1,720,958 research outputs found
Centrifuge modelling of rainfall-induced landslides in unsaturated soil slopes
Get PDFLandslides and debris flows are geohazards common to countries with mountainous terrains. A significant number of rainfall-induced landslides occurs in regions with tropical and subtropical climates where residual and colluvial soils are widespread. This study is in the context of the Innovative Training Network project MUMOLADE (Multiscale Modelling of Landslides and Debris Flows) which aims to study the multi-scale and multi-phase analysis of landslides and debris flows. The research presented here aims to investigate the initiation conditions of landslides in soil slopes under varied rainfall conditions.
A physical modelling experimental investigation on the initiation of rainfall-induced landslides in unsaturated soil slopes was performed in this study using the geotechnical centrifuge at the University of Nottingham. To accomplish the objectives of the study, a testing apparatus was designed and built. The tests were conducted in a plane-strain centrifuge box with embedded systems for controlling rainfall and groundwater conditions. The container has a transparent Perspex window at one side which provided the ability to measure subsurface ground displacements using digital image analysis. Pore Pressure Transducers (PPTs) were used for measuring pore water pressures within the slope models during the tests.
Initially, the study focused on the behaviour of a fine, uniform sand. Classification tests of the soil were performed in the laboratory, including the Soil Water Retention Curve (SWRC) determined using the tensiometer method. Sand slope models were built at a high inclination angle, about 1.5 times higher than the soil’s angle of friction, with suction providing the necessary cohesive component for stability at 1g. The simulation of two failure mechanisms was attempted in this study. The first mechanism involves the loss of suction due to wetting in a soil slope of an angle higher than the angle of friction due to wetting. The second mechanism
involves the collapse of the saturated voids due to increased pore water pressure which reduces the effective stresses within the soil mass. The second mechanism was not able to be reproduced in the tests conducted in initially unsaturated soil slopes with a low GWT, since the models showed unexpected resistance to failure. On the other hand, soil slope models failed under a condition of an elevated (i.e. shallow) GWT. In order to study further the unsaturated soil conditions under increased gravity tests were performed in which drainage of the soil under these conditions was enabled and measurements of the retained water content and degree of saturation at different g-levels were made. Results
indicated that even medium-grained soils retain an amount of water content in their mass and that the retained water content at each g-level strongly depends on the grain size distribution of the soil. The implication of these tests is that soil slopes retain their stability when gravity increases even at extreme conditions, i.e. at an angle 50% higher than the angle of friction.
In order to overcome the scaling issue of the increased seepage velocity, slope models were built from a slightly finer soil which was scaled down by . These soil slopes were tested under similar boundary conditions at the sand slopes. The results indicated that the reduction of the permeability caused instability initiation under different combinations of rainfall intensity and duration. Slope centrifuge tests were simulated using limit equilibrium analysis in order to validate the experimental results. The analysis revealed the previous statement, that the elimination of suction was not sufficient to reduce the shear strength of the soil to a level that would cause instability in the slopes. Failure was attributed to the small increase of positive pore water pressures which were generated only to the slopes with reduced seepage velocity (i.e. in slopes made of ). Therefore, grain size scaling in soil slope models was found to provide the necessary conditions for the simulation of rainfall-induced landslides in centrifuge tests. Rainfall thresholds for the occurrence of rainfall-induced landslides were obtained and compared to existing data from the literature. The comparison showed that the centrifuge tests conducted in this study provided an over-estimated rainfall threshold
Centrifuge modelling of rainfall-induced landslides in unsaturated soil slopes
Get PDFLandslides and debris flows are geohazards common to countries with mountainous terrains. A significant number of rainfall-induced landslides occurs in regions with tropical and subtropical climates where residual and colluvial soils are widespread. This study is in the context of the Innovative Training Network project MUMOLADE (Multiscale Modelling of Landslides and Debris Flows) which aims to study the multi-scale and multi-phase analysis of landslides and debris flows. The research presented here aims to investigate the initiation conditions of landslides in soil slopes under varied rainfall conditions.
A physical modelling experimental investigation on the initiation of rainfall-induced landslides in unsaturated soil slopes was performed in this study using the geotechnical centrifuge at the University of Nottingham. To accomplish the objectives of the study, a testing apparatus was designed and built. The tests were conducted in a plane-strain centrifuge box with embedded systems for controlling rainfall and groundwater conditions. The container has a transparent Perspex window at one side which provided the ability to measure subsurface ground displacements using digital image analysis. Pore Pressure Transducers (PPTs) were used for measuring pore water pressures within the slope models during the tests.
Initially, the study focused on the behaviour of a fine, uniform sand. Classification tests of the soil were performed in the laboratory, including the Soil Water Retention Curve (SWRC) determined using the tensiometer method. Sand slope models were built at a high inclination angle, about 1.5 times higher than the soil’s angle of friction, with suction providing the necessary cohesive component for stability at 1g. The simulation of two failure mechanisms was attempted in this study. The first mechanism involves the loss of suction due to wetting in a soil slope of an angle higher than the angle of friction due to wetting. The second mechanism
involves the collapse of the saturated voids due to increased pore water pressure which reduces the effective stresses within the soil mass. The second mechanism was not able to be reproduced in the tests conducted in initially unsaturated soil slopes with a low GWT, since the models showed unexpected resistance to failure. On the other hand, soil slope models failed under a condition of an elevated (i.e. shallow) GWT. In order to study further the unsaturated soil conditions under increased gravity tests were performed in which drainage of the soil under these conditions was enabled and measurements of the retained water content and degree of saturation at different g-levels were made. Results
indicated that even medium-grained soils retain an amount of water content in their mass and that the retained water content at each g-level strongly depends on the grain size distribution of the soil. The implication of these tests is that soil slopes retain their stability when gravity increases even at extreme conditions, i.e. at an angle 50% higher than the angle of friction.
In order to overcome the scaling issue of the increased seepage velocity, slope models were built from a slightly finer soil which was scaled down by . These soil slopes were tested under similar boundary conditions at the sand slopes. The results indicated that the reduction of the permeability caused instability initiation under different combinations of rainfall intensity and duration. Slope centrifuge tests were simulated using limit equilibrium analysis in order to validate the experimental results. The analysis revealed the previous statement, that the elimination of suction was not sufficient to reduce the shear strength of the soil to a level that would cause instability in the slopes. Failure was attributed to the small increase of positive pore water pressures which were generated only to the slopes with reduced seepage velocity (i.e. in slopes made of ). Therefore, grain size scaling in soil slope models was found to provide the necessary conditions for the simulation of rainfall-induced landslides in centrifuge tests. Rainfall thresholds for the occurrence of rainfall-induced landslides were obtained and compared to existing data from the literature. The comparison showed that the centrifuge tests conducted in this study provided an over-estimated rainfall threshold
The combined effect of clay and moisture content on very small strain stiffness of compacted sand-clay mixture
Get PDFThe very small strain shear modulus (stiffness) of soils, Gmax, is one of the most important parameters for predicting ground movements and dynamic responses of geo-structures. In this study, the combined effect of clay fraction and moisture content on shear stiffness of an unsaturated sand-clay mixture at very small strains was investigated using bender elements. Compacted soil specimens were prepared at three different clay contents of 10, 20, and 30%, and at four different initial moisture contents of 3, 6, 9 and 12%. Bender element tests were carried out under isotropic and constant moisture content conditions and inside a modified triaxial testing system equipped with a pair of piezoelectric bender-extender elements. Gmax was calculated based on the velocity measurement of shear waves propagated through the specimen. The tests results showed that Gmax decreases approximately linearly with an increase in moisture content, and non-linearly with an increase in clay content. A basic empirical equation was derived from an examination of trends in evolution of Gmax with clay and moisture content. Additional empirical correlations were also derived for estimation of moisture content and degree of saturation based on the compression wave velocity measurements
The combined effect of clay and moisture content on very small strain stiffness of compacted sand-clay mixture
Get PDFThe very small strain shear modulus (stiffness) of soils, Gmax, is one of the most important parameters for predicting ground movements and dynamic responses of geo-structures. In this study, the combined effect of clay fraction and moisture content on shear stiffness of an unsaturated sand-clay mixture at very small strains was investigated using bender elements. Compacted soil specimens were prepared at three different clay contents of 10, 20, and 30%, and at four different initial moisture contents of 3, 6, 9 and 12%. Bender element tests were carried out under isotropic and constant moisture content conditions and inside a modified triaxial testing system equipped with a pair of piezoelectric bender-extender elements. Gmax was calculated based on the velocity measurement of shear waves propagated through the specimen. The tests results showed that Gmax decreases approximately linearly with an increase in moisture content, and non-linearly with an increase in clay content. A basic empirical equation was derived from an examination of trends in evolution of Gmax with clay and moisture content. Additional empirical correlations were also derived for estimation of moisture content and degree of saturation based on the compression wave velocity measurements
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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