Zongji Yang

Regional landslide zonation based on entropy method in Three Gorges area, China

With the development and expansion of landslide research, quantitatively conducting regional landslide hazard assessment is demonstrated to be an effective method in assessing regional scale landslide hazard. This study sets up an index system for regional landslide hazard zonation by means of intensity factors and probability factors of landslide occurrence. Based on the information entropy method, subjective weight was calculated for landslides in Wanzhou County, located in the Three Gorges Reservoir area, and a qualitative landslide hazard degree zonation model was established. This model was then used in evaluating the hazard degree of the landslides in Wanzhou County, and the results were contrasted and confirmed through field investigation and density analysis. As a result, this model demonstrates a useful application of a new methodology and provides a basis for future warning and hazard evaluations of a regional scale landslide.

Assessment of prospective hazards resulting from the 2017 earthquake at the world heritage site Jiuzhaigou Valley, Sichuan, China

On August 8, 2017, a Ms = 7.0 magnitude earthquake occurred in the Jiuzhaigou Valley, in Sichuan Province, China (N: 33.20°, E: 103.82°). Jiuzhaigou Valley is an area recognized and listed as a world heritage site by UNESCO in 1992. Data analysis and field survey were conducted on the landslide, collapse, and debris flow gully, to assess the coseismic geological hazards generated by the earthquake using an unmanned aerial vehicle (UAV), remote-sensing imaging, laser range finders, geological radars, and cameras. The results highlighted the occurrence of 13 landslides, 70 collapses, and 25 potential debris flow gullies following the earthquake. The hazards were classified on the basis of their size and the potential property loss attributable to them. Consequently, 14 large-scale hazards, 30 medium-sized hazards, and 64 small hazards accounting for 13%, 28%, and 59% of the total hazards, respectively, were identified. Based on the variation tendency of the geological hazards that ensued in areas affected by the Kanto earthquake (Japan), Chi-chi earthquake (Taiwan China), and Wenchuan earthquake (Sichuan China), the study predicts that, depending on the rain intensity cycle, the duration of geological hazard activities in the Jiuzhaigou Valley may last over ten years and will gradually decrease for the following five to ten years before returning to pre-earthquake levels. Thus, necessary monitoring and early warning systems must be implemented to ensure the safety of residents, workers and tourists during the construction of engineering projects and reopening of scenic sites to the public.

Clarifying the hydrological mechanisms and thresholds for rainfall-induced landslide: in situ monitoring of big data to unsaturated slope stability analysis

The development of early warning systems for landslide hazards has long been a challenge because the accuracy of such systems is limited by both the complicated underlying mechanisms of landslides and the lack of in situ data. In this study, we implemented a multivariate threshold criterion that integrates in situ monitoring data and data from unsaturated hydro-mechanical analyses as an early warning system for rainfall-induced landslides in the Wenchuan earthquake region of China. The results indicate that rainfall intensity is closely correlated with the probability of landslide occurrence. Variations in matric suction and suction stress were obtained from in situ measurements and used to quantify the soil water retention curve, which presented clear hysteresis characteristics. The impacts of rainfall infiltration on slope failure in post-earthquake landslide areas under transient rainfall conditions were quantified by hydro-mechanical modelling theories. Variations in the suction stress of unsaturated soil were used to calculate the safety factor. The influence of hydrological hysteresis processes on the slope failure mechanism was analysed. Multivariate threshold criteria that include the intensity–probability (I-P) threshold, soil moisture and matric suction based on in situ big data and unsaturated slope stability analysis benchmarks are proposed for use in an early warning system for rainfall-induced landslides.

Hydro-mechanical analysis of rainfall-induced fines migration process within unsaturated soils

Seepage-induced fines migration under rainfall infiltration is a main cause leading to shallow failures in loose colluvial slopes. To describe the full process of fines migration within unsaturated soils during rainfall infiltration and the associated hydro-mechanical behaviors, a seepage-erosion-deformation coupled formulation is proposed in this paper. The governing equations proposed are implemented into a finite element code and used to investigate the influences of skeleton deformation on the rainfall infiltration process through unsaturated soil columns. The numerical results were presented in detail for a better understanding of the rainfall-induced fines migration process within unsaturated soils. Further, the obtained results are integrated into an infinite slope model for slope stability analysis. The results show that, the skeleton deformation will affect the rainfall infiltration rate and hence the timing of slope failures; meanwhile their influences are more evident if the fines deposition process is taken into account. Moreover, the slope stability could be reduced gradually due to the soil strength loss along with loss of fine particles. Therefore, particular attentions should be paid to analyzing the stability of soil slopes susceptible to internal erosion.

A Multi-Scaled Early Warning Method for Rainfall-Induced Mountain Hazards

The magnitude 8 (8M) earthquake (“Wenchaun earthquake”) that hit Sichuan province on May 12, 2008 highlighted the need for the development of large-scale early warning systems in the region surrounding the Longmenshan fault. Many of the deaths from this disaster were not from the earthquake itself, but from the landslides and other hazards that it caused. To this end, real-time monitoring and early warning of mountainous hazards (e.g. landslides, debris flows) are efficient non-engineering measures for disaster prevention and mitigation. Thus, to ensure effective disaster prediction and reduction within this region, a system of real-time regional and site-specific monitoring of mountainous hazards, coupled with early warning, is necessary. We propose here a multi-scaled, real-time early warning method. It combines large-scale regional hazard calculations and site-specific hazard monitoring with subsequent early warning. Real-time monitoring and early warning systems were constructed in five mountain hazard sites along the Longmenshan fault line, including three debris flow and two landslide zones. For early disaster warning, the threshold value of precipitation needed to trigger the post-earthquake mountainous hazard should be measured. However, identification of such threshold values is one of the most difficult issues for regional and site-specific mountainous hazard monitoring and early warning systems. To account for this difficulty, this study utilized a natural coseismic landslide from the Taziping Village of Hongkou County in Dujianyan City to identify the critical values and threshold parameters that resulted. After this experimental field test, the correlation of rainfall intensity with rainfall duration was calculated. The results indicated that the experimental field test was capable of identifying the threshold factors for the rainfall-induced mountain hazards.

Estimating the properties of weathered bedrock and pilerock interaction from the geological strength index

The effect of variable rock mass properties on pile-rock interaction poses a great challenge to the design of stabilizing piles and numerical analysis of pile-rock interaction. The paper presents a novel method to estimate the properties of weathered bedrock, which can be applied to routine design of landslide-stabilizing piles for collivial landslides. The Ercengyan landslide located in the Three Gorges Reservoir, China, is the area of interest for this study. A geological investigation and triaxial tests were conducted to estimate the basic parameters, including Geological Strength Index (GSI), uniaxial compressive strength σci and Hoek-Brown constant mi of intact bedrock in the study area. Hoek-Brown criterion was used to estimate mechanical properties of the weathered rock, including elastic modulus Em, cohesion c, friction angle Φ, and normal ultimate lateral resistance pmax. A parametric study was performed to evaluate the effect of parameterizations of GSI, σci and mi on the bedrock properties and p-y curves. The estimated rock mass properties were used with PLAXIS 2D software to simulate pile-rock interaction. Effect of GSI on stress at the pile-rock interface and in the rock, pile bending moment, pile shear force, and p-y curve were analysed.

Early warning model for slope debris flow initiation

Early warning model of debris flow is important for providing local residents with reliable and accurate warning information to escape from debris flow hazards. This research studied the debris flow initiation in the Yindongzi gully in Dujiangyan City, Sichuan province, China with scaled-down model experiments. We set rainfall intensity and slope angle as dominating parameters and carried out 20 scaled-down model tests under artificial rainfall conditions. The experiments set four slope angles (32°, 34°, 37°, 42°) and five rainfall intensities (60 mm/h, 90 mm/h, 120 mm/h, 150 mm/h, and 180 mm/h) treatments. The characteristic variables in the experiments, such as, rainfall duration, pore water pressure, moisture content, surface inclination, and volume were monitored. The experimental results revealed the failure mode of loose slope material and the process of slope debris flow initiation, as well as the relationship between the surface deformation and the physical parameters of experimental model. A traditional rainfall intensity-duration early warning model (I-D model) was firstly established by using a mathematical regression analysis, and it was then improved into ISD model and ISM model (Here, I is rainfall Intensity, S is Slope angle, D is rainfall Duration, and M is Moisture content). The warning model can provide reliable early warning of slope debris flow initiation.

Numerical and experimental study on the formation mode of a landslide dam and its influence on dam breaching

The deposition morphology of a landslide dam determines the position of initial breach and sequentially affects the direction of lateral erosion. Twenty-seven discrete element numerical simulations were carried out to investigate the deposition morphology of landslide dams concerning three factors, which include the valley shape, the valley bed inclination and the landslide velocity. The simulation results show that the valley shape and the landslide velocity affect the dam morphology in both transverse and longitudinal directions, while the valley bed inclination mainly affects the longitudinal morphology of dams. Further, a flume experiment was carried out to investigate the specific effects of the dam morphology on the overtopping erosion of landslide dams. The results of experiment show that for the dams in different deposition morphologies and with different initial breaches, the breach widths and the total sediment discharges induced by overtopping erosion are different.

Explore on Hydro-Mechanical Threshold for Early Warning of Rainfall induced Shallow Landslides

After the Wenchuan earthquake on May 12th, co-seismic landslides and fractured slopes were more susceptible to rainfall-induced shallow mass re-mobilization and post-earthquake disasters were gained widespread significance for the disaster mitigation. However, despite the rainfall thresholds, the hydrological parameters of rainfall induced mass re-mobilization in natural environment of Wenchuan earthquake regions is not well understood and widely used for disaster early warning. In this study, shallow rainfall triggered slope failures under partially saturated conditions in the hollows of the gully was proved by instrumental evidence of in situ experimental tests in a natural co-seismic landslide for simulating the rainfall triggered erosion process of shallow failures in debris flow catchment. In addition, the results revealed the transient process and unsaturated condition for mass movement in response to rainfall, and demonstrated the importance of hydrological parameters includes soil matrix suction and moisture content for shallow slope failure in the hollows, and the stability analysis suggested a hydro-mechanical thresholds including water contents and matrix suction based on the mechanism of slope failure for early warning of the mass-remobilization in hollows of debris flows. These findings were expecting for contribution effectively on improvement of early warning accuracy for rainfall induced shallow landslides and debris flows in earthquake hit region.

Experimental Identification on Thresholds for Early Warning of Rainfall-induced Failure on Fractured Slopes after Giant Earthquake

Prevention and mitigation of rainfall induced geological hazards after the Ms=8 Wenchuan earthquake on May 12th, 2008 were significant for rebuild of earthquake hit regions. After the Wenchuan earthquake, there were tens of thousands of fractured slopes which were broken and loosened by the ground shaking, they were very susceptible to heavy rainfall and change forms into potential debris flows. In order to carry out this disaster reduction and prediction effectively in Longmenshan region, careful real-time monitoring and pre-warning of mountain hazards in both regional and site-specific scales is reasonable as alternatives in Wenchuan earthquake regions. For pre-warning the failure of fractured slopes induced by rainfall, the threshold value or the critical value of the precipitation of hazards should be proposed. However, the identification of critical criterion and parameters to pre-warning is the most difficult issue in mountainous hazards monitoring and pre-warning system especially in the elusive and massive fractured slopes widespread in Wenchuan earthquake regions. In this study, a natural coseismic fractured landslide in the Taziping village, Hongkou County, Dujianyan City, was selected to conduct the field experimental test, in order to identify the threshold parameters and critical criterion of the fractured slopes of Taziping. After the field experimental test, the correlation of rainfall intensity, rainfall duration and accumulative rainfall was investigated. The field experimental test was capable of identifying the threshold factors for failure of rainfall-induced fractured slopes after the giant earthquake.