1,720,994 research outputs found
A microstructure-based elastoplastic model to describe the behaviour of a compacted clayey silt in isotropic and triaxial compression
Full text linkThe paper focuses on the hydro-mechanical behaviour of an unsaturated compacted clayey silt, accounting for fabric changes induced by drying-wetting cycles occurring at low-stress levels. The response along isotropic compression and triaxial compression (shear) at constant water content was investigated by laboratory tests on both as compacted and dried-wetted samples. Compaction induces a micro-structural porosity pertinent to clay peds and a macro-structural porosity external to the peds. Drying-wetting cycles decrease the micro-porosity and increase the macro-porosity, which reduces the water retention capacity, increases the compressibility and promotes higher peak strengths with more brittle behaviour during triaxial compression. A coupled double porosity elastic-plastic model was formulated to simulate the experimental results. A non-associated flow rule was defined for the macrostructure, modifying a stress-dilatancy relationship for saturated granular soils to account for the increase in dilatancy with suction observed in the experiments. The average skeleton stress and suction were adopted as stress variables. Consistently with model predictions, the shear strength at critical state is not significantly influenced by the degree of saturation or by the hydraulic history. On the other contrary, the higher peak strength, brittleness and dilatancy of the dried wetted samples are mostly explained by their reduced water retention capacity
Evaluating the capability of a critical state constitutive model to predict the collapse potential of loose sand
Full text linkMany catastrophic flow failures in granular soil slopes are believed to be caused by a rise in pore water pressure associated with substantial loss of soil shear strength. This failure mechanism is known as prefailure instability or static liquefaction. Constant shear (CS) and consolidated undrained (CU) triaxial tests can reproduce stress paths, in which such instability may occur before reaching the failure. In the present study, a previously proposed critical state constitutive model was first used to simulate the behavior of loose saturated sand in CU tests. It was then employed to predict the instability of loose sand subjected to the CS loading. Under such loading, loose dry sand initially experience small volume increase, and then start to contract substantially. In saturated sand, such contractions can lead to the generation of pore water pressure and sudden decrease of shear strength. The capability of the model to predict the onset of the volume contraction and collapse potential of loose dry sand was examined by comparing the model predictions with experimental results of CS tests. The comparison showed that the effect of initial void ratio, consolidation and deviatoric stresses on behavior of loose dry sand can be well predicted by the mode
Resistenza a liquefazione statica di rifiuti minerari in condizioni non sature
No full textLa nota presenta i risultati di una campagna di prove sperimentali realizzata su materiali di risulta di coltivazioni minerarie (tailings) con l’obiettivo di analizzarne il comportamento meccanico in condizioni non sature o prossime alla saturazione. Un’approfondita caratterizzazione geotecnica e la conoscenza del comportamento meccanico di tali materiali costituiscono elementi fondamentali nelle analisi di stabilità nel breve e lungo termine dei bacini di decantazione, in particolare con riferimento al fenomeno della liquefazione statica che risulta essere una delle maggiori cause di collasso dei bacini stessi. Le prove sono state effettuate sulla frazione limosa dei tailings fuoriusciti in seguito alla rottura dei bacini di decantazione di Stava (Italia) nel luglio 1985. Il comportamento meccanico di tali materiali è stato indagato presso il Laboratorio Geotecnico del Politecnico di Torino (Italia), principalmente in termini di curve sforzo - deformazione, sottoponendo i tailings a prove triassiali con fase di taglio eseguita in condizioni non drenate. Al fine di valutare alcuni fattori che si ritiene possano influenzare la risposta meccanica del materiale, la sperimentazione è stata effettuata su campioni preparati a differenti gradi di saturazione e densità iniziali
Effects of repeated hydraulic loads on microstructure and hydraulic behaviour of a compacted clayey silt
Full text linkSoils used in earth constructions are mostly unsaturated, and they undergo frequent drying-wetting cycles (repeated hydraulic loads) due to changes in climatic conditions or variations of the ground water level, particularly at shallow depths. After compaction, changes in water content can significantly influence the hydromechanical response of the construction material, which therefore has to be assessed for repeated hydraulic loads. This research investigates the effect of such loads on the microstructure and hydraulic behaviour of a silty soil, typically used in the construction of embankments and dykes, with the aim of providing a better understanding of the consequences of drying-wetting cycles on the response of the material over time. Experimental tests were performed to study the impact of drying-wetting cycles on the water retention, hydraulic conductivity and fabric of compacted specimens. Fabric changes are documented to take place even without significant volumetric strains, promoting an irreversible increase in the hydraulic conductivity and a reduction in the capacity to retain water compared to the as-compacted soil. The fabric changes are interpreted and quantified by means of a hydromechanical model, which accounts for the evolving pore size distribution at different structural levels. The proposed model reproduces quite well the microstructural observations, together with the evolution of the water retention behaviour and of the hydraulic conductivit
A water retention model accounting for the hysteresis induced by hydraulic and mechanical wetting-drying cycles
Full text linkA comprehensive description of the water retention behaviour of unsaturated soils requires accounting for the hysteresis caused by hydraulic and mechanical wetting-drying cycles. A hysteretic water retention model is proposed by introducing the liquid-solid contact angle to account for the dependency of the response on non-monotonic changes in suction and void ratio. The proposed model reproduces main drying and wetting surfaces and also nonlinear scanning curves during hydraulic or mechanical loading. Experimental tests and numerical simulations were carried out to study the water retention behaviour of a clayey silt. The model simulations captured the experimental results well
Release of metal ions from round and rectangular NiTi wires
Full text linkAbstract Background The aim of this study was to evaluate the amount of nickel and titanium ions released from two wires with different shapes and a similar surface area. Methods Forty round nickel-titanium (NiTi) arch wires with the diameter of 0.020 in. and 40 rectangular NiTi arch wires with the diameter of 0.016 × 0.016 in. were immersed in artificial saliva during a 21-day period. The surface area of both wires was 0.44 in.2. Wires were separately dipped into polypropylene tubes containing 50 ml of buffer solution and were incubated and maintained at 37 °C. Inductively coupled plasma atomic emission spectrometry (ICP-AES) was used to measure the amount of ions released after exposure lengths of 1 h, 24 h, 1 week, and 3 weeks. Repeated measures ANOVA and Tukey tests were used to evaluate the data. Results The results indicated that the amount of nickel and titanium concentrations was significantly higher in the rectangular wire group. The most significant release of all metals was measured after the first hour of immersion. In the rectangular wire group, 243 ± 4.2 ng/ml of nickel was released after 1 h, while 221.4 ± 1.7 ng/ml of nickel was released in the round wire group. Similarly, 243.3 ± 2.8 ng/ml of titanium was released in the rectangular wire group and a significantly lower amount of 211.9 ± 2.3 ng/ml of titanium was released in the round wire group. Conclusions Release of metal ions was influenced by the shape of the wire and increase of time
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
Apical surgery vs apical surgery with simultaneous orthograde retreatment: A prospective cohort clinical study of teeth affected by persistent periapical lesion
Full text linkAim: This prospective clinical study analyzed the 24-month outcome of conventional apical surgery retro-filled with calcium-silicate cement versus apical surgery with simultaneous orthograde retreatment by means of clinical and radiographic criteria. Materials and methods: This study included 83 teeth affected by persistent periapical lesions in 68 patients. Mean age was 52 years (median = 51 years; range 19–81 years). Twenty-eight cases were treated with apical surgery, 16 cases with apical surgery with simultaneous orthograde retreatment and 39 cases with orthograde retreatment in previously treated teeth established as control group. Periapical index score (PAI) was used as radiographic criteria. Teeth were examined at 6 months, 1 and 2 years and classified as healed (without any symptoms and PAI ≤ 2), healing (without any symptoms and PAI = 3) or diseased (with symptoms or PAI ≥ 4 and not functional) on the basis of radiographic and clinical criteria. At 24 months evaluation, healed and healing were considered as success and diseased and fracture as failure. Multilevel GLM model and an ordered logistic regression as statistical analysis was made with level of significance set at p < 0.05. Results: Total drop-out was 7% (n = 6). After 6–9 months, 6 teeth (3 from apical surgery, 2 from simultaneous treatment and 1 from orthograde retreatment) were extracted for root fracture. Twenty-four-month success rate of apical surgery group was 78% (n = 17), apical surgery with simultaneous orthograde retreatment presented 81% (n = 10) and orthograde retreatment success was 80% (n = 24). There was no statistically difference between the groups at 24 months (p = 0.890). Conclusions: Both surgical techniques revealed a high percentage of healing, similar to that reported by previous studies. Apical surgery with simultaneous orthograde retreatment showed a faster healing after 12 months comparing to the control group. Riassunto: Scopo: In questo studio sono stati confrontati pazienti trattati con chirurgia apicale convenzionale vs chirurgia apicale con ritrattamento ortogrado simultaneo. Come gruppo controllo, sono stati considerati pazienti sottoposti a ritrattamento ortogrado. Materiali e metodi: Questo studio ha incluso 83 elementi con lesioni periapicali persistente (68 pazienti). Ventotto casi di chirurgia apicale, 16 casi di chirurgia apicale con ritrattamento ortogrado simultaneo e 39 casi di ritrattamento ortogrado non chirurgico come gruppo controllo. L’indice periapicale (PAI) è stato utilizzato come criterio radiografico. Gli elementi sono stati esaminati ogni 6 mesi per 24 mesi e classificati in base a criteri clinici e radiografici come guarito (assenza di sintomi e PAI≤2), in guarigione (assenza di sintomi e PAI = 3) e non guarito (presenza di sintomi o PAI≥4). A 24 mesi gli elementi guariti e in guarigione sono stati classificati come successo clinico mentre gli elementi non guariti e fratturati come fallimento. È stata eseguita un’analisi statistica sul modello multilevel GLM e regressione logistica per valutare una possibile differenza significativa tra i gruppi (p < 0.05). Risultati: Il drop-out è stato del 7% (n = 6). Sei elementi (3 dal gruppo chirurgia apicale, 2 dal gruppo chirurgia apicale con ritrattamento ortogrado simultaneo e 1 dal gruppo ritrattamento ortogrado non chirurgico) sono stati estratti dopo 6-9 mesi per frattura radicolare e sono stati considerati come fallimento. Nel gruppo di chirurgia apicale si è osservato un successo clinico a 24 mesi del 78% (n = 17), nel gruppo chirurgia apicale con ritrattamento ortogrado simultaneo dell’ 81% (n = 10) e nel gruppo ritrattamento ortogrado non chirurgico dell’ 80% (n = 24). Conclusioni: Entrambe le tecniche chirurgiche hanno mostrato un’alta percentuale di successo a 24 mesi. Non ci sono state differenze significative tra i trattamenti (p = .890) a 24 mesi. È stato osservato una guarigione più rapida nel gruppo di chirurgia apicale con ritrattamento ortogrado simultaneo. Keywords: Apical surgery, Retreatment, Treatment, Endodontics, MTA, Parole chiave: Chirurgia apicale, Ritrattamento, Trattamento, Endodonzia, MT
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
- …
