Analyse microscopique des mecanismes de dessiccation et de gonflement des sols argileux

National audienceL'article présente quelques analyses menées sur une argile : la montmo-rillonite grecque. Elles ont été réalisées à l'aide d'un Microscope Electronique à Ba-layage Environnemental (MEBE). Dans ce dernier, on a imposé un cycle d'humidification-séchage et parallèlement un séchage à l'air libre sur une galette d'argile. Les premières observations semblent montrer une certaine réversibilité du gonflement. Toutefois une " pseudo-précipitation " de petits grains s'est initiée. Les formes géométriques des galettes d'argile ne semblent pas présenter d'influence sur le comportement au séchage à l'air libre de l'argile

Analyse à l'échelle micro et macroscopique des mécanismes de retrait et de gonflement des sols argileux

International audienceThis paper presents some experimental analyses performed, within an ESEM, of the shrinkage and swelling of a clay.Cette communication présente l'analyse expérimentale, menée au MEBE, du retrait et du gonflement d'une argile

Stifness and influence of non-saturation in granitic materials

Neste artigo, apresentam-se as características de deformabilidade para dois materiais com a mesma origem geológica: um agregado granítico (0/12.5; D50=0.52mm) e uma areia siltosa designada de areia de Perafita (0/6.35; D50=0.36mm). A rigidez, dos materiais compactados, no domínio das pequenas deformações (<10-5) é estudada através de ensaios triaxiais de precisão. A influência da não saturação na deformabilidade dos materiais é estudada através de ensaios triaxiais com medição das pressões intersticiais. Tais medidas permitiram a análise do comportamento mecânico dos materiais não saturados em termos de tensões efectivas, que foram calculadas através das aproximações de Terzaghi e Bishop. Os resultados obtidos mostram para cada material uma relação única entre o módulo de deformabilidade e a tensão efectiva (para e=0.5), independentemente do grau de saturação das amostras ensaiadas.In this paper, the stiffness of two materials with the same geological origin is presented, a granite aggregate (0/12.5, D50=0.52mm) and a silty sand, called Perafita sand (0/6.35; D50=0.36mm). The stiffness in very small strains (<10-5) of the compacted materials is studied by means of precision triaxial tests. The influence of non-saturation on the stiffness is also studied through triaxial tests with measurement of negative pore water pressures. Such measurements allow the analysis of the mechanical behaviour of the unsaturated material in terms of effective stresses. For this purpose two approaches were used: the Terzaghi’s and Bishop’s approaches. The results obtained show, for each material, a unique relationship between vertical stiffness and vertical effective stress (constant void ratio=0.5), independently of the saturation degree of the studied samples.Universidade de Coimbra. Faculdade de Ciências e Tecnologia (FCT-UC)GRICES/CNRS – “Security of road and railway infrastructures versus climatic changes

Influencia de la no saturación y granulometría en la rigidez de materiales de naturaleza granítica

In this paper, the stiffness of two materials with the same geological origin is presented, a silty sand, called Perafita sand (D50=0.36mm) and a granite aggregate (0/12.5, D50=0.52mm). The stiffness in very small strains ( <10-5) of the compacted materials is studied by means of precision triaxial tests (with local strain measurements). The influence of non-saturation on the stiffness is also studied through triaxial tests with measurement of negative pore water pressures. Such measurements allow the analysis of the mechanical behaviour of the unsaturated material in terms of effective stresses. For this purpose two approaches were used: the Terzaghi’s and Bishop’s approaches. The results obtained show, for each material, a unique relationship between vertical stiffness and vertical effective stress (constant void ratio = 0.5), independently of the saturation degree of the studied samples.En este artículo se presenta la rigidez de dos materiales con el mismo origen geológico, una arena con limo, llamada arena Perafita (D50=0.36mm) y un agregado granítico (0/12.5, D50=0.52mm). La rigidez para deformaciones muy pequeñas ( <10-5) de los materiales compactados es estudiada mediante ensayos triaxiales de precisión (con medidas locales de deformación). La influencia de la no-saturación en la rigidez se ha estudiado también mediante ensayos triaxiales con medición de la presión negativa del agua en los poros. Estas medidas permiten el análisis del comportamiento mecánico de materiales no saturados por lo que se refiere a tensiones efectivas. Con este propósito fueron usados dos procedimientos: el de Terzaghi y el de Bishop. Los resultados obtenidos revelan, para cada material, una relación única entre la rigidez vertical y la tensión efectiva vertical (ratio de vacíos constante = 0.5), independientemente del grado de saturación de las muestras estudiadas

Influence of Compaction and Loading Conditions of the Dynamic Properties of a Silty Sand

Elastic properties of a compacted silty sand were measured in a precision triaxial cell at the École Centrale de Paris in the range of very small strains (between 10-6 and 10-4), using improved Hall effect-based local strain gauges and data acquisition system. The dynamic properties of the soil were also derived from resonant column tests at the IST, in Lisbon, as functions of shear strain, compaction water content and confining pressure. Elastic limits as low as 5.10-6 were found in both devices, with Poisson’s ratio from triaxial tests ranging from 0.05 at very strains (\u3c 10-4) to 0.37 at larger strains. Comparison between quasi-static and dynamic values were found to be in good agreement, with a reasonable value of Poisson’s ratio, despite the complex properties of this soil such as viscosity, ageing effects and anisotropy

Influence of suction on the properties of two granular road materials

The results of an experimental work on two road granular materials are presented, including small strains precision triaxial tests under cyclic loading, large strains triaxial tests with measurement of the negative pore water pressure (suction) and wetting tests. The influence of different initial conditions of density, water content and fines content was studied. The two materials differ by their fine contents (7 % for MHC and 10% for HFC). The specimens are compacted at different water content and at a density corresponding to 97% of the Modified Proctor maximum density. The interpretation of the results, in the quasi-elastic domain, is based on an effective stress analysis that allows to take into account both the effects of total stresses and negative pressure, in the perspective of a more rational design of pavement layers

Uma abordagem à modelação de solos não saturados

Os geomateriais das camadas dos pavimentos e das vias-férreas, bem como os das respectivas fundações, encontram- se no estado não saturado nas boas condições de serviço destas infra-estruturas. Torna-se por isso necessário considerar o aspecto não saturado destes materiais na modelação e no dimensionamento destas infra-estruturas. O presente artigo propõe a generalização do conceito de tensão efectiva para solos não saturados. De modo a descrever o comportamento exibido pelos solos não saturados, nomeadamente os fenómenos de colapso durante a humidificação, é proposto um modelo elastoplástico. Este modelo, descrito em termos de tensões efectivas, é calibrado para o caso de uma areia siltosa com base em resultados de ensaios triaxiais clássicos e de precisão com medição da pressão intersticial negativa

Cyclic behavior of Bogotá unbound aggregates

The results of an experimental work on three road granular materials from La Sabana de Bogotá are presented. These works include small strains precision triaxial tests under cyclic loading with measurement of the negative pore water pressure (suction). The influence of different initial conditions of density, water content and fines content was studied. The interpretation of the results, in the quasi-elastic domain, is based on total stress and effective stress analyses , the latter allowing to take into account the effects of both total stresses and negative pressure, in the perspective of a more rational design of pavement layers.En este artículo se presentan los resultados de un trabajo experimental sobre tres diferentes materiales granulares de la Sabana de Bogotá. El trabajo experimental incluye ensayos triaxiales con cargas cíclicas y con medida de la presión intersticial negativa (succión). Sobre los tres materiales se estudió la influencia de diferentes parámetros tales como la densidad, la humedad y el contenido de finos. La interpretación de los resultados en el dominio de deformaciones quiasi-elástico se hace tanto en términos de esfuerzos totales como de esfuerzos efectivos lo cual permite tener en cuenta la influencia de la presión intersticial negativa con la perspectiva de utilizarlos en diseños de pavimentos más racionales

Método del papel de filtro para la medida de la succión del suelo

The capillary pressure of the soil (i.e., the pressure difference between air and water components in soil voids) is a key variable in the analysis of the hydro-mechanical behavior of unsaturated soils. Therefore a simple and economical laboratory method for the measurement of the capillary pressure of the soil (also known as soil matric suction, the reference being the atmospheric pressure), even if a degree of approximation is involved, is of considerable value. The filter paper method calculates soil suction indirectly by measuring the gravimetric water content of the filter paper at equilibrium that is related to soil suction through a predetermined calibration curve. The advantages of the method are simplicity, economy and reasonable accuracy. It can be used to measure suctions from 10 to 30000 kPa. In this paper, the authors use the contact filter paper method for matric suction measurements of an unsaturated compacted silty sand (formed by the weathering of granite) which has been used as a building material for a road in the north of Portugal. The matric suctions inferred from filter paper measurements depend on the calibration between the water content of the filter paper and suction. Therefore, three calibration curves proposed at the literature (Chandler et al. 1992; ASTM D 5298; and Oliveira & Marinho 2006) for the Whatman 42 filter paper are used to interpret the measured filter paper gravimetric water contents. The results of these tests are compared to other techniques (i.e., tensiometers, and the osmotic technique) used to measure or control the negative pore water pressure in the compacted soil specimens and the results obtained are reasonably accurate.La presión capilar del suelo (es decir, la diferencia de la presión entre el aire y los componentes del agua en vacíos del suelo) es una variable llave en el análisis del comportamiento hidromecánico de suelos no saturados. Un método por lo tanto simple y económico del laboratorio para la medida de la presión capilar del suelo (también conocido como la succión matrica del suelo, la referencia que es la presión atmosférica), mesmo si un grado de aproximación está implicado, es de valor considerable. El método del papel de filtro calcula la succión indirectamente utilizando curvas de calibración. Las ventajas del método son simplicidad, economía y exactitud razonable. El método del papel de filtro se puede utilizar para medir succiones a partir del 10 al 30000 kPa. En este artículo, los autores utilizan el método del papel de filtro para la medida de la succión matric de una arena limosa compactada no saturada (formada por la meteorización del granito) que se ha utilizado como material de construcción para un camino en el norte de Portugal. Las succiones matric deducidas de medidas del papel de filtro dependen de una calibración entre el humedade del papel de filtro y la succión. Por lo tanto, tres curvas de calibración propuestas en la literatura (Chandler et al. 1992; ASTM D 5298; y Oliveira & Marinho 2006) para el papel de filtro de Whatman 42 se utilizan para interpretar lãs humedades gravimétricas medidas del papel de filtro. Los resultados de los ensayos se comparan a otras técnicas (es decir, tensiómetros, y la técnica osmótica) usadas para medir o controlar la presión negativa en lãs muestras compactadas del suelo y los resultados obtenidos sea razonablemente exacto