Shear strength and slip rate dependence of weathered volcanic ash soil controlled by water adsorption ability

Our research aims to clarify the process of slip acceleration in landslides in order to mitigate landslide disasters. Here we especially focus on what factors control the shear strength of rocks and soils that compose the landslide slip zone and what factors generate variety in sliding features. This is because knowing those factors could provide hints for predicting the onset of runaway slip. Although a method to predict the onset of slope failure has been proposed (Fukuzono, 1985), which is based on the inverse of surface slip rate converging to zero as the failure time approaches, it doesn’t always work. There have been reports that the slip rate turned to decrease, and the slide did not induce to the failure, even after obtaining enough slip acceleration (Matsuura et al., 2015; Doi et al., 2020).

We here bring the concept of rate-dependent shear strength, which has been developed in seismology and is related to fault slip stability (e.g., Dieterich, 1979). Whether the slip exhibits further acceleration or deceleration depends on whether the shear strength of the shear zone material shows negative rate-dependence or positive rate-dependence. The former is called velocity-weakening, and the latter is called velocity-strengthening, respectively. Thus, such materials could cause the sliding feature that turns to deceleration during slip acceleration, meaning the slip velocity will have an upper limit value. In this study, the concept of rate-dependent shear strength was applied to describe the sliding properties of clay-rich soils as simulants the landslide. Moreover, the clay-rich soils are naturally thought to be one of the causes of slope failure because of their low frictional property (Bromhead, 2013; Schulz and Wang 2014). We conducted the shear experiments on clay-rich soils to measure the shear strength and rate-dependence. Additionally, we measured various properties of the soils, such as mineral composition and content, liquid limit (WL), plasticity index (PI) and specific surface area (SSA), at the viewpoint what determine the lowness of strength and the variety of rate-dependence.

The samples we used were collected from eight locations in the landslide-prone area in western part of the Aso Caldera, Kumamoto Prefecture. The rotary shear experimental apparatus we used was set at Geological Survey of Japan, AIST (Togo and Shimamoto 2012). We varied the slip velocity from 10⁻⁴ to 10 mm/min (10⁻³ - 10² μm/s) that provided the rate-dependent shear strength functioned by the velocity. The samples were saturated in water (drainage) at room-temperature, and the normal stress was set at approximately 1 MPa

Samples with larger SSA showed the trend of negative rate-dependence at lower velocities (< 1 mm/min) but positive rate-dependence at higher velocities (> 1 mm/min), indicating they have the potential to suppress the acceleration at around 1 mm/min. On the other hand, the rate-dependence was always negative for the samples with small SSA, meaning they have the potential of runaway slip generation. Thus, it can be said that how much SSA is in the slip zone material might constrain the variety in slip at landslide.