Modelling landslides in unsaturated slopes subjected to rainfall infiltration using material point method

This paper presents a dynamic fully coupled formulation for saturated and unsaturated soils that undergo large deformations based on material point method. Governing equations are applied to porous material while considering it as a continuum in which the pores of the solid skeleton are filled with water and air. The accuracy of the developed method is tested with available experimental and numerical results. The developed method has been applied to investigate the failure and post‐failure behaviour of rapid landslides in unsaturated slopes subjected to rainfall infiltration using two different bedrock geometries that lie below the top soil. The models show different failure and post‐failure mechanisms depending on the bedrock geometry and highlight the negative effects of continuous rain infiltrations. Copyright © 2016 John Wiley & Sons, Ltd.     

Identification of shear strength and seismic coefficient by back analyzing surficial slides in the 2004 Mid-Niigata prefecture earthquake

Sliding of natural and artificial slopes generally occurs during or following strong earthquakes. Such sliding is greatly affected by a combination of geological conditions and earthquake loading. Earthquake-induced landslides often cause more damage to infrastructure and human lives than the earthquake itself. Pseudo-static analysis is widely implemented as one of several design methods used in engineering practice to assess the seismic stability of natural and artificial slopes. However, the most important issue of pseudo-static analysis is to select the most appropriate method for measuring seismic coefficient. In order to investigate this, back analysis was conducted for surficial slides subjected to strong ground motion during the 2004 Mid-Niigata prefecture earthquake in Japan. This paper surveyed the stochastic properties of earthquake-induced surficial slides and clearly showed that the obtained results were applicable to back analysis of shear strength and seismic coefficient. In back analysis, soil properties such as soil strength and density and sliding depth were assumed as random variables owing to their uncertainties. Seismic coefficient is also assumed to be a random variable and varies with distance from the epicenter fault line. The analysis of 4504 recorded surficial slides clearly shows a unique relationship of landslide occurrence ratio with slope angle and distance from the epicenter fault line. This study’s results enhance the calculation of the shear strength of weathered soil covering slopes and the horizontal seismic coefficient through back analysis procedure. By considering possible stochastic properties of variables, some case studies were implemented in the back analysis.     

Heavy metal abundances in the Kandy lake—An environmental case study from Sri Lanka

The Kandy lake, situated in the heart of Sri Lanka's second largest city with a population of nearly 120,000, has been monitored to probe the extent of heavy metal pollution. Although the lake is a source of drinking water to the city, a large number of effluent canals drain into the lake carrying a continuous flow of industrial and domestic waste matter. A total of 66 surface water samples were analyzed for their Fe²⁺, total Fe, total V, SO ₄ ²⁻ , Cd²⁺, and Pb²⁺ contents. Pb and Cd were found in high concentrations averaging 150 μg/l and 77 μg/l, respectively, and exhibit a marked positive correlation with each other (r=+0.94). Vehicular emissions and industrial waste matter contribute largely to the Pb and Cd contents of the lake, the anthropogenic influence outweighing the contributions made by geological materials. All field observations and laboratory experiments indicate a tendency of the Kandy Lake towards eutrophicity.     

Drainage density and effective precipitation

Since the work of Horton in 1945, several geomorphologists have made attemps to explain the factors controlling the regional variations of drainage density. One of the most important contributions to this endeavour was made by Melton (1957) who demonstrated that drainage density has a significantly high negative correlation with Thornthwaite's P/E index. Melton linterpreted this as a result which shows the efficacy of the natural vegetative cover in controlling erosional processes.Recently the author, in a study of the morphometry of dissection in the central hills of Sri Lanka, found that the relationship between drainage density and effective precipitation in this area is significantly positive. This result when considered in combination with Melton's finding could be interpreted as an indication of the fact that above a certain critical level of effective precipitation (i.e. 80–90) the relationship between drainage density and P/E index turns positive. Such a conclusion is in agreement with the results of recent work by Langbein and Schumm (1958) and Hadley and Schumm (1961), who demonstrated that sediment yield reaches a maximum under grassland conditions and possibly reaches another peak where the impeding effect of vegetation cannot be increased by further increase of precipitation.     

Spatial distribution of landslides induced by the 2004 Mid-Niigata prefecture earthquake,Japan

On October 23, 2004, an earthquake with a moment magnitude of 6.8 occurred in the Chuetsu area of Niigata prefecture in Japan. This earthquake is known as the 2004 Mid-Niigata prefecture earthquake; the event was followed by severe aftershocks and caused many types of landslides such as surficial slides, shallow slides, and deep slides. A large number of landslides occurred in the upland village of Yamakoshi, destroying the entire village; in addition, a huge number of houses collapsed in Kawaguchi town. This study investigates the correlations between each type of landslide and the bedding plane orientation and dip, and other geomorphologic conditions. The landslide occurrence ratio (LOR) is used as an index to determine the correlation between the 2004 Mid-Niigata prefecture earthquake-induced landslides and the slope angle, slope aspect, rock type, and bedding plane orientation and dip. This work also proposes a methodology to determine the geometric alignment between the topography and the orientation of geological bedding planes. The method provides an efficient means of estimating the topography/bedding plane relationship over large areas.     

Crushing and embedment of proppant packs under cyclic loading:An insigh to enhanced unconventional oil/gas recovery

Crushing and embedment are two critical downhole proppant degradation mechanisms that lead to a significant drop in production outputs in unconventional oil/gas stimulation projects.These persistent production drops due to the non-linear responses of proppants under reservoir conditions put the future utilization of such advanced stimulation techniques in unconventional energy extraction in doubt.The aim of this study is to address these issues by conducting a comprehensive experimental approach.According to the results,whatever the type of proppant,all proppant packs tend to undergo significant plastic deformation under the first loading cycle.Moreover,the utilization of ceramic proppants(which retain proppant pack porosity up to 75％),larger proppant sizes(which retain proppant pack porosity up to 15.2％)and higher proppant concentrations(which retain proppant pack porosity up to 29.5％)in the fracturing stimulations with higher in-situ stresses are recommended to de-escalate the critical consequences of crushing associated issues.Similarly,the selection of resin-coated proppants over ceramic and sand proppants may benefit in terms of obtaining reduced proppant embedment.In addition,selection of smaller proppant sizes and higher proppant concentrations are suggested for stimulation projects at depth with sedimentary formations and lower in-situ stresses where proppant embedment pre-dominates.Furthermore,correlation between proppant embedment with repetitive loading cycles was studied.Importantly,microstructural analysis of the proppant-embedded siltstone rock samples revealed that the initiation of secondary induced fractures.Finally,the findings of this study can greatly contribute to accurately select optimum proppant properties(proppant type,size and concentration)depending on the oil/gas reservoir char-acteristics to minimize proppant crushing and embedment effects.     

Proppant Crushing Mechanisms Under Reservoir Conditions: Insights into Long-Term Integrity of Unconventional Energy Production

Proppant crushing, a major proppant failure mechanism, occurs in geothermal energy and oil/gas stimulation production stages when the level of net stress exerted on the proppant exceeds the actual crush resistance of the material. Loss of effective reservoir conductivity due to proppant crushing can result in significant loss in productivity, and so it is crucial to understand the realistic proppant mechanical performances under deep reservoir conditions. This review provides a comprehensive overview of proppant crushing at the micro- to macro-levels by analyzing single proppant breakage, as well as re-arrangement and breakage mechanisms of proppant packs under in situ fracture environments. The choice of an appropriate proppant type based on the fracture treatment plays a key role in effective geothermal and oil/gas recovery. In addition, injection of proppants with better characteristics (higher sphericity, lower size, better gradation and lower granular porosity) can significantly influence the reduction of the extent of proppant crushing. Moreover, this study compares the performances and responses of different types of proppants upon proppant interaction with geothermal and oil/gas reservoir environments. Furthermore, this paper discusses various proppant types and their enhanced characteristics, which can be utilized as controlling measures for proppant crushing during unconventional energy extraction.

     

Grain-scale analysis of proppant crushing and embedment using calibrated discrete element models

Acta Geotechnica - Proppant crushing and embedment in hydraulically-induced fractures is a major drawback to the recovery of unconventional oil/gas and geothermal energy production. This study...