| Abstract | The mechanical stabilization of soil slopes by means of tree roots is mainly dependent on root strength properties and root morphological parameters. In particular area, the selection of tree species on the basis of root properties is essential in b i otechnical slope protection. This study focussed on the effects of tree roots on soil slopes in Khao Luang mountain area , a tropical rain forest area of southern Thailand, which was recently affected by a catastrophic landslide and debri s f l ow event . The Khao Luang mountain area consists mainly o f a granitic batholith, and most parts of the hillslopes are covered with a relatively thin residual soil cover . Due to the major storm event of November, 1988, hundreds of land~ ides and extensive soil erosion occurred in mountainous slopes and mobilized into debris flows inundating the valley areas. The main facto~s influenced the slope failures can be generalized as the intensity and duration of rainfall, geological conditions and properties of slope forming materials, slope condit i ons, and vegetation . The strength properties as well as physical parameters of the slope soil indicate that the slopes are vulnerable to failures and soil erosion, and subsequent mobilization into flow forms. The intensity of slope failures on moderate to steep slopes was generall y h i gher in the slopes occupying vegetation with shallow and weak root systems than t hat in t he natural forest area which usually consists of indigenous trees with hard deep penetrating root systems . According t o the abundance , rate of growth, general root properties, etc. seven tree species including three indigenous species (Dipterocarpus alatus, Hopea odorata , and Ficus benjamina), three pioneer species (Alangium kurzii, Hibiscus macrophyl l us, and Alstonia macro phylla) , and para rubber (Hevea brasiliensis) were selected to analyze the root morphology and strength proper ties. Seedlings of these species were p l anted in a plot and cultivated in order to measure the rate of growth and survival. After one - year growth period, the plants were pulled out to determine the pullout resistance, and to study the root morphology . The pullout resistance of a plant is mainly governed by the root strength properties and root morphological parameters . For a given species, the pullout resistance increases wi th the increase of the Cumulative Root Length Factor and the Cumulative Root Crookedness Factor which are the factors defined t o estimate the influence of root length and the degree of crookedness on the root depth and orientation, respectively . To assess the mechanisms of root anchorage and root reinforcement, tree pullout tests and root permeated soil shear tests were conducted in the field, and the test results were incorporated with strength properties and morphological parameters of tree roots . The root tensi le strengths of the test species vary in a large range for a given root diameter, and decrease with increasing root diameter for a given species . Finally, for each species, the root anchorage and reinforcement properties and transpiration capacities were evaluated with reference to different vegetated soil slope conditions in order ( '' ') I ' l.l.l. to determine the effects of root on slope stability. Except para rubber, the test species proved to be positive for stabilization of soil slopes depending on the slope steepness, mode of potential slope failures, thickness of the soil cover, and the nature of the underlying rock . Neither root strength properties nor the root morphological parameters favor para rubber to be considered in slope stabilization under any kind of vegetated soil slope condition which is vulnerable to failure . |