Key factors influencing the mechanism of rapid and long runout landslides triggered by the 2008 Wenchuan earthquake, China
© Wang et al.; licensee Springer 2014
Received: 9 April 2014
Accepted: 10 July 2014
Published: 4 October 2014
The 2008 Wenchuan earthquake triggered many rapid and long runout landslides, which directly caused great loss of property and human lives and were responsible for a large percentage of total damages caused by the earthquake. It is very important for the purposes of landslide disaster prevention and mitigation to understand the earthquake triggered mechanism of initiation and motion of rapid and long runout landslides, which can potentially be the deadliest of ground failures.
In this paper, field investigations of some highly damaging landslides caused by the Wenchuan earthquake are introduced first, and followed by data from ring shear tests used to simulate the initiation and motion of one landslide in particular, the Donghekou complex landslide.
It was found that groundwater and valley water played key roles in the rapid motion and long runout process of this landslide during the great earthquake.
As summarized by Bird and Bommer (), large landslides triggered by earthquakes can potentially cause massive loss of life and injuries and leave an indiscriminate trail of devastation. In this paper, some typical landslides, which moved rapidly and caused large numbers of fatalities, will be briefly introduced and compared with each other. The examples are Xiejiadian landslide at Jiufeng in Pengzhou City, the rapid and long runout Chengxi landslides and the Xinbei Mid-school rockslide in Beichuan County, and the Donghekou complex slide in Qingchuan County (Figure 1). These landslides killed all of the people and destroyed all villages in their travel paths (Wang et al. ; Yin et al. ; Cui et al. ; Sato and Harp ). Secondly, a detailed study of the Donghekou complex landslide will be presented. The detailed study includes field investigation, soil sampling, and a geo-simulation test on the soil samples from this slide, with the application of actual seismic motion from the earthquake. This study will contribute to an understanding of the mechanism of the initiation and motion of rapid and long runout landslides, especially the key factors controlling the travel distance.
2Methods, results and discussions
2.1 Field investigations for some typical landslides
In the year following the Wenchuan earthquake, the authors investigated the earthquake affected areas for geo-hazards at four different points in time. The first time was in June 2008, one month after the earthquake occurred. At that time, most of the landslides remained in their original shape after initial occurrence and the local people had a recent memory about the topographic change around them, making the observation and investigation more closely attuned to conditions which occurred immediately after the initiation of the landslides. Investigations were conducted on Chengxi landslide, Xinbei Mid-school rockslide, and Xiejiadian landslide. The second investigation was conducted in July 2008. At that time, the first access to Donghekou complex slide was made. Two landslide dams caused by this slide were not yet excavated or modified. After that time, the landslide dams were leveled, and a memorial park was built at the lower part of the slide. The third investigation was conducted at the end of July 2008 for a duration of one week, and debris flows caused by rainfall in the earthquake-affected area were observed. During the fourth investigation conducted in February 2009 (a dry season in this area), it was found that groundwater was flowing out of the slide surface and source area of the Chengxi landslide, the Donghekou complex slide, and the Xiejiadian landslide. During the field investigations, there was a strong indication that the hydrogeological conditions associated with the landslides controlled the initiation and motion of the landslides. In order to verify this hypothesis, characteristics of some of these typical landslides, i.e., the Xiejiadian landslide, the Chengxi landslide and the Xinbei Mid-school rockslide, and the Donghekou complex slide are presented here to show the importance of hydrogeological conditions to the landslide process.
2.1.1 Xiejiadian rockslide – rock flow in Jiufeng Village
2.1.2 The Chengxi complex landslide (debris fall/slide/flow) and the Xinbei Mid-school rockslide in Qushan Town
In the fourth investigation, we confirmed that groundwater existed in the weathered shale in the source area of the landslide, and the underground soil located 30 cm beneath the surface had high water contents, while the surface was quite dry.
The cause of the rapid sliding in the Chengxi complex slide, in addition to the steepness of the slope at the source area, was possible due to the existence of a shallow groundwater table in the basin beneath Qushan Town. As indicated by a longitudinal cross section on the Chengxi slide made by Yin et al. (), the ground level before the earthquake was just 3–5 m higher than the water level in the Jian River, a river flowing through Qushan Town. We had planned to excavate the slide mass to determine the water content in the slide surface, however, this area has been reserved for an earthquake memorial park, and we could not get permission.
The Xinbei Mid-school rockslide occurred in dolomite/limestone. Because the displaced rock mass was almost dry and of large boulder size, it slowed and stopped at about 25 degrees. The permeability of dolomite block is high because of its fractured structure. In addition, the travel path of this slide is at a much higher elevation than the Jian River. The groundwater table is estimated to be far beneath the slide surface of the rockslide. There is no water affecting the sliding process, so the debris could stop and deposit at 25 degrees. The Xinbei Mid-school was destroyed and about 500 students and teachers were killed, because the school was located near the foot of the slope, and in the direct path of the landslide. Additionally, the school was in very close proximity to the earthquake fault rupture, and most likely experienced a damaging level of shaking from the earthquake.
2.1.3 A complex landslide in Donghekou Village
As can be seen from Figure 6b, the landslide destroyed all of the houses in this area, and dammed the rivers. Four sub-villages and an elementary school were destroyed, and about 700 people were killed. The landslide dam in the main stream of the Xiasi River is not very thick. Because of the long travel distance, the debris flow deposits spread wide and thin.
The slickenside faces on the mountain at the source of the landslide and the adjacent mountain, which also experienced the rockslide motion, indicate intense energy and high velocity of movement of the sliding mass. Local eyewitness said three buses full of local travelers were buried by the rapidly-moving debris because the buses had no time to escape. The flat deposits of the complex slide showed characteristic of a flowslide. There are almost no large blocks in the travel path, showing the highly fractured property of the sliding mass before it formed the landslide.
2.2 Simulation tests for the initiation and motion mechanism of the Donghekou complex slide for use as a case study
We selected Donghekou complex slide as a prototype model for laboratory tests to study the initiation and motion mechanisms of landslides triggered by the Wenchuan earthquake, because the Donghekou complex slide moved for a long distance and showed a high rate of speed. Long travel distance implies a very low shear resistance during the landsliding. We try to find the mechanism to make the low shear resistance possible during the landslide motion. After the earthquake, many analyses and discussions have been conducted for this landslide, and the most comprehensive comments were given by Qi et al. (). They mentioned that besides the influence of the earthquake, the high relief and steep inclination in the source slope and plentiful saturated loose deposits formed by a river or gully in the travel path had a great contribution to the rapid and long runout landslides. In this paper, we explain the initiation and motion mechanisms using the concept of undrained shearing, although other mechanisms may also exist.
Using the undrained ring shear apparatus developed at Kyoto University, Japan (Sassa ; Sassa et al. ), two types of simulation tests were conducted. They are: (a) Simulation test to clarify the initiation mechanism of the Donghekou complex slide triggered by the Wenchuan earthquake and (b) Test to simulate the movement when the failed sliding mass loaded onto the valley deposits, and clarify the motion mechanism for long runout and in high speeds.
2.2.1 The geotechnical simulation test for the Donghekou slide with the actual seismic motion
In a geotechnical simulation test, the initial stress level, i.e., the initial normal stress, shear stress, and pore-water pressure existing in the slope before earthquake should be reproduced first to simulate the initial stress condition in the slope, and then the cyclic loading caused by a seismic motion can be applied to observe the effect on the slope caused by the earthquake. Because the Donghekou complex slide is so large and the earthquake was so strong, it is impossible to simulate both the initial stress condition of the slope and the cyclic loading from the earthquake at the same time. As commonly understood in slope stability evaluations, the slope angle is much more important than the thickness of sliding mass. That is, the relationship between the initial normal stress and shear stress is more important than the absolute value of the normal stress and shear stress. In this test, with consideration of the stress capacity of the ring shear apparatus, the initial slope angle of the Donghekou complex slide was maintained, while the thickness of the sliding mass was reduced significantly.
For the initial stress condition of the Donghekou rockslide, the slope angle of 25 degrees was used at the source area. The initial normal stress and shear stress were obtained from the slope angle, thickness of the sliding mass, and the unit weight of the soil/rock. With the consideration of the capacity of the apparatus, a thickness of 20 m was used to replace the actual average thickness of 75 m. The unit weight of the sliding mass was assumed to be 18 kN/m3. Statistically, under the same slope condition, larger scale landslides always show a higher mobility (Okuda ). Theoretically, for an infinite long slope consisting of non-cohesive soils, the slope stability has no relationship with gravitational acceleration. Considering that the stress level in the simulation test is about 27% of the actual, the initiation mechanism and motion mechanism of the Donghekou complex slide can be qualitatively interpreted. The actual failure should have a higher mobility than that shown in the laboratory test.
Generally, seismic intensity is attenuated by distance, and amplified by standing elevation and loose deposits. To apply the seismic record to Donghekou slide, modifications would have to be made. However, when considering the following factors, we assumed the seismic intensity at the Donghekou slide can be treated as the same as that recorded in SFB station. They are: 1) the SFB station is on the hanging wall of the secondary fault, and Beichuan fault is located near the source area of the Donghekou complex slide (Yuan et al. ); and 2) the elevation difference between source area and toe of Donghekou slide is about 700 m. The source area of the Donghekou slide should have been affected by the seismic amplification. The SFB station is located in alluvium, so the recorded waves also should have been amplified. As both sites are located in the same seismic intensity zone of VIII, and the amplification by standing elevation in the Donghekou complex slide and the alluvium deposit at the SFB station can be treated as equivalent qualitatively. Although we know that the geological and topographic conditions and the distance to the source fault of the earthquake may cause amplification and attenuation of the seismic shaking in different site, the coefficient of the amplification and attenuation is very difficult to be determined. So for simplification, the seismic wave monitored at SFB station was used to form the seismic loading input signal for the simulation test, without any amplification and attenuation (Wang et al. ).
- (2)Determination of the two components in parallel direction and normal direction to the sliding surface, by summing the horizontal component aHR (t) and the vertical component aUD (t) (Figure 12b). The component in the normal direction to the sliding surface aNR (t), and the component in the shear direction of the sliding surface, aSH (t), are obtained as shown in Equations (2) and (3). Where, is the slope angle of the landslide.(2)
The test was conducted with disturbed sample A from the source area. The sample was dried in an oven at 105°Cat first, and then separated into grains by gently applying a wooden hammer. By means of free fall deposition method (Ishihara ), the dry sample grains were set in the shear box, and saturated by carbon dioxide gas and de-aired water. The saturation degree was confirmed by the BD value suggested by Sassa (). When the BD value is higher than 0.95, the sample is recognized as fully saturated. In this test, BD was 0.97, showing a fully saturated condition. After normal consolidation at 295.7 kPa, the initial shear stress 137.9 kPa was loaded gradually while keeping the sample in a drained condition to avoid excess pore-water pressure generation. At this moment, the void ratio of the soil sample reached 0.650. Then, the sample was changed to an undrained condition, and the input signals of seismic loading (Figure 13) were loaded to simulate the condition of the original slope when the earthquake motion was applied as the initiation factor.
2.2.2 Results of the simulation test showing the impact of failed slide mass loading into the valley deposit
As mentioned previously, the angle from the toe of the Donghekou complex slide and the source area is about 11 degrees. Caution is required to compare the actual apparent friction value with that obtained in ring shear tests. The most important difference is that the total normal stress in ring shear test is constant, while in the actual case, the normal stress generally decreases to lower level because the sliding mass always becomes thinner and thinner. Other reasons causing this difference may include (1) energy dissipation caused by the impact of the collision of the debris among the sliding mass, (2) some parts may not be in a fully saturated condition mobilizing a higher shear resistance. On the other hand, the test in laboratory was conducted in a simple and ideal situation with a fully saturated sample in undrained conditions. These factors may cause the difference between the actual apparent friction angle and test value. Moreover, it is clear that the test shows that the existence of the water is a very important factor for the rapid movement and long travel distance.
Through field investigation of the long runout landslides triggered by the Wenchuan earthquake and experimental study of the Donghekou complex landslide, in particular, it was found that the valley water and groundwater played key roles in the long runout and rapid landslide motion during the great earthquake. In reality, there is only limited information on the actual rainfall, valley water, and ground water conditions during the earthquake. It is suggested that in the source area, the landslide gained velocity when the “sliding surface liquefaction” phenomenon occurred as a result of undrained shearing in a saturated sliding zone. In turn, the displaced sliding mass can exert an impact on the valley deposit. When the valley deposit is also saturated, liquefaction can occur causing the valley deposit to work as a sliding zone with low shear resistance, transporting the sliding mass from the source area for a long distance. There may be other mechanisms for rapid and long runout landslides triggered by the earthquake, however, the hydrogeological condition in the source area and traveling path is certainly a key factor in controlling the shear resistance, and should be considered seriously in risk assessments for landslide disaster reduction. Impacts and losses depended upon whether the landslide moved but was abruptly stopped, and whether the landslides moved for a long distance with high speed. It would be useful if more earthquake-induced landslide case studies using methods outlined in this paper were to be analyzed. We could then be closer to concluding whether there are the same and/or additional variables that affect rate of movement, extent of movement and their relationships to the rates of death and damage.
The field investigation and sampling was partially supported by Chinese State Key Fundamental Research Program Project (2008 CB425802, representative: Peng Cui). Dr. Laizhen Pei of Institute of Mountain Hazards and Environment, Chinese Academy of Sciences guided the field investigation. Dr. Hongshuai Liu of Institute of Engineering Mechanics, China Earthquake Administration supplied the seismic record of the Wenchuan earthquake. The authors deeply appreciated the discussions with Prof. Xiyong Wu of Southwest Jiaotong University, China, Prof. Masahiro Chigira of Kyoto University, Prof. Kazuo Konagai of University of Tokyo, and Prof. Satoshi Tsuchiya of Shizuoka University, Japan in the field investigation. The field investigation and ring shear tests were conducted when the first author worked in Disaster Prevention Research Institute of Kyoto University, Japan. The authors acknowledge Dr. Janusz Wasowski of CNR-IRPI, Italy, Dr. Julian Bommer of Imperial College London, and Beena Ajmera of California State University, Fullerton, USA for their helpful and constructive comments to improve the manuscript.
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