9:30 AM - 9:45 AM
[HDS07-03] Shear strength and slip rate-dependence of altered volcanic ash soil controlled by water adsorption ability
Keywords:landslide, Shear strength, slip rate-dependence, clay minerals, adsorption ability, friction
Although various models have been proposed to predict the onset of catastrophic landslide slip (e.g. Saito, 1968; Fukuzono, 1985; Voight, 1989), the slip does not always accelerate as predicted. Some reports indicate that the slip can decelerate, and stop, even after obtaining sufficient speed (e.g. Doi et al., 2020). Further slip acceleration is constrained if slip rate-dependence of friction is positive (Tika et al., 1996; Dieterich, 1979). However, there are few reports on slip rate-dependence for soils that comprise the landslide slip zone, and dominant factors controlling the slip rate-dependence are not clear.
In this study, shear experiments were conducted on altered volcanic ash soils to simulate the landslide slip and to investigate the slip rate-dependence of friction strength. In addition, we compared various factors, such as clay mineral content, adsorbed water content (AWC), liquid limit (WL), plasticity index (PI), and total specific surface area (TSSA), with shear strength and its slip rate dependence to identify the dominant factor controlling them.
The eight samples were from the landslide-prone are in western part of the Aso caldera, Kumamoto Prefecture, Japan. XRD profile indicated they contained smectite, kaolinite, alunite, amorphous silica and glass. The AWC and clay mineral content was estimated using TG-DTA. In TGDTA, AWC was obtained as weight loss until the temperature reached to 250℃, and clay mineral content was determined based on the weight loss due to the volatilization of each mineral content between 250℃ and 1000℃. The TSSA was measured using the EGME method (Eltantawy & Arnold, 1973). It is capable of measuring not only the external SSA but also the interlamellar space of smectite.
Shear experiments were conducted using the rotary shear apparatus set at Geological Survey of Japan, AIST (Togo & Shimamoto, 2012). Shear strength was measured in the range of slip velocity from 10-4 - 10 mm/min at room temperature, under wet and c.a. 1 MPa in normal stress condition. Steady-state shear strength was determined as the value at the end of 24-hours shearing at a constant slip velocity (0.28 mm/min).
The samples were broadly classified into smectite-rich and kaolinite-rich groups. The smectite richest sample had 74 wt.% of that. Kaolinite richest sample had 31 wt.% of that. AWC was 18 wt.% in maximum. The maximum values of WL and PI were 133% and 94%, respectively, and they were mostly plotted around the A-line on the plasticity chart (Bardet, 1997). TSSA was 549 m2/g in maximum. Steady-state shear strength ranged from 0.1 to 0.5 MPa. We found negative correlations between TSSA, AWC, WL, PI, and the steady-state shear strength but no clear correlation between those factors and the smectite content. Samples with large TSSA indicated negative rate-dependence at lower velocities (10-4 - 1 mm/min) but positive rate-dependence at higher velocities (1 – 10-1 mm/min). On the other hand, the rate-dependence was always negative for the samples with small TSSA.
Therefore, the ability of water adsorption of the soils would be controlled by TSSA and that the adsorbed water might reduce the shear strength low. Therefore, it can be said that the sample with larger TSSA is more likely to behave plastically and be less likely to growth into the catastrophic landslide slip. Contrary to expectations, a clear correlation between TSSA and the smectite content was not observed. Thus, what determines the water adsorption ability of soils remains an open question, and how this influences on sliding behavior in landslides remains a topic for future research.
In this study, shear experiments were conducted on altered volcanic ash soils to simulate the landslide slip and to investigate the slip rate-dependence of friction strength. In addition, we compared various factors, such as clay mineral content, adsorbed water content (AWC), liquid limit (WL), plasticity index (PI), and total specific surface area (TSSA), with shear strength and its slip rate dependence to identify the dominant factor controlling them.
The eight samples were from the landslide-prone are in western part of the Aso caldera, Kumamoto Prefecture, Japan. XRD profile indicated they contained smectite, kaolinite, alunite, amorphous silica and glass. The AWC and clay mineral content was estimated using TG-DTA. In TGDTA, AWC was obtained as weight loss until the temperature reached to 250℃, and clay mineral content was determined based on the weight loss due to the volatilization of each mineral content between 250℃ and 1000℃. The TSSA was measured using the EGME method (Eltantawy & Arnold, 1973). It is capable of measuring not only the external SSA but also the interlamellar space of smectite.
Shear experiments were conducted using the rotary shear apparatus set at Geological Survey of Japan, AIST (Togo & Shimamoto, 2012). Shear strength was measured in the range of slip velocity from 10-4 - 10 mm/min at room temperature, under wet and c.a. 1 MPa in normal stress condition. Steady-state shear strength was determined as the value at the end of 24-hours shearing at a constant slip velocity (0.28 mm/min).
The samples were broadly classified into smectite-rich and kaolinite-rich groups. The smectite richest sample had 74 wt.% of that. Kaolinite richest sample had 31 wt.% of that. AWC was 18 wt.% in maximum. The maximum values of WL and PI were 133% and 94%, respectively, and they were mostly plotted around the A-line on the plasticity chart (Bardet, 1997). TSSA was 549 m2/g in maximum. Steady-state shear strength ranged from 0.1 to 0.5 MPa. We found negative correlations between TSSA, AWC, WL, PI, and the steady-state shear strength but no clear correlation between those factors and the smectite content. Samples with large TSSA indicated negative rate-dependence at lower velocities (10-4 - 1 mm/min) but positive rate-dependence at higher velocities (1 – 10-1 mm/min). On the other hand, the rate-dependence was always negative for the samples with small TSSA.
Therefore, the ability of water adsorption of the soils would be controlled by TSSA and that the adsorbed water might reduce the shear strength low. Therefore, it can be said that the sample with larger TSSA is more likely to behave plastically and be less likely to growth into the catastrophic landslide slip. Contrary to expectations, a clear correlation between TSSA and the smectite content was not observed. Thus, what determines the water adsorption ability of soils remains an open question, and how this influences on sliding behavior in landslides remains a topic for future research.