Sable | EPFL Graph Search

thumbnail|right|Dunes de la vallée de la Mort en Californie. vignette|Les charroyeurs de sable Stanislao Pointeau 1861. vignette|La Seine à Port-Marly, tas de sable par Alfred Sisley (1875), Chicago, Art Institute of Chicago. Le sable est une matière solide granulaire constituée de petites particules provenant de la désagrégation de matériaux d'origine minérale (essentiellement des roches) ou organique (coquilles, squelettes de coraux) dont la dimension est comprise entre (limon) et (gravier) selon la définition des matériaux granulaires en géologie. Sa composition peut révéler jusqu'à 180 minéraux différents (quartz, micas, feldspaths, etc.) ainsi que des débris calcaires. Le sable a de nombreuses applications en tant que matériau granulaire, dont la principale est la fabrication du béton. C'est une ressource non renouvelable. Caractéristiques physicochimiques Une particule individuelle est appelée grain de sable. Les sables sont identifiés grâce à la granulométri

Dynamics of cavitation bubbles near granular boundaries

Mohamed Farhat, Davide Bernardo Preso, Armand Baptiste Sieber

We study the dynamics of a cavitation bubble near beds of sand of different grain sizes. We use high-speed imaging to observe the motion of the bubble and the sand for different values of the stand-off parameter gamma (dimensionless bubble-boundary distance) between 0.3 and 5.3. Compared with a rigid boundary, we find that a granular boundary leads to bubbles with shorter lifetimes and reduced centroid displacements. Above gamma approximate to 1.3, the behaviour of the bubble is almost independent of the granularity of the sand. When the stand-off parameter lies between 0.6 and 1.3, a mound of sand develops beneath the bubble, which can force the latter to assume a conical shape as it collapses. For gamma less than or similar to 0.6, the bubble develops a bell-shaped form, leading to the formation of thin and surprisingly fast micro-jets (v(jet) > 1000 m s(-1)). Moreover, between gamma approximate to 1.3 and gamma approximate to 0.3, granular jets erupt from the sand surface following the bubble collapse. We additionally develop a simple numerical model, based on the boundary integral method, to predict the dynamics of a cavitation bubble near a bed of sand, which we replace by an equivalent liquid. The simulations are remarkably consistent with experimental observations for values of gamma down to 1.3. We also show how the anisotropy parameter a dimensionless version of the Kelvin impulse, can be adapted for the case of a nearby bed of sand and predict the displacement of the bubble centroid for zeta less than or similar to 0.08.

CAMBRIDGE UNIV PRESS2022

Gaseous tracer diffusion from a point source as a site investigation method

David Werner

Transport modelling, risk assessment, and the evaluation of remediation strategies at sites contaminated with volatile organic compounds (VOCs) require the knowledge of gas-phase diffusion coefficients in unsaturated porous media. This thesis presents a new in situ method for the simultaneous measurement of effective and sorption-affected diffusion coefficients. The method relies on the injection of a gaseous compound and a conservative gaseous tracer using a soil gas probe. Concentrations are monitored at the tip of the probe during 8 hours and evaluated with an analytical equation for reactive transport in a homogeneous medium. Results are reported for volatile organic pollutants in a sand-filled lysimeter and at a field site with a sandy unsaturated zone. The determination of the effective diffusion coefficients showed good reproducibility and robustness with respect to analytical error, with an estimated overall error of 13%. The determination of the sorption-affected diffusion coefficients also showed good reproducibility. However, batch experiments suggest a systematic overestimation by 15-20%, because the tracers were not truly conservative. The in situ method avoids considerable uncertainties associated with estimating apparent diffusion coefficients from empirical relationships and avoids problems with establishing representative soil conditions in laboratory experiments. Another key issue for risk assessment at contaminated sites is the localization and quantification of nonaqueous phase liquids (NAPLs) such as gasoline, kerosene or heating oil in the subsurface. This thesis proposes the theory and practical application of a new diffusive partitioning tracer test (DPTT) for NAPL detection in the vadose zone. The general approach is the same as for the determination of apparent diffusion coefficients. A mixture of chlorofluorocarbons as gaseous tracers is injected into the vadose zone using a soil gas probe. While the tracers diffuse away, small volumes of gas are withdrawn from the injection point. The quantitative determination of the NAPL saturation is based on a comparison of the concentration decline of tracers with different air-NAPL partitioning coefficients. An alternative approach is the measurement of diffusive breakthrough curves in some distance from the injection point. The test has been evaluated in laboratory sand columns, at a field site with an artificial contamination embedded in a sandy unsaturated zone and in a sand-filled lysimeter, where the NAPL distribution was heterogeneous. NAPLs in saturations between 0 and 4 % of the total porosity have been reliably detected and quantified in a wide range of different water contents. NAPL saturations determined by DPTTs and by extraction from soil cores agree within a factor 2 or better. Results from the lysimeter study suggest that the NAPL saturation derived from the tracer concentrations at the injection point represent a small soil volume in the vicinity of the injection point, whereas the NAPL saturation determined from diffusive tracer breakthrough curves represent the soil between injection and monitoring point. These innovative investigation methods for the vadose zone are inexpensive, require little technical equipment on site, and allow the simultaneous determination of different parameters at one location from a few single measurements: as demonstrated in situ for two sandy soils, the effective and the apparent diffusion coefficient or the NAPL saturation, the effective diffusion coefficient and the soil gas concentration of several VOCs can be determined simultaneously by the proposed methods. This new approach has been tested in sandy, unsaturated porous media. The performance of the methods in heterogeneous environments or in porous media other than sand needs to be further investigated.

EPFL2002

Cyclic properties of sand

Emilie Rascol

Seismic wave propagation in granular soils can induce large strain amplitudes in case of strong earthquakes. Seismic motions are irregular in frequency content and in amplitude, and have three different components in orthogonal directions. In this context, the main objective of this PhD research deals with nonlinear effects observed in granular soils under such complex loadings. The assumptions and simplifications usually considered for representing seismic loadings are evaluated, focusing on two main aspects: (i) cyclic stress frequency applied to the sample (ii) superposition of two independent stresses. For that purpose, the nonlinear behaviour of two different sands, Leman Sand and Fonderie Sand, is explored with cyclic and seismic triaxial tests. These tests are performed with unidirectional or bidirectional loadings, at medium to high strain amplitude, and in the earthquake frequency range. A dynamic triaxial press was developed to perform such tests, with dry and undrained saturated sand samples. Axial and lateral stresses can be applied independently with large amplitudes for various loading shapes. An innovative non-contact measurement technique was developed to continuously monitor the sample radius; this testing equipment is based on three laser sensors, set up around the triaxial cell, which detect the position of the sample surface thanks to optical triangulation. The obtained data are processed through a complex calibration system to provide the radial strain evolution at mid-height of the sample. The mounting structure supporting the sensors allows precise positioning and is equipped for manual vertical scanning of the sample profile. The first triaxial tests are performed with classical cyclic loadings, to characterize the behaviour of the two sands in pseudo-dynamic conditions. These dry and undrained saturated tests allow to describe the decrease of stiffness which leads to failure of the sand sample. Failure of undrained saturated sand occurs by liquefaction. Dry and undrained cyclic tests performed on Leman Sand at various frequencies from 0.1 to 6.5 Hz show that the behaviour of this granular material is frequency-dependent at medium to large strains. Sand stiffness, which depends on stress conditions, seems to influence the extent of frequency effects on soil behaviour: for tests with lower stiffness, the soil response to low frequency is significantly amplified (i.e. higher strain amplitude, more pore pressure increase, etc.) compared to the high frequency range. The overall rate-sensitivity may be enhanced by the angularity of the grains. Other cyclic undrained saturated tests on Leman Sand demonstrate that the superposition of two different loadings, one axial and one lateral (bidirectional tests), induce coupling effects in the nonlinear soil response. Bidirectional effects result in an amplification of the sand response until the occurrence of cyclic liquefaction. The phase angle between axial and lateral stresses is the key parameter influencing the coupling. Moreover, the comparison between unidirectional and bidirectional irregular seismic loadings show that bidirectional conditions slightly influence undrained sand response, with conditions of amplification very similar to cyclic tests. Experimental results are finally modelled with the linear equivalent method and with a multi-mechanism elastoplastic model (ECP Hujeux). Nonlinear effects observed in laboratory experiments, and particularly the increase of strain amplitude leading to cyclic liquefaction of dense sand, are well captured by the elastoplastic model. The linear equivalent method gives a very crude approximation, even at medium strain level, and is not suitable for accurate evaluation of stiffness degradation observed during our cyclic tests. To conclude, assessing the behaviour of granular soils under earthquake loadings requires to take into account the nonlinear features of sand behaviour in terms of pore pressure generation and strain amplitude. In particular, frequency content and bidirectional loadings influence the sand response for medium to large strains. These experimental results could be considered for improving the analysis of strong ground motions. They constitute an important contribution for promoting more accurate nonlinear modelling of site effects in natural sands.

EPFL2009