Morphometric analysis of the Andaman outer shelf and upper slope——Implications for the recent slope failure events

The devastating 2004 tsunamis that hit the southwestern coast of Thailand pose a serious threat to people along the coastal zone. A major aim for the tsunami hazard prediction is better prediction of the next tsunamis and their impacts. In this paper, we present the first implications of recent slope failure events of the Andaman outer shelf and upper slope based on a new detailed bathymetric data and subbottom profiler records acquired during two cruises of the MASS project in 2006 and 2007. Morphometric analysis reveals a variety of anomalous features,including: three large plateaus surrounded by moats, ruggedness and unevenness of slope morphology, and two translational submarine landslides. Two submarine landslides are studied from the detailed bathymetric data and subbottom profiler record covering the upper slope of the Andaman Sea shelf break within Thai exclusive economic zone. Maximum approximated volumes of both displaced masses are 4.8×10~7 m~3 and 2.2×10~7 m~3.Considering the data, there is no evidence that landslides have been the sources for tsunami hazard potential in recent geological time. These prerequisites will allow better study of slope failure events in the area. Further investigation is required to better understand obvious geotectonic phenomena.

Bayesian machine learning-based method for prediction of slope failure time

The data-driven phenomenological models based on deformation measurements have been widely utilized to predict the slope failure time(SFT).The observational and model uncertainties could lead the predicted SFT calculated from the phenomenological models to deviate from the actual SFT.Currently,very limited study has been conducted on how to evaluate the effect of such uncertainties on SFT prediction.In this paper,a comprehensive slope failure database was compiled.A Bayesian machine learning(BML)-based method was developed to learn the model and observational uncertainties involved in SFT prediction,through which the probabilistic distribution of the SFT can be obtained.This method was illustrated in detail with an example.Verification studies show that the BML-based method is superior to the traditional inverse velocity method(INVM)and the maximum likelihood method for predicting SFT.The proposed method in this study provides an effective tool for SFT prediction.

Dissociation of gas hydrates by hydrocarbon migration and accumulation-derived slope failures:An example from the South China Sea

The mechanism of slope failure associated with overpressure that is caused by hydrocarbon migration and accumulation remains unclear.High-resolution seismic data and gas hydrate drilling data collected from the Shenhu gas hydrate field(site SH5)offer a valuable opportunity to study the relations between submarine slope failure and hydrocarbon accumulation and flow that is associated with a~2 kmdiameter gas chimney developed beneath site SH5 where none gas hydrates had been recovered by drilling and sampling despite the presence of distinct bottom simulating reflectors(BSRs)and favorable gas hydrate indication.The mechanism of submarine slope failure resulted from buoyancy extrusion and seepage-derived deformation which were caused by overpressure from a~1100 m-high gas column in a gas chimney was studied via numerical simulation.The~9.55 MPa overpressure caused by hydrocarbons that migrated through the gas chimney and then accumulated beneath subsurface gas hydratebearing impermeable sediments.This may have resulted in a submarine slope failure,which disequilibrated the gas hydrate-bearing zone and completely decomposed the gas hydrate once precipitated at site SH5.Before the gas hydrate decomposition,the largely impermeable sediments overlying the gas chimney may have undergone a major upward deformation due to the buoyancy extrusion of the overpressure in the gas chimney,and slope failure was initiated from plastic strain of the sediments and reduced internal strength.Slope failure subsequently resulted in partial gas hydrate decomposition and sediment permeability increase.The pressurized gas in the gas chimney may have diffused into the overlying sediments controlled by seepage-derived deformation,causing an effective stress reduction at the base of the sediments and significant plastic deformation.This may have formed a new cycle of submarine slope failure and finally the total gas hydrate dissociation.The modeling results of buoyancy extrusion and seepage-derived deformation of the overpressure in the gas chimney would provide new understanding in the development of submarine slope failure and the link between slope failure and gas hydrate accumulation and dissociation.

Probabilistic assessment of slope failure considering anisotropic spatial variability of soil properties

Anisotropic spatial variability of soil properties is frequently encountered in geotechnical engineering practice due to the complex depositional process.To quantitatively evaluate the response of slope failure related to anisotropic spatial variability of soil properties and reveal the underlying influence of anisotropic spatial variability of soil properties on the slope reliability,this study integrates the random finite difference method(RFDM)into a probabilistic assessment framework and adopts general spatial variability and a cohesive-frictional soil slope example for illustration.A parametric analysis is carried out to investigate the influence of general anisotropic spatial variability of soil properties on slope failure probability and failure characteristics.The results show that the directional angles of scales of fluctuation of general anisotropic spatial variability significantly affect the slope failure probability.The dominant failure mode is the intermediate type in most cases of general anisotropic spatial variability,which is distinguished from the shallow failure mode occurring in the homogenous state.Overestimation of cross-correlation between c and u(qc;u),scales of fluctuation(dmax and dmin)in general anisotropic spatially variable soils significantly influences the average slip mass volumes of deep and multi-slip failure mode.Compared with transverse anisotropic spatial variability,general anisotropic spatial variability significantly ampli-fies the effects of qc;u,dmax and dmin on slope reliability.

Deformation and failure mechanism of Yanjiao rock slope influenced by rainfall and water level fluctuation of the Xiluodu hydropower station reservoir

With the construction of the Xiluodu hydropower station on the Jinsha River,the reservoir impoundment began in 2013 and the water level fluctuates annually between 540 m and 600 m above sea level.The Yanjiao rock slope which is located on the left bank of the Jinsha River 75 km upstream of the Xiluodu dam site,began to deform in 2014.The potential failure of the slope not only threatens Yanjiao town but also affects the safe operation of the Xiluodu reservoir.This paper is to find the factors influencing the Yanjiao slope deformation through field investigation,geotechnical reconnaissance,and monitoring.Results show that the Yanjiao slope can be divided into a bank collapse area(BCA)and a strong deformation area(SDA)based on the crack distribution characteristics of the slope.The rear area of the slope has been experiencing persistent deformation with a maximum cumulative displacement(GPS monitoring point G4)of 505 mm and 399 mm in the horizontal and vertical directions,respectively.The potential failure surface of the slope is formed 36 m below the surface based on the borehole inclinometer.The bank collapses of the Yanjiao slope are directly caused by the reservoir impoundment while the deformation area of the slope is affected by the combination of the rainfall and reservoir water level fluctuation.Based on mechanism of the Yanjiao slope,prestressed anchor combined with the surface drainage and slope unloading are recommended to prevent potential deformation.

Slope failure simulations with MPM

The simulation of slope failures,including both failure initiation and development,has been modelled using the material point method（MPM）.Numerical case studies involving various slope angles,heterogeneity and rainfall infiltration are presented.It is demonstrated that,by utilising a constitutive model which encompasses,in a simplified manner,both pre-and post-failure behaviour,the material point method is able to simulate commonly observed failure modes.This is a step towards being able to better quantify slope failure consequence and risk.

Geotechnical forensic investigation of a slope failure on silty clay soil--A case study

Qila Bala Hisar is one of the noteworthy places of Peshawar,Khyber Pakhtunkhwa.The fort was constructed on a flled ground during the 18th century and it was renovated several times by the occupants ever since.Recently,due to an earthquake of magnitude 7.3,the upper part of the south-western wall of the fort collapsed.The collapse of the wall was attributed to the failure of the retained slope.This research was undertaken to characterize the slope material,study causal factors of failure and evaluate remedial strategy.The investigation involved conventional field and laboratory testing and geophysical investigation using electrical resistivity technique to evaluate the nature of stratum.Also,X-ray Diffraction and Scanning Electron Microscopy was used to study the slope material at a molecular level to evaluate the existence of swelling potential.The analysis has shown that excessive seepage of water caused by the poor maintenance of runoff and sewage drains is the causal factor triggered by the seismic event.A remedial strategy involving soil nails,micro piles and improvement of the surface drainage is recommended.

Numerical analysis of slope collapse using SPH and the SIMSAND critical state model

Geological disasters such as slope failure and landslides can cause loss of life and property.Therefore,reproducing their evolution process is of great importance for risk assessment and mitigation.The recently developed SIMSAND critical state sand model combined with the smoothed particle hydrodynamics(SPH)method is adopted in this work to study slope failure under large deformations.To illustrate the efficiency and accuracy of the SIMSAND-SPH approach,a series of slope collapse studies using the discrete element method(DEM)considering various particle shapes(i.e.spherical,tetrahedral and elongated)is adopted as benchmarks.The parameters of the SIMSAND model are calibrated using DEM triaxial tests.In comparison to the DEM simulations,the runout distance and final slope height are well characterized with the SIMSAND-SPH approach with less computational cost.All comparisons show that the SIMSAND-SPH approach is highly efficient and accurate,which can be an alternative numerical tool to simulate real scale granular flow.

Boundary effect of toppling failure based on three-dimensional mechanical model

Previous researches on the mechanical model of toppling failure mainly concentrated on twodimensional mechanical model(TwDM) analysis. The TwDM analysis assumes the width of the slab beam is unit width without considering the lateral constraint force. The assumed conditions are obviously different from the site conditions, thus there is a certain difference between the calculated results and the field work. A three-dimensional mechanical model(ThDM)of toppling failure was established, considering that the slab beam was mainly subject to self-weight, the frictional resistance of interlayer and lateral constraint force. Due to the progressive characteristics of toppling failure, the concept and the formula of the first fracture depth(FFD) of toppling was raised and constructed. The case study indicates that the ThDM is more effective and can be accurately used to calculate the toppling fracture depth of the slab beam. The FFD decreases proportionally with the increase of slab beam width. FFD grows fast when the slab beam width is less than 2.0 m and it tends to be stable when the slab beam width is above 2.0 m. The FFD decreases with the increase of the lateral constraint coefficient, indicating that the boundary condition of the free space is positively correlated with the stability and depth of toppling. This is a good explanation of the free space effect. This study provides a reference for the stability evaluation and prevention-control design of toppling slope in the future.

Stability of High Slope Interbedded Strata with Low Dip Angle Constituted by Soft and Hard Rock Mass

Slopes consisting of interbedded strata of soft and hard rock mass, such as purplish red mudstone and grey brown arkosic sandstone of Jurassic age, are very common in Sichuan basin of China. The mudstone is soft while the sandstone is hard and contains many opening or closing joints with a high dip angle. Some are nearly parallel and the others are nearly decussated with the trend of the slopes. Many natural slopes are in deformation or sliding because of those reasons. The stability of cutting slopes and supporting method to be taken for their stability in civil engineering are important. In this paper, the stability and deformation of the slopes are studied. The methods of analysis and support design principle are analyzed also. Finally, the method put forward is applied to study Fengdian high cutting slope in Sichuan section of the express way from Chengdu to Shanghai. The results indicate that the method is effective.

Sediment-Loading Processes in a Forested Catchment: Modeling and Observations

In order to investigate sediment-loading processes in a catchment, the daily time series of river discharge and sediment load were applied to a semi-distributed model, the Soil and Water Assessment Tool (SWAT). The time series of discharge and sediment load were obtained by monitoring the river stage and water turbidity of the Oikamanai River, Hokkaido, Japan, in the rainfall season (April-November) of 2011-2014. The catchment is forested (ca 90% area) but underlain by the Neogene sedimentary rocks with currently active faults and forest soils with tephra layers, which tend to frequently produce slope failure such as landslide and bank collapse by rainfall or snowmelt. The water turbidity, T, in ppm was converted into suspended sediment concentration, SSC, in g/L by applying the linear relationship between T and SSC. The acquisition of the time series of discharge, Q (m3/s) and sediment load, L (=Q·SSC in g/s) of the river allowed us to distinguish the fluvial sediment transport, accompanied by slope failure in the upstream, from that under no slope failure. The SWAT was used to simulate soil erosion and identify the region prone to the soil erosion in the Oikamanai River basin. The model’s results showed a satisfactory agreement between daily observed and simulated sediment load as indicated by the high Nash-Sutcliffe efficiency. This evidences that the upper mountainous region of the catchment provides a main sediment source, accompanied by slope failure.

Large-Scale Test Model of the Progressive Deformation and Failure of Cracked Soil Slopes

A large-scale test bed(LWH=6 m×3 m×2.8 m)instrumented with various sensors is used to examine the effects of rainfall infiltration and evaporation on the deformation and failure of cracked soil slopes,taking the Anhui area along the Yangtze River as a field example.The results indicate that(1)during rainfall,the soil around the shallow shrinkage fissures attains transient saturation,and the attendant decrease of matric suction is the primary cause of the shallow slope failure;(2)slope deformation continues during post-rainfall evaporation;(3)if a period of evaporation is followed by heavy rainfall,soil creep is concentrated near the deepest cracks,and two zones of steep gradients in pore pressure form at the crest and toe of the slope.Finally,a saturated zone forms near each crack base and gradually enlarges,eventually forming a continuous saturated layer that induces the slope instability or failure.

A Comparative Study of the Seismic Performances and Failure Mechanisms of Slopes Using Dynamic Centrifuge Modeling

In this study, dynamic centrifuge model tests were performed for sand slopes under different earthquake ground motions and slope angle to characterize the seismic performance of slopes. Four groups of tests under varying seismic input amplitude were conducted. Under the action of increasing earthquake intensity, the rigidity of the soil decreases and the damping ratio increases, both of the dynamic response and the predominant period of slopes are increased. Three types of seismic waves with the same seismic intensity were applied in the model tests. It shows that the variability in the ground motion leads to the acceleration response spectra of the slopes being completely different and the Northridge seismic wave with low-frequency component is closest to the predominant period of the slope model. In addition, the effect of slope angle on the seismic performance of slopes were also clarified. The results reveal how the slope angle affects the acceleration recorded on the ground surface of the slope, both in terms of the peak ground-motion acceleration(PGA) amplification factor and the predominant period. Finally, the permanent displacement of the model slopes under different earthquake intensities were further analyzed. Based on the nonlinear growth of the permanent displacement of the slope, the test results demonstrated the failure process of the slope, which can further provide a basis for theperformance-based seismic design of slopes.

Geometry and Maximum Width of a Stable Slope Considering the Arching Effect

The stability of an arching slope in deformable materials above strong rocks strongly depends on the shape and width of the span.Equations for a free surface problem that incorporate these two parameters were derived using a simplified two-dimensional arching slope model,and were validated using physical model tests under 1 g and centrifugal conditions.The results are used to estimate the maximum excavation width for a weak claystone slope in a lignite mine,for which we calculate a safety factor of 1.31.

Multi-wing butterfly effects on catastrophic rockslides

The catastrophic rockslide,which frequently triggers numerous severe disasters worldwide,has drawn much attention globally;however,understanding the initiation mechanism of catastrophic rockslides in the absence of typical single triggering factors related to strong seismic activity or torrential precipitation continues to be challenging within the global scientific community.This study aims to determine the mechanism of the three largest catastrophic rockslides in the eastern Tibetan Plateau,Yigong,Xinmo,and Baige,over the past 20 years using field investigation,remote sensing,and runoff analysis.Instead of the conventional driving factors of heavy rainfall and strong earthquakes,the multi-wing butterfly effects(MWBE)of climatic factors and weak earthquakes are for the first time identified as drivers of the catastrophic rockslide disasters.First,strong tectonic uplift,fast fluvial incision,high-density faults,and large regional water confluence formed the slopes in the critical regime,creating the source conditions of rockslide.Second,the MWBE of early dry-heat events and antecedent rainfall,combined with imminent weak earthquakes,initiated rockslide.Third,the delayed amplified runoff moving toward the sliding surface and lowering the strength of the locking-rock segment constituted the fundamental mechanism of the MWBE on rockslide.The catastrophic rockslide was ultimately inferred to be a nonlinear chaotic process;however,prediction and forecasting of rockslide based on the MWBE in the early stages are possible and essential.This finding presents a new perspective concerning forecasting progressive landslides.