1:45 PM - 3:15 PM
[O11-P54] Risk assessment of shallow landslides in school forest
Keywords:Shallow landslide, geological analysis, School forest
[Background]
Jishu no Mori, a school forest owned by Sendai Sanko High School, contains numerous hazardous areas such as surface collapse sites and steep slopes, which hinder the school’s fieldwork activities.
[Objective]
The aim is to identify the factors causing surface collapse in Jishu no Mori and investigate the details of hazardous areas within the forest. This will assist in predicting potential future surface collapses and contribute to improving the safety of Jishu no Mori.
[Data]
Surface collapses are often caused by the accumulation of large amounts of rainwater or steep slopes with gradients exceeding 60 degrees.
[Method]
Geological analysis was conducted at exposed rock outcrops near Jishu no Mori, and columnar and geological charts were created. Jishu no Mori was divided into nine sections (A–G, the summit, and the collapse site). For each site, 25 grams of soil were sampled, mixed with 50 grams of distilled water, heated, and the air within the soil was removed. Distilled water was then added until the total volume reached 100 mL, and the mass was measured to calculate the particle density of the soil. Jishu no Mori was approximated as a hexagon, and its area and volume were calculated using maps from the Geospatial Information Authority of Japan. Slopes for each site were measured, and the total weight and cohesion of the soil were determined using the average particle density. Using Coulomb's equation, the cohesion was represented as W, the vertical stress on the failure surface as X, and the internal friction angle as Y. These values were used to calculate the soil strength constant of the failure surface and the shear stress at each site.
[Results]
In creating the columnar and geological charts, a clay layer was discovered at a depth of 480 cm below the surface. In the particle density experiment, the following values were obtained: Site A: 1.36 g/cm³, Site B: 1.24 g/cm³, Site C: 1.86 g/cm³, Site D: 1.64 g/cm³, Site E: 1.65 g/cm³, Site F: 1.87 g/cm³, Site G: 1.67 g/cm³, Summit: 1.99 g/cm³, Collapse Site: 1.86 g/cm³. The strength constant of the failure surface was calculated as 23.00 kN/m², and the shear stresses for each site were as follows: Site A: 9.95 kN/m² (12 degrees), Site B: 10.72 kN/m² (20 degrees), Site C: 10.49 kN/m² (18 degrees), Site D: 9.71 kN/m² (8 degrees), Site E: 9.82 kN/m² (8 degrees), Site F: 9.58 kN/m² (4 degrees), Site G: 19.14 kN/m² (52 degrees), Summit: 9.53 kN/m² (2 degrees).
[Discustion]
In the geological map, the clay layer did not overlap with the locations of surface collapses, suggesting that clay was not a primary factor in these collapses. In particle density experiments, the density near the collapsed surface was higher compared to other locations. However, the soil composition predicted by the particle density was identical across all locations, indicating no significant advantage in terms of soil composition. Regarding slopes and cross-sectional shear stress at each location, point G showed a value of 19.14kN/m², which was closest to the strength constant. This value suggests a high likelihood of collapse if the area is subjected to heavy rainfall, such as a typhoon, which increases soil weight and load. Additionally, from a cross-sectional perspective, point G exhibits a bulging terrain. Considering its shear stress, point G can be identified as a location highly prone to surface collapses.
[Conculusion]
From the above, it can be concluded that the likelihood of future surface collapses is highest at point G in Jishu Forest, making it a hazardous location. Therefore, we have marked dangerous areas on the geological map created in this study to visually identify these hazardous zones, similar to a hazard map. By utilizing this map for fieldwork and other activities, we aim to make Jishu Forest a safer and more student-friendly environment.
Jishu no Mori, a school forest owned by Sendai Sanko High School, contains numerous hazardous areas such as surface collapse sites and steep slopes, which hinder the school’s fieldwork activities.
[Objective]
The aim is to identify the factors causing surface collapse in Jishu no Mori and investigate the details of hazardous areas within the forest. This will assist in predicting potential future surface collapses and contribute to improving the safety of Jishu no Mori.
[Data]
Surface collapses are often caused by the accumulation of large amounts of rainwater or steep slopes with gradients exceeding 60 degrees.
[Method]
Geological analysis was conducted at exposed rock outcrops near Jishu no Mori, and columnar and geological charts were created. Jishu no Mori was divided into nine sections (A–G, the summit, and the collapse site). For each site, 25 grams of soil were sampled, mixed with 50 grams of distilled water, heated, and the air within the soil was removed. Distilled water was then added until the total volume reached 100 mL, and the mass was measured to calculate the particle density of the soil. Jishu no Mori was approximated as a hexagon, and its area and volume were calculated using maps from the Geospatial Information Authority of Japan. Slopes for each site were measured, and the total weight and cohesion of the soil were determined using the average particle density. Using Coulomb's equation, the cohesion was represented as W, the vertical stress on the failure surface as X, and the internal friction angle as Y. These values were used to calculate the soil strength constant of the failure surface and the shear stress at each site.
[Results]
In creating the columnar and geological charts, a clay layer was discovered at a depth of 480 cm below the surface. In the particle density experiment, the following values were obtained: Site A: 1.36 g/cm³, Site B: 1.24 g/cm³, Site C: 1.86 g/cm³, Site D: 1.64 g/cm³, Site E: 1.65 g/cm³, Site F: 1.87 g/cm³, Site G: 1.67 g/cm³, Summit: 1.99 g/cm³, Collapse Site: 1.86 g/cm³. The strength constant of the failure surface was calculated as 23.00 kN/m², and the shear stresses for each site were as follows: Site A: 9.95 kN/m² (12 degrees), Site B: 10.72 kN/m² (20 degrees), Site C: 10.49 kN/m² (18 degrees), Site D: 9.71 kN/m² (8 degrees), Site E: 9.82 kN/m² (8 degrees), Site F: 9.58 kN/m² (4 degrees), Site G: 19.14 kN/m² (52 degrees), Summit: 9.53 kN/m² (2 degrees).
[Discustion]
In the geological map, the clay layer did not overlap with the locations of surface collapses, suggesting that clay was not a primary factor in these collapses. In particle density experiments, the density near the collapsed surface was higher compared to other locations. However, the soil composition predicted by the particle density was identical across all locations, indicating no significant advantage in terms of soil composition. Regarding slopes and cross-sectional shear stress at each location, point G showed a value of 19.14kN/m², which was closest to the strength constant. This value suggests a high likelihood of collapse if the area is subjected to heavy rainfall, such as a typhoon, which increases soil weight and load. Additionally, from a cross-sectional perspective, point G exhibits a bulging terrain. Considering its shear stress, point G can be identified as a location highly prone to surface collapses.
[Conculusion]
From the above, it can be concluded that the likelihood of future surface collapses is highest at point G in Jishu Forest, making it a hazardous location. Therefore, we have marked dangerous areas on the geological map created in this study to visually identify these hazardous zones, similar to a hazard map. By utilizing this map for fieldwork and other activities, we aim to make Jishu Forest a safer and more student-friendly environment.