Presentation Information
[O12-P29]The relation of the landslides' repose angle and sands for its analysys of soil and collapse
*Yujiro Sasaki1 (1. Miyagi Rifu high school)
Keywords:
Slope collapse,Sand,Angle of repose
(1) Background
Japan, while blessed with abundant water resources and varied terrain and climate due to its steep topography, is also fragile and prone to collapse due to factors such as water content and seismic activity. As a result, landslides and other sediment-related disasters, particularly slope failures, occur almost every year. Miyagi Prefecture, centered around the Sendai Plain, is no exception. Not only do sediment-related disasters occur along the mountains, but river terrace formations represented by the Hirose River and Natori River are also well developed, and when viewed from the side, the slopes of these terraces are quite steep. For this reason, raised embankments are common in urban areas.
In general, in structures such as retaining walls and landslide prevention works, it is important to ensure that the ground has an angle below a certain limit. This angle is called the angle of repose and is generally belower than around 30 degrees. Retaining walls built with embankments are also installed to maintain slope stability and prevent landslides, and slopes steeper than the angle of repose are considered to have a high risk of collapse.
(2) Method
I set an incline on a 1 cm thick plywood at various angles and placed 240 g of sand with a fixed particle size of 1 mm in a square frame. The plywood was marked every 1 cm to facilitate video analysis from the side. In this setup, a camera (PENTAX WG-1000) was used to start video recording, and immediately after, the square frame was removed and record the sand sliding down the slope. From the video recording, the time for the fastest sand particles to completely fall on the plywood and the distance along the path were measured with a ruler from the count on the timer and the 1 cm interval markings in the video. The average velocity of 1 mm sand particles on the plywood was then calculated.
The above procedure was carried out at slope angles of 15°, 30°, and 45°. Additionally, the same experiment was conducted with polystyrene placed on the plywood to compare the falling speeds.
(3) Results
Generally, at the angle of repose of 30° on an embankment, the falling velocity of 1 mm sand grains was 14.07±0.258 cm/s on polystyrene and 13.68±0.458 cm/s on plywood. The error was evaluated using the least squares method (here, N=10). On plywood, the velocity was reduced by 3% compared to polystyrene, indicating that the dynamic friction force acts more strongly.
Furthermore, at an incline of 15°, the falling velocity of 1 mm sand grains was 6.99±0.344 cm/s on polystyrene and 6.12±0.378 cm/s on plywood, a 13% difference in velocity. This indicates that at the gentler angle of 15°, it takes time for the sand to start sliding, and not only dynamic friction but also static friction acts strongly.
Experiments are also ongoing to observe how collapses vary with slopes of different particle sizes now. From these results, it was observed that on cliffs at the 30° angle of repose, even if they appear stable at certain times, the risk of collapse rapidly increases with even slight changes in weight, such as water percolate. In contrast, on a 15° slope, a certain load can be applied without immediate collapse. Additionally, by covering the surface with irregularly shaped particles or heterogeneous soil—similar to the increased dynamic friction observed on plywood—it may be possible to prevent instantaneous collapse more effectively than on polystyrene.
Japan, while blessed with abundant water resources and varied terrain and climate due to its steep topography, is also fragile and prone to collapse due to factors such as water content and seismic activity. As a result, landslides and other sediment-related disasters, particularly slope failures, occur almost every year. Miyagi Prefecture, centered around the Sendai Plain, is no exception. Not only do sediment-related disasters occur along the mountains, but river terrace formations represented by the Hirose River and Natori River are also well developed, and when viewed from the side, the slopes of these terraces are quite steep. For this reason, raised embankments are common in urban areas.
In general, in structures such as retaining walls and landslide prevention works, it is important to ensure that the ground has an angle below a certain limit. This angle is called the angle of repose and is generally belower than around 30 degrees. Retaining walls built with embankments are also installed to maintain slope stability and prevent landslides, and slopes steeper than the angle of repose are considered to have a high risk of collapse.
(2) Method
I set an incline on a 1 cm thick plywood at various angles and placed 240 g of sand with a fixed particle size of 1 mm in a square frame. The plywood was marked every 1 cm to facilitate video analysis from the side. In this setup, a camera (PENTAX WG-1000) was used to start video recording, and immediately after, the square frame was removed and record the sand sliding down the slope. From the video recording, the time for the fastest sand particles to completely fall on the plywood and the distance along the path were measured with a ruler from the count on the timer and the 1 cm interval markings in the video. The average velocity of 1 mm sand particles on the plywood was then calculated.
The above procedure was carried out at slope angles of 15°, 30°, and 45°. Additionally, the same experiment was conducted with polystyrene placed on the plywood to compare the falling speeds.
(3) Results
Generally, at the angle of repose of 30° on an embankment, the falling velocity of 1 mm sand grains was 14.07±0.258 cm/s on polystyrene and 13.68±0.458 cm/s on plywood. The error was evaluated using the least squares method (here, N=10). On plywood, the velocity was reduced by 3% compared to polystyrene, indicating that the dynamic friction force acts more strongly.
Furthermore, at an incline of 15°, the falling velocity of 1 mm sand grains was 6.99±0.344 cm/s on polystyrene and 6.12±0.378 cm/s on plywood, a 13% difference in velocity. This indicates that at the gentler angle of 15°, it takes time for the sand to start sliding, and not only dynamic friction but also static friction acts strongly.
Experiments are also ongoing to observe how collapses vary with slopes of different particle sizes now. From these results, it was observed that on cliffs at the 30° angle of repose, even if they appear stable at certain times, the risk of collapse rapidly increases with even slight changes in weight, such as water percolate. In contrast, on a 15° slope, a certain load can be applied without immediate collapse. Additionally, by covering the surface with irregularly shaped particles or heterogeneous soil—similar to the increased dynamic friction observed on plywood—it may be possible to prevent instantaneous collapse more effectively than on polystyrene.