1,720,977 research outputs found
Cyclic behaviour and liquefaction potential of silty sand: experimental and numerical investigations
Full text linkZugleich: Dissertation, Universität Kassel, Fachbereich Bau- und Umweltingenieurwesen, 202
PWP Tester - A simplified cyclic shear test for pore water pressure evolution in granular soils
No full textThe build-up of excess pore water pressure (PWP) usually takes place in loose, saturated, coarse-grained soils when subjected to external shear loading (monotonic or cyclic) under prevented water drainage. When the amount of the excess PWP reaches the value of the total stress, the grain structure collapses and the shear strength and stiffness of the soil disappear - the soil behaves as a liquid. Such soil phenomenon is commonly referred to as soil liquefaction. Experimental laboratory testing of the excess PWP build-up in coarse-grained soils is almost restricted to sophisticated undrained cyclic triaxial and simple shear tests.
A novel experimental method, the PWP Tester, enabling a simple and fast testing of the excess PWP accumulation in coarse-grained soils has been designed and presented in this thesis. The evolution of the excess PWP during cycling shearing of a cylindrical sand specimen in undrained conditions is measured during the test, and parameters characterising this evolution are determined. Besides an extensive repeatability study, the influence of several soil state variables, e.g. soil density, consolidation pressure, or loading amplitude on the accumulation of the excess PWP was tested. It was shown that the generation of the excess PWP decreased with rising density and consolidation stress, while increasing loading amplitude resulted in faster excess PWP build-up, which confirmed the plausibility of the experimental method. A comparison with the results of undrained cyclic triaxial tests in the case of eight different sands demonstrated a good agreement between both experimental methods, which further corroborated the suitability of the proposed test for a fast investigation of the excess PWP build-up in coarse-grained soils. Specimens prepared according to the proposed procedure in the PWP Tester are pluviated with a funnel under water. The extraction of the sand fabric from the µCT images acquired after the underwater deposition of different sands demonstrated a similar geometrical fabric for different grain size distibutions. Despite having a very different relative density after underwater pluviation, which ranged from a loose to a dense state, a similar resistance to the accumulation of the excess PWP was obtained for different sands. These results have indicated that the same soil fabric prescribed by the same specimen preparation method has at least the same or even stronger impact on the excess PWP accumulation than (different) relative densities. The influence of different sand fabrics on the excess PWP evolution was further tested in the case of a coarse and a fine sand that were prepared via several different procedures. A large impact of the sand different fabrics on the excess PWP build-up was identified for the coarse sand, while the tests on fine sand resulted in a negligible influence of different grain compositions on the evolution of the excess PWP.
In total, 42 different materials, including natural sands as well as mixtures of glass beads and crushed sand were tested with the PWP Tester. The results on these materials have shown no clear trend of the excess PWP evolution with the main soil physical properties, e.g. uniformity coefficient, mean grain diameter, and fines content. However, observing one soil property and keeping all other similar has shown that the resistance to the excess PWP accumulation rises with the mean grain diameter but reduces with an increase in the uniformity coefficient and fines content. The influence of different grain shapes and grain angularities was evaluated by performing tests on three sands with similar grain size distribution curves. Here, a sand consisting of elongated and angular grains with a rather rough surface, exhibited lower relative density when deposited under water and showed a larger resistance to the excess PWP generation with growing density than sand with a more round grains having smoother grain surface. Different grain descriptors of these sands were determined using the dynamic image analysis and digital microscopy. The same analysis was performed for natural sand and mixtures of glass beads and crushed sand with the same grain size distribution curves. Glass beads and crushed sand were both more resistant to the excess PWP build-up than natural sand. This was attributed to a stronger fabric in the case of glass beads and a high angularity (and thus grain interlocking) of the crushed sand grains.
Furthermore, an idealized application of the PWP Tester results for in situ conditions was presented, where the calculation of the excess PWP evolution in a sand layer under speci c density, stress and loading conditions was shown
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
PWP Tester - A simplified cyclic shear test for pore water pressure evolution in granular soils
Full text linkThe build-up of excess pore water pressure (PWP) usually takes place in loose, saturated, coarse-grained soils when subjected to external shear loading (monotonic or cyclic) under prevented water drainage. When the amount of the excess PWP reaches the value of the total stress, the grain structure collapses and the shear strength and stiffness of the soil disappear - the soil behaves as a liquid. Such soil phenomenon is commonly referred to as soil liquefaction. Experimental laboratory testing of the excess PWP build-up in coarse-grained soils is almost restricted to sophisticated undrained cyclic triaxial and simple shear tests.
A novel experimental method, the PWP Tester, enabling a simple and fast testing of the excess PWP accumulation in coarse-grained soils has been designed and presented in this thesis. The evolution of the excess PWP during cycling shearing of a cylindrical sand specimen in undrained conditions is measured during the test, and parameters characterising this evolution are determined. Besides an extensive repeatability study, the influence of several soil state variables, e.g. soil density, consolidation pressure, or loading amplitude on the accumulation of the excess PWP was tested. It was shown that the generation of the excess PWP decreased with rising density and consolidation stress, while increasing loading amplitude resulted in faster excess PWP build-up, which confirmed the plausibility of the experimental method. A comparison with the results of undrained cyclic triaxial tests in the case of eight different sands demonstrated a good agreement between both experimental methods, which further corroborated the suitability of the proposed test for a fast investigation of the excess PWP build-up in coarse-grained soils. Specimens prepared according to the proposed procedure in the PWP Tester are pluviated with a funnel under water. The extraction of the sand fabric from the µCT images acquired after the underwater deposition of different sands demonstrated a similar geometrical fabric for different grain size distibutions. Despite having a very different relative density after underwater pluviation, which ranged from a loose to a dense state, a similar resistance to the accumulation of the excess PWP was obtained for different sands. These results have indicated that the same soil fabric prescribed by the same specimen preparation method has at least the same or even stronger impact on the excess PWP accumulation than (different) relative densities. The influence of different sand fabrics on the excess PWP evolution was further tested in the case of a coarse and a fine sand that were prepared via several different procedures. A large impact of the sand different fabrics on the excess PWP build-up was identified for the coarse sand, while the tests on fine sand resulted in a negligible influence of different grain compositions on the evolution of the excess PWP.
In total, 42 different materials, including natural sands as well as mixtures of glass beads and crushed sand were tested with the PWP Tester. The results on these materials have shown no clear trend of the excess PWP evolution with the main soil physical properties, e.g. uniformity coefficient, mean grain diameter, and fines content. However, observing one soil property and keeping all other similar has shown that the resistance to the excess PWP accumulation rises with the mean grain diameter but reduces with an increase in the uniformity coefficient and fines content. The influence of different grain shapes and grain angularities was evaluated by performing tests on three sands with similar grain size distribution curves. Here, a sand consisting of elongated and angular grains with a rather rough surface, exhibited lower relative density when deposited under water and showed a larger resistance to the excess PWP generation with growing density than sand with a more round grains having smoother grain surface. Different grain descriptors of these sands were determined using the dynamic image analysis and digital microscopy. The same analysis was performed for natural sand and mixtures of glass beads and crushed sand with the same grain size distribution curves. Glass beads and crushed sand were both more resistant to the excess PWP build-up than natural sand. This was attributed to a stronger fabric in the case of glass beads and a high angularity (and thus grain interlocking) of the crushed sand grains.
Furthermore, an idealized application of the PWP Tester results for in situ conditions was presented, where the calculation of the excess PWP evolution in a sand layer under speci c density, stress and loading conditions was shown
PWP Tester - A simplified cyclic shear test for pore water pressure evolution in granular soils
No full textThe build-up of excess pore water pressure (PWP) usually takes place in loose, saturated, coarse-grained soils when subjected to external shear loading (monotonic or cyclic) under prevented water drainage. When the amount of the excess PWP reaches the value of the total stress, the grain structure collapses and the shear strength and stiffness of the soil disappear - the soil behaves as a liquid. Such soil phenomenon is commonly referred to as soil liquefaction. Experimental laboratory testing of the excess PWP build-up in coarse-grained soils is almost restricted to sophisticated undrained cyclic triaxial and simple shear tests.
A novel experimental method, the PWP Tester, enabling a simple and fast testing of the excess PWP accumulation in coarse-grained soils has been designed and presented in this thesis. The evolution of the excess PWP during cycling shearing of a cylindrical sand specimen in undrained conditions is measured during the test, and parameters characterising this evolution are determined. Besides an extensive repeatability study, the influence of several soil state variables, e.g. soil density, consolidation pressure, or loading amplitude on the accumulation of the excess PWP was tested. It was shown that the generation of the excess PWP decreased with rising density and consolidation stress, while increasing loading amplitude resulted in faster excess PWP build-up, which confirmed the plausibility of the experimental method. A comparison with the results of undrained cyclic triaxial tests in the case of eight different sands demonstrated a good agreement between both experimental methods, which further corroborated the suitability of the proposed test for a fast investigation of the excess PWP build-up in coarse-grained soils. Specimens prepared according to the proposed procedure in the PWP Tester are pluviated with a funnel under water. The extraction of the sand fabric from the µCT images acquired after the underwater deposition of different sands demonstrated a similar geometrical fabric for different grain size distibutions. Despite having a very different relative density after underwater pluviation, which ranged from a loose to a dense state, a similar resistance to the accumulation of the excess PWP was obtained for different sands. These results have indicated that the same soil fabric prescribed by the same specimen preparation method has at least the same or even stronger impact on the excess PWP accumulation than (different) relative densities. The influence of different sand fabrics on the excess PWP evolution was further tested in the case of a coarse and a fine sand that were prepared via several different procedures. A large impact of the sand different fabrics on the excess PWP build-up was identified for the coarse sand, while the tests on fine sand resulted in a negligible influence of different grain compositions on the evolution of the excess PWP.
In total, 42 different materials, including natural sands as well as mixtures of glass beads and crushed sand were tested with the PWP Tester. The results on these materials have shown no clear trend of the excess PWP evolution with the main soil physical properties, e.g. uniformity coefficient, mean grain diameter, and fines content. However, observing one soil property and keeping all other similar has shown that the resistance to the excess PWP accumulation rises with the mean grain diameter but reduces with an increase in the uniformity coefficient and fines content. The influence of different grain shapes and grain angularities was evaluated by performing tests on three sands with similar grain size distribution curves. Here, a sand consisting of elongated and angular grains with a rather rough surface, exhibited lower relative density when deposited under water and showed a larger resistance to the excess PWP generation with growing density than sand with a more round grains having smoother grain surface. Different grain descriptors of these sands were determined using the dynamic image analysis and digital microscopy. The same analysis was performed for natural sand and mixtures of glass beads and crushed sand with the same grain size distribution curves. Glass beads and crushed sand were both more resistant to the excess PWP build-up than natural sand. This was attributed to a stronger fabric in the case of glass beads and a high angularity (and thus grain interlocking) of the crushed sand grains.
Furthermore, an idealized application of the PWP Tester results for in situ conditions was presented, where the calculation of the excess PWP evolution in a sand layer under speci c density, stress and loading conditions was shown
Explizites Akkumulationsmodell für nichtbindige Böden unter zyklischer Belastung
No full textZyklische Belastungen führen im Boden zu einer Akkumulation der Dehnungen bzw. zu Spannungsänderungen. Die Akkumulation der Dehnung in nichtbindigen Böden unter hochzyklischer Belastung (d.h. bei einer großen Zyklenanzahl und mit kleinen Amplituden) wurde in Triaxialversuchen und Einfachscherversuchen untersucht. Variiert wurden die Spannweite, die Polarisation und die Form der Spannungsschleifen, der mittlere Spannungszustand, die Anfangsporenzahl, die statische Vorbelastung und die Kornverteilungskurve. Auf Basis der Versuchsergebnisse wurde ein explizites Akkumulationsmodell entwickelt, welche die Akkumulation im Boden analog zum Problem des Kriechens unter konstanten Lasten behandelt. Mit diesem Modell und der Finite Elemente Methode wurden Randwertprobleme mit Flachgründungen und Pfählen unter zyklischer Belastung gelöst
Variations on the Author
Full text link“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
Full text linkWe 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
Full text linkWe 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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