of tree roots on slope stability, information about the amount of roots and their strength should be known. A simulation model for the root distribution of Cryptomeria japonica was proposed where the number of roots in each 0.5.cm diameter class can be calculated at arbitrary depths. The pull-out strength of roots was used to analyze the stability of four different types of forested slopes. Root reinforcement is important on slopes where roots can extend into joints and fractures in bedrock or into a weathered transitional layer between the soil and bedrock. Root reinforcement of soil increases quickly after afforestation for about the first 20 years, then remains Sediment disasters by debris flows, mud flows, and landslides occur almost every year during the rainy July to October Typhoon season in Japan. In July 1982, a heavy rainfall of 488 mm in a day, with a maximum intensity of 127.5 mm per hour, caused 4300 debris flows in Nagasaki prefecture, Kyushu Island. This storm destroyed 2200 houses and killed 299 people. During July 1983, intensive rainfall initiated many debris flows and 199 people were killed in Shimane prefecture, along Japan Sea on western Honshu Island. 1Presented at the Subject Group S1.04 Technical Session on Geomorphic Hazards in Managed Forests, XIX World Congress, International Union of Forestry Research Organizations, August 5-11, 1990, Montreal, Canada. 2Research Scientist, Forestry and Forest Products Research Institute, Tsukuba, Ibaraki, 305 Japan; and Principal Research Hydrologist, Pacific Southwest Research Station, Forest Service, United States Department of Agriculture, Arcata, CA. U.S.A. 95521 The cause of so much destruction and death might be unprecedented intensive rainfall. In addition, expansion of cities, resorts, and roads onto hillslopes and mountain areas has been also thought to be a primary cause. In response, national and local governments have adopted a program of aggressively constructing many erosion control works at great expense. But,, even so, it is impossible for such construction to protect all mountain hillslopes from debris flows. An important cause of increased frequency of debris flows is the removal of forests to accommodate urbanization and road construction in mountainous areas. In monsoon areas, like Japan, where steep mountains are covered with forests, mass wasting is the prevailing type of erosion. There is a fragile balance of stability on such steep hillslopes where the forest cover interacts with soil moisture, soil strength, geological condition, historical rainfall, and other factors to stabilize the regolith on a slope. From a viewpoint of soil mechanics, on many hillslopes the factor of safety of a slope (FS) approaches 1.0 during a rainfall event that occurs once every several years. Under conditions of such delicate balance, removal of the trees by logging may result in a reduction in soil strength sufficient to cause landslides. The influence of forests on slope stability has been one of the most important subjects of study--especially, the role of tree roots on reinforcing soil shear strength. To evaluate the mechanical effect of roots in strengthening soil, however, the quantity and distribution of roots in subsurface soil layers must be quantified. In this paper, a simulation model for the distribution and stabilizing effect of roots is investigated using an infinite slope stability analysis model.