The topic of rainfall thresholds for landslide occurrence was thoroughly investigated, producing abundance of case studies at different scales of analysis and several technical and scientific advances. We reviewed the most recent papers published in scientific journals, highlighting significant advances and critical issues. We collected and grouped all the information on rainfall thresholds into four categories: publication details, geographical distribution and uses, dataset features, threshold definition. In each category, we selected descriptive information to characterize each one of the 115 rainfall threshold published in the last 9 years. The main improvements that stood out from the review are the definition of standard procedures for the identification of rainfall events and for the objective definition of the thresholds. Numerous advances were achieved in the cataloguing of landslides too, which can be defined as one of the most important variables, together with rainfall data, for drawing reliable thresholds. Another focal point of the reviewed articles was the increased definition of thresholds with different exceedance probabilities to be employed for the definition of warning levels in landslide early warning systems. Nevertheless, drawbacks and criticisms can be identified in most part of the recent literature on rainfall thresholds. The main issues concern the validation process, which is seldom carried out, and the very frequent lack of explanations for the rain gauge selection procedure. The paper may be used as a guide to find adequate literature on the most used or the most advanced approaches followed in every step of the procedure for defining reliable rainfall thresholds. Therefore, it constitutes a guideline for future studies and applications, in particular in early warning systems. The paper also aims at addressing the gaps that need to be filled to further enhance the quality of the research products in this field. The contribution of this manuscript could be seen not only as a review of the state of the art, but also an effective method to disseminate the best practices among scientists and stakeholders involved in landslide hazard management.

A review of the recent literature on rainfall thresholds for landslide occurrence

American Society for Testing and Materials

Loess geohazards research in China: Advances and challenges for mega engineering projects

Preface Chapter 1 What is debris flow? Various sediment moving phenomena Definition of debris flow Classification and characteristics of debris flows Stony type debris flow Turbulent muddy type debris flow Viscous debris flow The significance of the mechanical classification of debris flows Classifications on the other points of view Chapter 2 Models for mechanics of flow 2.1 Models for solids and fluid mixture as the multi-phase flow 2.2 Single-phase continuum models 2.2.1 Visco-plastic fluid model 2.2.2 Dilatant fluid model 2.3 Two-phase continuum models (Mixture theory) 2.3.1 Stress equilibrium equations 2.3.2 Coulomb mixture theory (Quasi-static debris flows) 2.4 Theory for subaerial rapid granular flows 2.4.1 Particle collision stress 2.4.2 Kinetic stress 2.4.3 Skeletal stresses 2.4.4 Constitutive relations 2.4.5 Application of the theory to dry granular flow 2.4.6 Comparison with other constitutive relations for inertial range 2.5 Role of interstitial fluid in inertial debris flows 2.6 The mechanism of immature and turbulent muddy debris flows 2.6.1 Immature debris flow 2.6.2 Turbulent muddy debris flow 2.7 Generalized theory for inertial debris flows 2.7.1 Theoretical considerations 2.7.2 Verification by experimental data 2.7.3 Approximate solution for solids concentration and resistance to flow 2.8 Newtonian fluid model for viscous debris flow 2.8.1 Theoretical considerations 2.8.2 Verification by experiments Chapter 3 Initiation and development of debris flow Initiation and development of debris flow due to gully bed erosion The formation of incipient debris flow by the effects of surface water runoff The development of stony debris flow on sediment bed Verification of the theory by experiments Landslide-induced debris flow Model for the transformation into debris flow Mathematical model for the one-dimensional motion of an earth block Numerical simulation of earth block and debris flow motions across three-dimensional terrain Debris flow and flood flow induced by collapse of natural dam Formative conditions and shapes of natural dam Failure in entire channel width and the resulting debris flow Prediction of debris flow/flood flow induced by the overflow in partial width Chapter 4 Characteristics of fully developed flow Translation of debris flow surge and the shape of the snout The case of stony type debris flow The case of viscous type debris flow Boulder accumulation at the forefront of stony debris flow Various concepts for the mechanism The theory of Takahashi (1980) Competence to transport large boulders The cause of intermittency Debris flow around bend Routing of debris flow in the transferring reach Kinematic wave method Dynamic wave method Chapter 5 Processes and geomorphology of deposition One-dimensional stoppage/depositing processes of stony debris flow The arrival distance at the abrupt change in channel slope Topography of deposit formed at an abrupt slope change Numerical simulation of depositing process One-dimensional depositing process of turbulent muddy debris flow Formation of debris flow fan Description of the experimental results for stony debris flow and empirical presentations of the feature of debris flow fan Numerical simulation of fan formation process and its verification Numerical simulation of fan formation by turbulent debris flow Particle size distribution in the fan formed by stony debris flow General situations found in the field and experimental data Mathematical model for particle size distributions Erosion and deformation of debris flow fan Experiments for the process of erosion Model and its verification for the fan comprised of uniform material Model and its verification for the fan comprised of heterogeneous material Chapter 6 Debris flow disasters and their reproduction by computer simulations The rain storm disasters at Okuetsu Outline of the disaster The natural dam formation and the damages done by backwater Processes of destruction of the natural dam and damages downstream Horadani debris flow disasters Outline of the disaster Hydrograph estimation of the debris flow Reproduction of debris flow depositing area on the fan Collapse of the tailings dam at Stava, Northern Italy Outline of the disasters Reproduction of the debris flow in the Stava River and its verification Disasters caused by the eruption of Nevado del Ruiz Volcano Outline of the disasters Reproduction of the phenomena Sediment disasters in Venezuela Outline of the disasters Debris flow routing under an arbitrary rainfall condition Reproduction of debris flow hydrograph and others in the Camuri Grande River Reproduction of sediment flooding on the Camuri Grande fan Debris flow disasters at Atsumari, Hougawachi, Minamata City Outline of the disasters Reproduction of the processes of debris flow Chapter 7 Countermeasures for debris flow disasters Methods to prevent from debris flow generation Hillside works Drainage works Groundsill and bed girdle Debris flow control by closed type check dam Sediment depositing process behind check (sabo) dam Erosion process of the deposit behind sabo dam Effects of sediment control by sabo dam to downstream Debris flow control by open-type sabo dams Kinds and sediment checking mechanism of open-type sabo dams Blocking model of grid-type dam Model for debris flow controlling by grid-type dabo dam Determination of the optimum spacing and the optimum position to install Making debris flow harmless by channel works and training walls Design of countermeasures on the fan of the Camuri Grande River Management of debris flow by training dike Design debris flows for the countermeasure planning Method based on the previous data Prediction of total sediment runoff by field investigation Theoretical prediction of debris flow scale Debris flow prone ravines and hazardous area Debris flow prone ravine Hazardous zone by debris flow Prediction of debris flow occurrence by rainfall References Notations Index

Debris flow : mechanics, prediction and countermeasures

Scientific challenges in disaster risk reduction for the Sichuan–Tibet Railway

It is widely acknowledged that the mechanical behavior of a soil is significantly influenced by the soil microstructure; and the microstructure can evolve as a result of any mechanical, hydraulic, chemical or thermal change taken place on soil sample. The present study aimed to investigate the microstructures of intact loess and remolded loess and to explore the evolution of the microstructure and PSD (pore-size distribution) due to consolidation for both intact and remolded loess. A loess from the Loess Plateau of China was used as test material. A series of intact loess specimens were consolidated to various vertical pressures in oedometer cells. The same loess was remolded at the optimum water content state to various compaction degrees using static compaction method. It is equivalent to consolidate the remolded loess with the least compaction degree under various stress levels. The microstructures of intact loess specimens after consolidation tests and remolded loess specimens are characterized using the SEM (scanning electron microscope) and MIP (mercury intrusion porosimetry) techniques. The micrographs and PSDs under various consolidation pressures (or compaction energies) were compared to investigate the evolution of the microstructure and PSD induced by mechanical loading for each kind of soil. The microstructure of intact loess is more homogeneous and is less dependent on consolidation stress than remolded loess. In both soils, the pores with entrance diameter smaller than 6 μm are almost not influenced by consolidation. In intact loess, inter-aggregate pores with entrance diameter greater than 6 μm are compressed randomly due to loading. However, in remolded loess, the pores are compressed until all larger pores have been compressed. The cumulative pore volume curve of remolded loess of any compaction degree can be divided into two segments, large-pore segment (6–50 μm) and small-pore segment (< 6 μm). The large-pore segment is simplified into a horizontal line and a straight line which slope is independent on the compactness. Remolded loess has very different microstructure and PSD from intact loess even though they may have the same GSD (grain-size distribution), mineralogical composition, and some other physical properties. The microstructural evolution induced by mechanical loading also varies in both kinds of soils. Based upon the measured PSDs, a method is proposed for predicting the PSD of remolded loess of any compaction degree using a reference PSD.

Evolution of pore-size distribution of intact loess and remolded loess due to consolidation

Secondary succession has great impact on plant and soil characteristics, however, the trends of microbial patterns and the influencing factors during grassland succession without human disturbance remains unclear. Therefore, we investigated the changes of bacterial community in sloped farmlands abandoned for 0, 20, 30, and 40 years (GL-0 yr, GL-20 yr, GL-30 yr, and GL-40 yr). Additionally, plant traits (coverage, diversity, richness, evenness, biomass, and biomass carbon) and soil nutrients were also determined. The results showed that soil bacterial alpha diversity was positively and significantly correlated with the succession time, and the secondary succession greatly affected soil bacterial beta diversity, in contrast, the effects on soil bacterial beta diversity at the late succession time (GL40 and GL30) were larger than that at the early succession time (GL20). For the bacterial taxa, the dominant phyla including Actinobacteria (34.8%), Proteobacteria (26.0%), Acidobacteria (15.0%), Chloroflexi (7.5%), Gemmatimonadetes (8.7%), Nitrospirae (1.6%), Bacteroidetes (2.1%), Verrucomicrobia (1.1%), and Planctomycetes (1.0%) were found. Particularly, the relative abundance of Proteobacteria was higher at the late time (RP40), while the Actinobacteria was higher at the early time (RP20). Such different responses of bacterial diversity and taxa were largely explained by plant traits and soil nutrients, especially for TOC and TN. Collectively, our results indicate that plant secondary succession shifts the bacterial community structure, largely driven by changes in soil nutrients and plant diversity and composition, and also supported the growing view that soil bacterial community are the key determinants of aboveground and belowground linkages that functionally control terrestrial ecosystems.

Change in soil bacterial community during secondary succession depend on plant and soil characteristics

It is well known that the hydromechanical behavior of both saturated and unsaturated loess soils is significantly influenced by the soil fabric. However, there is limited understanding of how the soil fabric or structure evolves due to mechanical, hydraulic and chemical changes on loess soils. Information of the microstructural evolution or change in pore-size distribution (i.e., PSD) of loess soils along different stress paths is valuable for proposing an advanced constitutive model that considers the microstructure and can better model the hydromechanical behavior of loess soils. For this reason, in the present study, the microstructure is characterized on intact and saturated loess specimens before and after oedometer consolidation tests, using scanning electron microscopy and mercury intrusion porosimetry methods. The results suggest that the loading-induced change in PSD varies with stress level and saturation state of the loess soil. A reduction arises in the cumulative intrusion void ratio due to an increase in vertical stress, which accounts for compression of inter-aggregate pores greater than 6 μm. However, loading saturated loess leads to transformation from a bimodal PSD into a trimodal one that defines three major pore series, namely large-pore series (i.e., more than 6 μm), medium-pore series (i.e., between 0.1 and 6 μm) and small-pore series (i.e., less than 0.1 μm). The trimodal nature of PSD is, however, destructed under higher vertical stresses. Both large pores and medium pores are compressed under higher vertical stresses (i.e., > 600 kPa). The inundating-induced change in PSD is dependent on loading condition and can be discerned to take place in the same three pore series. Not only large pores but also medium pores collapse upon inundating under higher vertical stresses (i.e., > 600 kPa). The microstructural evolution is consistent with the mechanical responses of both intact and saturated loess.

Change in pore-size distribution of collapsible loess due to loading and inundating

Surface multi-hazard effect of underground coal mining

Understanding the creeping behavior of loess is of great importance as large-scale loess landslides are closely related with creep behavior. At present, it is still challenging to predict and estimate the long-term stability of such landslides. This is in a large degree due to the poor understanding of moisture control on creep behavior of loess. The purpose of this study is to decipher the loess creep behavior under various moisture contents (MCs) using loess specimens obtained from Baqiao landslide, Xi’an of China, using multi-loading triaxial creep tests under different MCs of 9%, 12%, 15%, 18% and 21%. Based on the laboratory test results, a series of relationships between the creep rate at the steady-state creep stage and the initial strain and initial shear modulus are revealed. Meanwhile, a method for obtaining the long-term strength of loess specimens, namely, the Steady-state Creep Rate Slope Method (SCRSM), is proposed. SCRSM resolves the issue in several conventional methods such as the Isochronous Stress-Strain Curve Method, the Tangent Method of Steady Creep Rate when MCs are of concern. Such an improvement is primarily due to a better way of finding the inflection point of the steady-state rate. It is found that SCRSM is robust and accurate to determine the long-term strength of loess specimens. Furthermore, we propose a modified Burgers model with a newly introduced nonlinear parameter n to overcome deficiencies of conventional creep models. This modified Burgers model is flexible to fit the creep test curves of loess, and can describe the curves at the accelerated creep stage more accurately. Lastly, the main factors triggering the Baqiao landslide is analyzed considering stratum lithology, rainfall and excavation. In general, this study provides a basis for understanding the evolutional process of loess landslides as well as guidelines for prevention, controlling and prediction of loess landslides.

Moisture content effect on the creep behavior of loess for the catastrophic Baqiao landslide

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