Displaced Landslide Materials Research Articles

The debris avalanche in Donghekou area triggered by the 2008 Wenchuan (M8.0) earthquake: Features and possible transportation mechanisms

In 2008, the Wenchuan earthquake triggered many large landslides with rapid movement and long runouts, resulting in a great number of casualties. Although there have been many studies of the geographical features and initiation mechanisms of some catastrophic landslides, the movement mechanisms for many remain unclear. In this paper, we present a case study of a large landslide (debris avalanche) triggered by the 2008 Wenchuan earthquake in the Donghekou area, Sichuan Province, China. We made detailed field surveys of the geographical features of the landslide and carried out subsurface investigations of the landslide deposits using microtremor array measurement and electrical resistivity tomography (ERT). Based on the observed surficial features, shear-wave velocity (Vs) profiles and 2D electrical resistivity profiles of the landslide deposits, we estimated the possible thickness of landslide deposits at different locations, and also analyzed the possible landsliding mechanisms. We inferred that this landslide resulted from retrogressive failures on the source area, and the displaced landslide materials underwent transitional spreading with further entrainment of debris along the travel path. Multiple mud waves might have been formed in the substrate soil layers along the travel path due to the entraining of landsliding materials, and the landsliding materials might have presented channelized movements, indicating that different parts may have moved at different speeds. This kind of transportation mechanism may provide information for elevating the numerical simulation of landsliding, and also for reuse of deposit area of large landslides.

Investigation and dynamic analysis of the catastrophic rockslide avalanche at Xinmo, Maoxian, after the Wenchuan Ms 8.0 earthquake

At 5:38:58 a.m. on June 24, 2017, a catastrophic rockslide avalanche occurred at the Xinmo village, Diexi town, Maoxian county, Sichuan Province, and the area could have been deeply affected by many strong earthquakes in history. The landslide killed 83 people and destroyed the whole village, with the runout horizontal distance of about 2800 m and the elevation difference of 1200 m, which was equivalent to a Fahrboschung of about 23°, and the accumulative volume reached nearly 18 million m3. The field investigation and preliminary analysis of landslide seismic signals indicated that, after sliding from the ridge crest, the rock mass continuously impacted and accumulated on the upper part of the slope, which caused the substrate, i.e., old rockslide accumulation, to slide and disintegrate. It was transformed rapidly into a pipeline-type debris avalanche that then turned into a fan-like debris flow due to the terrain, which also had the typical dynamic characteristics of high speed and long runout. However, in fact, as the cause and effect of the kinematic and dynamic processes of the Xinmo landslide is so complicated and difficult to understand, the Fast Lagrangian Analysis of Continua (FLAC) and the slope-stability analysis module of GeoStudio were firstly combined to analyze the failure process of the old rockslide accumulation body when suffering the added forces from the landslide. Then, the Engineering Discrete Element Method (EDEM) was used to simulate the runout behavior of the displaced landslide materials, the calculated velocity value of which was also compared with those using other dynamic modeling approaches (e.g., sled model, rheological model, and liquefied model). The results demonstrate that the dynamic characteristics and the kinetic type of the Xinmo landslide were successfully simulated. Besides, the velocity values simulated by EDEM and by the rheological model were close to the value of around 60 m/s based on the distance of debris avalanche and the corresponding duration recognized through landslide seismic signals. Also, their velocity variation curves with motion were rational. The above proposed method can be further applied for studies on early recognition and hazard zonation of similar large-scale rockslide avalanches in areas affected by strong earthquakes.

Volcaniclastic debris avalanche on Motomachi area of Izu-Oshima, Japan, triggered by severe storm: Phenomenon and mechanisms

Heavy rainfall and strong wind associated with Typhoon Wipha hit Izu-Oshima Island, Japan, and triggered catastrophic landslides on the western slope of Oshima Volcano on 16 October 2013. These landslides were shallow but large in scale, and the displaced landslide materials were characterized by rapid and long runout movement. A massive landslide resulted in 36 people dead and 3 missing, with 46 buildings being completely destroyed in the downstream area of Motomachi district. To understand the initiation and movement of these shallow landslides, we surveyed the landslide area and found that in addition to heavy rainfall, strong wind may also have played a key role in increasing the instability of the slopes. We took samples from the source areas and examined their static and dynamic shear behaviors under naturally drained or undrained conditions. Test results showed that high pore-water pressure can be built up and maintained within the displaced landslide material and increase landslide mobility, even though the thickness of displaced landslide materials is 0.7–1.2 m. This may then give a rational explanation on the fluidization process of the displaced landslide materials. We also examined the windthrow features of trees subjected to different wind speed through monitoring the ground motion by seismometer and portable anemometer, and then performed cyclic shear tests on the materials laying above the sliding surface. The test results suggest that the windthrow may have promoted the buildup of excess pore-water pressure within the saturated soil layer and thus favoured the occurrence of slope failure. Therefore, it is suggested that in the prediction of shallow landslides on forested slopes subjected to heavy storm, both the effects of rainfall and strong wind should be taken into account.

A fluidized landslide occurred in the Loess Plateau: A study on loess landslide in South Jingyang tableland

The South Jingyang tableland is located on the southern Loess Plateau of China. Ever since the introduction of the water division project in the 1970s, more than 40 loess landslides have occurred in this tableland-terrace slope topography here. On the morning of March 8, 2016, a loess landslide induced by irrigation occurred in Jiangliu Village on the South Jingyang tableland, and around 20,000m3 of loess rapidly slid along a 65-m high slope for 260m. The moisture content of the slip mass displayed a dramatic change along the slip direction: the water saturation degree in the landslide depression was relatively high, and the leading edge was in a flow state; there were some local natural sub-areas, but the overall area showed a characteristic fluidized. A field investigation indicated that while macropores developed in the loess and deep joints provided a preferential passage for the infiltration of irrigation water, which became a major water source for the slip mass, the softening of the toe part of the slope caused by irrigation became the major factor resulting in the instability of the slip mass. Undrained triaxial shear tests on loess in natural water content and fully saturated state show that the enhanced saturation degree not only lowered the soil mass strength but also increased pore water pressure, resulting in the zoning status of the Jiangliu landslide. The angle of reach formed during this landslide was approximately 14°, which demonstrated motion characteristics of long-runout. Further, considering the standing water at the slope foot and unique properties of slip soils after the landslide occurred, it is speculated that the motion mechanism of this high-speed and long-runout landslide was associated with loess liquefaction that led to sharp decrease in shear strength of the displaced landslide materials during the shear process.

Shear wave velocity imaging of landslide debris deposited on an erodible bed and possible movement mechanism for a loess landslide in Jingyang, Xi’an, China

The South Jingyang Plateau, with a total area of 70 km2, is located in Shaanxi Province, China. Since 1976, more than 50 landslides of different types have occurred repeatedly on the edge slopes of the plateau due to the start of diversion irrigation on the plateau, resulting in great loss of lives and property. To better understand the initiation and movement mechanisms of these loess landslides, we surveyed them and carried out a detailed investigation of a large landslide in the Xihetan area. Our field survey results revealed that although most of these landslides had a long runout with high mobility, most of the landslide materials originating from the edge slopes may have been in an unsaturated state when the landslide occurred. This suggests that the materials at the toe of the edge slope as well as on the travel path along the river terrace might have played a key role in landslide movement. To examine how the materials on the travel path were involved in the landsliding, we used a multichannel surface wave technique and surveyed shear wave velocity (V s ) profiles of the landslide deposits. We also examined the internal geometry of the deposits that outcropped on the right-side slope of the landslide foot. The longitudinal profile of V s along the direction of movement showed that terrace deposits near the toe of the edge slope may have been sheared upward, indicating that at the toe, the surface of rupture might be located inside the terrace deposits. The V s contours showed an A-shaped fold within the landslide deposits in the middle part of the travel path and became greater in the most distal toe part. The V s profile across the deposits showed a U-shaped belt, in which the soil layers have smaller V s . This belt may be the boundary between the sliding landslide debris and terrace deposits. The observed internal geometry of the landslide deposits indicates that a sliding surface developed within the sandy layer underlying the gravel layer. Therefore, we inferred that after failure, the displaced landslide materials overrode and sheared the terrace deposits along its main sliding direction, resulting in the formation of thrust folds within the terrace deposits, and greater V s on the distal toe part of the landslide.

Characteristics and numerical runout modeling of the heavy rainfall-induced catastrophic landslide–debris flow at Sanxicun, Dujiangyan, China, following the Wenchuan Ms 8.0 earthquake

The 2008 Ms 8.0 Wenchuan earthquake triggered a large number of extensive landslides. It also affected geologic properties of the mountains such that large-scale landslides followed the earthquake, resulting in the formation of a disaster chain. On 10 July 2013, a catastrophic landslide–debris flow suddenly occurred in the Dujiangyan area of Sichuan Province in southeast China. This caused the deaths of 166 people and the burying or damage of 11 buildings along the runout path. The landslide involved the failure of ≈1.47 million m3, and the displaced material from the source area was ≈0.3 million m3. This landslide displayed shear failure at a high level under the effects of a rainstorm, which impacted and scraped an accumulated layer underneath and a heavily weathered rock layer during the release of potential and kinetic energies. The landslide body entrained a large volume of surface residual diluvial soil, and then moved downstream along a gully to produce a debris flow disaster. This was determined to be a typical landslide–debris flow disaster type. The runout of displaced material had a horizontal extent of 1200 m and a vertical extent of 400 m. This was equivalent to the angle of reach (fahrboschung angle) of 19° and covered an area of 0.2 km2. The background and motion of the landslide are described in this study. On the basis of the above analysis, dynamic simulation software (DAN3D) and rheological models were used to simulate the runout behavior of the displaced landslide materials in order to provide information for the hazard zonation of similar types of potential landslide–debris flows in southeast China following the Wenchuan earthquake. The simulation results of the Sanxicun landslide revealed that the frictional model had the best performance for the source area, while the Voellmy model was most suitable for the scraping and accumulation areas. The simulations estimated that the motion could last for ≈70 s, with a maximum speed of 47.7 m/s.

Layered internal structure and breaching risk assessment of the Higashi-Takezawa landslide dam in Niigata, Japan

The internal structure and geotechnical properties of landslide dams have rarely been examined in detail, although they largely control the longevity and potential failure mechanisms of the dams. During the 2004 Mid-Niigata Prefecture earthquake, thousands of landslides were triggered and many landslide dams were formed in the Imo River basin. Among these landslides dams, the biggest one that occurred in the Higashi-Takezawa district threatened both the upstream and downstream areas. Emergency and eventual permanent countermeasures were successfully performed to mitigate the hazards presented by the dam. The preserved landslide dam provides a good field study area for improving understanding of the formation and stability of similar features. To characterize its internal structure and to provide more reliable information for stability analysis, we used a multiple-channel surface-wave technique and microtremor array measurements to survey the dam's S-wave velocity (Vs) profile. Our results show that the integrated use of these methods is an effective approach for determining the S-wave velocity structure of landslide dams, irrespective of the mountainous environment in which they typically occur. The Vs profile suggests that the displaced landslide materials had been little disturbed during movement, although movement was rapid and over a long distance. We also discuss herein the landsliding mechanism and assess the breaching risk of the landslide dam due to overtopping.

Investigation and dynamic analysis of a catastrophic rock avalanche on September 23, 1991, Zhaotong, China

At 6:10 p.m. on September 23, 1991, a catastrophic rock avalanche occurred in Zhaotong, Yunnan, southwestern China. Over 216 people were killed when the Touzhai village was overwhelmed directly in the path of the landslide. The landslide involved the failure of about 12 Mm3 of jointed basaltic rock mass from the source area. The displaced materials ran out a horizontal distance of 3650 m over a vertical distance of 960 m, equivalent to a Fahrboschung of 14.7°, and covered an area of 1.38 km2. To provide information for hazard zonation of similar type of potential landslides in the same area, we used a dynamic model (DAN-W) with three alternative rheological models to simulate the runout behaviour of the displaced landslide materials and found that a combination of the frictional model and Voellmy model could provide the best performance in simulating this landslide. The simulated results indicated that the duration of the movement is estimated at about 175 s for a mean velocity 21 m/s.

On characteristics and dynamic analysis of the Niumian valley rock avalanche triggered by the 2008 Wenchuan earthquake, Sichuan, China

On May 12, 2008, a large catastrophic rock avalanche was triggered by the Wenchuan earthquake on Niumian valley in Yingxiu town, Wenchuan County, China. The displaced landslide materials traveled about 2,640 m with a descent of about 850 m and buried several houses and caused nearly 20 casualties. Based on detailed field investigation, some basic characteristics of this landslide are introduced. Field investigation showed that the displaced landslide materials experienced three times of run-up and twice of superelevation along the run-out path, and downslope enlargement due to the entrainment of run-out path materials. A numerical model (DAN3D) is used to simulate the landsliding process. By means of trial and error, it can be seen that the Voellmy model and values of friction coefficient f = 0.19 (dimensionless) and turbulent coefficient \(\xi\) = 2,000 m/s2 can provide the best performance in the prediction of sliding and depositing areas. The simulated results show that the duration of the rock avalanche is about 120 s with an average velocity of 22 m/s.

Dynamic analysis and field investigation of a fluidized landslide in Guanling, Guizhou, China

On June 28, 2010, a large catastrophic landslide was triggered by a heavy rainfall in Guanling, Guizhou, China. This catastrophic event destroyed two villages and caused 99 casualties. The landslide involved the failure of about 985,000m3 of sandstone from the source area. The displaced materials traveled about 1300m with a descent of about 400m, covering an area of 129,000m2 with the final volume being accumulated to be 1,840,000m3, approximately. To provide information for hazard zonation of similar type of landslides in the same area, we used a dynamic model (DAN3D) to simulate the runout behavior of the displaced landslide materials, and found that a combined frictional–Vollemy model could provide the best performance in simulating this landslide and the runout is precisely duplicated with a dynamic friction angle (ϕ) of 30° and a pore pressure ratio (ru) of 0.55 for the materials at the source area and with Vollemy parameters of friction coefficient f=0.1 (dimensionless) and turbulent coefficient ξ=400m/s2. The simulated results indicated that the duration of the movement is estimated at about 60s for a mean velocity 23m/s. To examine the effectiveness of simulation by means of DAN3D and also to evaluate the reactivation potential of these displaced landslide materials depositing on the valley, we used electrical resistivity tomography (ERT) method to survey the depth and internal structure of landslide deposits. The ERT results showed that DAN3D gave a good prediction on the shape and runout distance of the landslide deposits, although the predicted maximum depths of landslide deposit on some areas were differing from those obtained by ERT method.

Some fluidized landslides triggered by the 2011 Tohoku Earthquake (Mw 9.0), Japan

The 2011 Tohoku Earthquake off the Pacific coast of Japan generated a large tsunami and many landslides, resulting in a great number of casualties. Although almost all casualties resulted from the tsunami, some long-travel, fluidized small-scale landslides also killed 13 people. After the earthquake, we surveyed seven of these catastrophic landslides triggered by the earthquake. We found that most of them have nearly identical geological features, with slopes consisting of pyroclastic deposits formed at different times, and with a palaeosol layer that outcropped in most cases after the landslide. Above the palaeosol there are layers of pumice and scoria. The palaeosol had a natural moisture content of ~160%, and the pumice and scoria had a moisture content of ~145%. From field observations we concluded that the sliding surface originated in the very upper part of palaeosol, and liquefaction occurred in both layers, resulting in the fluidization of displaced landslides. To examine the trigger and movement mechanism of these landslides, we monitored the ground motion of one landslide area during the many aftershocks, and compared the results with records obtained by a national seismic station nearby. We inferred that strong seismic motion occurred in the landslide area during the main shock. We sampled the palaeosol and pyroclastic deposits, and performed undrained static/cyclic shear tests on the materials both in a saturated state and at natural moisture content. The results indicate that high pore-water pressure was generated, resulting in decreased shear strength even in samples with the natural moisture content. The shear strength of the palaeosol lowered to a very small value with continuous increase of shear rate, enabling the high mobility of the displaced landslide materials.

The internal structure of a rockslide dam induced by the 2008 Wenchuan (Mw7.9) earthquake, China

The internal structure of landslide dams plays a key role in their stability; however, it has not been much studied, probably due to the difficulty in obtaining information on internal structure in most cases. Here, we examined the shear-wave-velocity structure of a rockslide dam by a surface-wave technique called multichannel analysis of surface waves (MASW). During the 2008 Wenchuan earthquake (Mw7.9), more than 60,000 landslides were triggered and 800 landslide dams formed. Those dams with a high risk of collapse threatened rescue activities, and almost all of the large landslide dams were treated by digging a sluiceway immediately after the earthquake. Although the risk of collapse of many landslide dams was removed or lowered, not all of the countermeasures were based on well controlled methods. To analyze the internal of landslide dams to assist in carrying out reliable countermeasures, we made detailed investigations on some of the dams, and here describe one landslide dam that occurred in the Tianchi area. Grain-size analysis revealed that the displaced landslide materials experienced fragmentation and segregation during the long movement. The shear-wave-velocity profile of the dam revealed that the dam consisted of three facies (carapace, body and basal facies). The boundary between facies is distinct. The body facies had a greater shear-wave velocity (compared to those landside dams that had suffered collapse failure during the construction of a sluiceway), showing that the dam consisted of more densely deposited materials. This kind of dam body had a lower permeability, capable of retarding seepage that triggers collapse failure of the dam body due to piping. Big blocks on the surface also enabled the dam body to have greater resistance to overflow and thus reducing possible collapse failure in the immediate aftermath of overtopping.