Presentation Information
[O12-P98]Plasticity of Soils and Landslide Risk
*Yuna Aoki1, *Keigo Kanazawa1, *Ayaka Komae1, *Risa Nishimura1, *Kotaro Matsumoto1, *Ryo Murakami1 (1. Hyogo Prefectural Kakogawa Higashi High School)
Keywords:
smectite,Plasiticity Index
Background
With the increasing frequency of extreme weather events in recent years, landslides, caused by a combination of rainfall, soil, and various other factors which are particularly likely to occur in Japan where has steep mountains and a lot of rivers. Additionally, a clay mineral known as smectite has recently attracted attention. Smectite is a highly swellable clay mineral, and we hypothesized that it might be involved in the susceptibility of landslides.
Experiment 1 Check of the Validity of the Test Method
We developed a simplified version of the plasticity test actually used at construction sites and similar locations. To check the validity of this simplified test, we applied it to pure smectite to measure its plasticity index and compared the result with the actual value. If the result falls within the range of the actual plasticity index values, the simplified test will be deemed valid. The simplified test is described in detail below.
1. Simplified Liquid Limit Test
Place the sample through a 425-micrometer sieve and water in a petri dish and mix. Create a groove 1 centimeter deep and 1.5 centimeters wide and drop the mixture 25 times from a height of 10 centimeters. The mass ratio of water to sample when the groove is filled is defined as the liquid limit (LL).
2. Simplified Plastic Limit Test
Place the sample through a 425-micrometer sieve and water in a petri dish and mix. Once the sample begins to form a cohesive mass, gradually stretch it from a stick measuring 5 mm wide by 5 cm long. The water-to-sample mass ratio at which the sample reaches a width of 3 mm without cracking or splitting is defined as the plastic limit (PL).
Last, the Plasticity Index, PI is the difference between LL and PL.
Results1
The PI value of pure smectite was measured to be 780.
Since this value was consistent with the PI values for smectite reported in previous studies, we concluded that this measurement method was valid and used it in subsequent experiments.
Experiment 2 Measurements Using Actual Soil
We experimented using actual soil with the method confirmed in Experiment 1. The soil samples were collected from four locations. Our school grounds, Yokawa in Miki City, Mount Rokko where landslides have occurred and in Mt. Rokko with no landslides. We conducted measurements of these four soils.
In cases where water seeped out when added to the sample or the sample failed to hold its shape, we determined that the sample’s PI value was too small to measure. In this experiment, this is denoted as “unmeasurable.”
Results 2
The sample collected from the landslide-prone area of Mount Rokko yielded a value of 19.4.
The samples collected from the non-landslide-prone area of Mount Rokko yielded the following values
Sample 1 was unmeasurable, Sample 2 was 31.8, Sample 3 was unmeasurable, and Sample 4 was unmeasurable.
Samples collected from the landslide-prone area in Yokawa in the Miki City, yielded the following values.
Sample 1 was 23.6, Sample 2 was 43.8, Sample 3 was 20.4, and Sample 4 was 17.8.
Decomposed soil from our school grounds yielded was calculated as unmeasurable.
Discussion
Clear PI values were measured in all samples collected from the landslide-prone areas. Both sites contain clay layers, which are expected to act as impermeable layers in the landslides.
Conclusion
Layers of clayey soil may contribute to landslides by acting as an impermeable layer. It is considered that landslides are more likely to occur in areas with higher PI values.
With the increasing frequency of extreme weather events in recent years, landslides, caused by a combination of rainfall, soil, and various other factors which are particularly likely to occur in Japan where has steep mountains and a lot of rivers. Additionally, a clay mineral known as smectite has recently attracted attention. Smectite is a highly swellable clay mineral, and we hypothesized that it might be involved in the susceptibility of landslides.
Experiment 1 Check of the Validity of the Test Method
We developed a simplified version of the plasticity test actually used at construction sites and similar locations. To check the validity of this simplified test, we applied it to pure smectite to measure its plasticity index and compared the result with the actual value. If the result falls within the range of the actual plasticity index values, the simplified test will be deemed valid. The simplified test is described in detail below.
1. Simplified Liquid Limit Test
Place the sample through a 425-micrometer sieve and water in a petri dish and mix. Create a groove 1 centimeter deep and 1.5 centimeters wide and drop the mixture 25 times from a height of 10 centimeters. The mass ratio of water to sample when the groove is filled is defined as the liquid limit (LL).
2. Simplified Plastic Limit Test
Place the sample through a 425-micrometer sieve and water in a petri dish and mix. Once the sample begins to form a cohesive mass, gradually stretch it from a stick measuring 5 mm wide by 5 cm long. The water-to-sample mass ratio at which the sample reaches a width of 3 mm without cracking or splitting is defined as the plastic limit (PL).
Last, the Plasticity Index, PI is the difference between LL and PL.
Results1
The PI value of pure smectite was measured to be 780.
Since this value was consistent with the PI values for smectite reported in previous studies, we concluded that this measurement method was valid and used it in subsequent experiments.
Experiment 2 Measurements Using Actual Soil
We experimented using actual soil with the method confirmed in Experiment 1. The soil samples were collected from four locations. Our school grounds, Yokawa in Miki City, Mount Rokko where landslides have occurred and in Mt. Rokko with no landslides. We conducted measurements of these four soils.
In cases where water seeped out when added to the sample or the sample failed to hold its shape, we determined that the sample’s PI value was too small to measure. In this experiment, this is denoted as “unmeasurable.”
Results 2
The sample collected from the landslide-prone area of Mount Rokko yielded a value of 19.4.
The samples collected from the non-landslide-prone area of Mount Rokko yielded the following values
Sample 1 was unmeasurable, Sample 2 was 31.8, Sample 3 was unmeasurable, and Sample 4 was unmeasurable.
Samples collected from the landslide-prone area in Yokawa in the Miki City, yielded the following values.
Sample 1 was 23.6, Sample 2 was 43.8, Sample 3 was 20.4, and Sample 4 was 17.8.
Decomposed soil from our school grounds yielded was calculated as unmeasurable.
Discussion
Clear PI values were measured in all samples collected from the landslide-prone areas. Both sites contain clay layers, which are expected to act as impermeable layers in the landslides.
Conclusion
Layers of clayey soil may contribute to landslides by acting as an impermeable layer. It is considered that landslides are more likely to occur in areas with higher PI values.