Presentation Information
[O12-P64]Effects of soil structure in coniferous and broadleaf forests on slope stability and the mechanisms of slope failure
*Koki Sakamoto1 (1. University of Tsukuba Senior High School at Otsuka,Tokyo)
Keywords:
slope failure,shallow landslide,soil structure,coniferous forests
Abstract
Sediment-related disasters have frequently been observed in satoyama forests near the author’s home, particularly in coniferous stands. This raised the question of why some slopes within the same mountain area are more susceptible to failure than others. Field observations suggested that soil properties differ markedly between coniferous and broadleaf forests. Based on this observation, this study investigated how differences in soil structure between these forest types may contribute to the occurrence of rainfall-induced slope failures and shallow landslides.
The study focused on the physical characteristics of forest soils and their relationship to slope instability. Soil samples were collected from sites classified by forest type (coniferous or broadleaf) and by the presence or absence of past slope failure. The sampling area was Okaki District, Tsuga Town, Tochigi City, Tochigi Prefecture, and samples were collected on October 8. The collected soils were analyzed for aeration, water retention, drainage, moisture content, pH, and surface response to water. In addition, infiltration behavior was visually observed by applying water to dry soil surfaces and recording the immediate and delayed responses.
The results revealed clear differences between the two forest types. Broadleaf forest soils showed markedly higher mean aeration (15.044 cm³/100 g), water retention (30.933 g/100 g), and drainage (0.250 g/s) than coniferous forest soils, which showed values of 3.703 cm³/100 g, 11.109 g/100 g, and 0.062 g/s, respectively. Water infiltrated immediately into broadleaf forest soils, whereas in all coniferous forest samples, water remained on the surface as droplets and did not infiltrate even after 10 minutes, indicating strong water repellency.
These findings suggest that broadleaf forest soils generally develop a well-aggregated structure with high porosity and water-holding capacity, functioning effectively as a “green dam.” However, at failed broadleaf sites, exceptionally high aeration and drainage values were recorded, implying that excessive permeability may promote rapid pore-water pressure increase during heavy rainfall. In contrast, coniferous forest soils appear to be in a physically obstructed state due to fine particles and strong water repellency. Furthermore, variations in moisture content among failed coniferous samples suggest that slip surfaces may form at the boundary between a water-repellent layer and an underlying saturated layer. The significance of this study lies in demonstrating that slope stability is controlled not simply by the quantity of water in the soil, but by structural imbalances in soil properties such as aeration and drainage.
Sediment-related disasters have frequently been observed in satoyama forests near the author’s home, particularly in coniferous stands. This raised the question of why some slopes within the same mountain area are more susceptible to failure than others. Field observations suggested that soil properties differ markedly between coniferous and broadleaf forests. Based on this observation, this study investigated how differences in soil structure between these forest types may contribute to the occurrence of rainfall-induced slope failures and shallow landslides.
The study focused on the physical characteristics of forest soils and their relationship to slope instability. Soil samples were collected from sites classified by forest type (coniferous or broadleaf) and by the presence or absence of past slope failure. The sampling area was Okaki District, Tsuga Town, Tochigi City, Tochigi Prefecture, and samples were collected on October 8. The collected soils were analyzed for aeration, water retention, drainage, moisture content, pH, and surface response to water. In addition, infiltration behavior was visually observed by applying water to dry soil surfaces and recording the immediate and delayed responses.
The results revealed clear differences between the two forest types. Broadleaf forest soils showed markedly higher mean aeration (15.044 cm³/100 g), water retention (30.933 g/100 g), and drainage (0.250 g/s) than coniferous forest soils, which showed values of 3.703 cm³/100 g, 11.109 g/100 g, and 0.062 g/s, respectively. Water infiltrated immediately into broadleaf forest soils, whereas in all coniferous forest samples, water remained on the surface as droplets and did not infiltrate even after 10 minutes, indicating strong water repellency.
These findings suggest that broadleaf forest soils generally develop a well-aggregated structure with high porosity and water-holding capacity, functioning effectively as a “green dam.” However, at failed broadleaf sites, exceptionally high aeration and drainage values were recorded, implying that excessive permeability may promote rapid pore-water pressure increase during heavy rainfall. In contrast, coniferous forest soils appear to be in a physically obstructed state due to fine particles and strong water repellency. Furthermore, variations in moisture content among failed coniferous samples suggest that slip surfaces may form at the boundary between a water-repellent layer and an underlying saturated layer. The significance of this study lies in demonstrating that slope stability is controlled not simply by the quantity of water in the soil, but by structural imbalances in soil properties such as aeration and drainage.