Investigation of shallow landslides triggered by heavy rainfall during typhoon Wipha (2013), Izu Oshima Island, Japan
© Yang et al. 2015
Received: 23 October 2014
Accepted: 5 May 2015
Published: 9 June 2015
Typhoon Wipha struck Izu Oshima Island on 16 October 2013, bringing heavy rainfall. It triggered widespread landslides on the western slopes of Izu Oshima Island, and caused significant loss of life and serious property damage. Authors have conducted both field investigations and laboratory experiments in an effort to understand the initiation mechanism of the shallow landslides triggered by heavy rainfall.
Pyroclastic-fall deposits on the slopes are well-graded fine sand with silt, and with high specific gravity and void ratio. These soil properties will affect the mechanical and hydraulic characteristics of soil. The results of consolidated-undrained triaxial tests show that the effective internal friction angle of soil is 38.7 degrees. The results of triaxial tests using pore-water pressure control show that static liquefaction can occur in porous pyroclastic-fall deposit layers due to rainfall infiltration.
The effective strength of pyroclastic-fall deposits on the upper slope is quite high. Even though the slope is very steep (over 30 degrees), it can remain stable while in an unsaturated condition. Due to heavy rainfall and the porosity of the pyroclastic-fall deposits, rainfall can quickly infiltrate into soil layer. Moreover, the interface above the underlying basalt will stop groundwater infiltration, acting as an impervious boundary. With increase of groundwater level, the effective strength of the porous soil will decrease. Finally, static liquefaction can be triggered, leading to the generation of shallow landslides on the upper slopes.
Several researchers and organizations carried out field investigations after this geo-disaster event (Sakurai and Disaster Research Team of Kanto Branch 2014; Ikeya 2014; Disaster Prevention Division of Tokyo Metropolitan Government (TMG) (2014)). Their investigation reports describe the occurrences of the landslides, and the damage caused by the debris flows. These studies provide key background information for further research. Authors had conducted both field investigations and laboratory experiments in an effort to understand the initiation mechanism of shallow landslides triggered by heavy rainfall during Typhoon Wipha. Consolidated-undrained triaxial tests were conducted to determine the effective soil strength. This paper also presents the results of triaxial tests with pore-water pressure control, which were conducted to simulate the initiation mechanism of the shallow landslides on the upper slopes due to rainfall infiltration.
Soil samples were collected from the pyroclastic-fall deposit layer above the basalt at location S1 to study soil properties (Fig. 9). Conventional laboratory experiments on the soil sample were conducted to obtain the basic parameters (grain size distribution, specific gravity, in-situ dry density and void ratio) which will be used to control the parameters of specimens for triaxial tests.
Triaxial text using strain control
Consolidated-undrained triaxial tests were conducted to determine the effective soil strength. According to the in-situ soil dry density, dry soil passed 2 mm sieving was used to make a cylindrical specimen. In order to get a homogenous specimen, the dry soil sample was divided into several parts to fill the cylindrical specimen tube with rubber membrane. After filled the all soil into the specimen tube, the cylindrical surface of the sample is covered by the rubber membrane sealed by rubber O-rings on the top and base of load system. After that, the specimen was fully saturated. Carbon dioxide (CO2) was slowly supplied from the base of the specimen to gradually replace the air within it. Then de-aired water was slowly supplied to replace and absorb the CO2 to achieve a saturated state. The specimen was confirmed to be fully saturated when the Skempton’s B value, which is called pore-pressure coefficients (Skempton 1954), was higher than 0.95. With the fully saturated specimens, four consolidated-undrained compression tests were conducted under four different confining stresses (25, 50, 75 and 100 kPa). After normal consolidation, the specimens were compressed at 1.0 % of axial strain per minute under the undrained condition. The data of deviatoric stress, pore-water pressure and axial strain was obtained by data logging system. Through these tests, the shear strength parameters can be obtained.
Triaxial text using pore-water pressure control
Triaxial tests with pore-water pressure control were conducted to simulate the initiation mechanism of the shallow landslide on the upper slopes due to rainfall infiltration.
The process of specimen preparation and saturation was the same as that of the triaxial tests using strain control. The confining pressures were σ1 = 40 kPa and σ3 = 15 kPa. After consolidation, de-aired water is supplied through a pore-water pressure controller to increase the pore-water pressure. The rate of increase in pore-water pressure was 0.2 kPa per minute. Through this step, the situation of pore-water pressure accumulation on the potential sliding surface is simulated.
Specific gravity, G s
Coefficient of uniformity, C u
Coefficient of curvature, C c
Mean grain size, D 50 (mm)
Dry density, ρ d (kg/m3)
Void ratio, e
Two types of failure mode
H is the thickness of the pyroclastic-fall deposit layer;
h is the height of groundwater table;
α is the slope angle;
γ is the average unit weight of soil, 17 kN/ m3;
γ w is the unit weight of water, 9.8 kN/m3;
c′ is the effective cohesion of soil;
ϕ′ is the effective friction angle of soil, 38.7 degrees.
The effective strength of pyroclastic-fall deposits on the upper slope is quite high, and the effective internal friction angle is 38.7 degrees. Consequently, even though the slope is very steep (over 30 degrees), it can remain stable while in an unsaturated condition. Due to heavy rainfall and the porosity of the pyroclastic-fall deposits, rainfall can quickly infiltrate into soil layer. Moreover, the interface above the underlying basalt will stop groundwater infiltration, acting as an impervious boundary. With increase of groundwater level, the effective strength of the porous soil will decrease. Finally, static liquefaction can be triggered, leading to the generation of shallow landslides on the upper slopes.
Slope cutting for road construction on the soft-hard slope structure (porous and shallow soil layer mantling the hard basalt layer) should be considered very carefully. Without any preventive measures, the upper layer soil can easily slide along the interface between the soft and hard layers.
The drainage system along the road, especially in areas of low terrain, provides an environment which easily gathered runoff water. An amount of water will continuously infiltrate into the slope below the road, leading to shallow landslides.
From the above, it is very important to evaluate the threshold rainfall for shallow landslide occurrences on Izu Oshima Island. This will be useful to build early warning systems to protect the local people during heavy rainfall events.
The authors would like to sincerely thank Dr. Barry Roser for his helpful and constructive comments to improve the manuscript. Valuable and constructive comments from anonymous reviewers are also appreciated. This work was financially supported by JSPS KAKENHI Grant Number A-2424106 for landslide dam failure prediction.
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