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The high-capacity tensiometer developed by Ridley and Burland in 1993 is a milestone in experimental unsaturated soil mechanics. This device, which relies on development of tension in an enclosed water body, permits direct measurement of negative water potential. Many tensiometers have been built since 1993, all being based on the same principle although the design may differ slightly from the original. In particular, the common characteristic is a very small water reservoir that is believed to be the location of bubble nucleation, a phenomenon referred to as cavitation that impedes the sensor from functioning properly. Many have studied cavitation and different explanations or cavitation mechanisms have been proposed. However, all the considerations put forward were derived without being able to capture what happened inside the water reservoir at cavitation. This is now achieved with the new tensiometer specifically designed to 'see' inside the water reservoir during suction measurement. For the first time, cavitation has been captured via high-speed photography and the mechanisms of cavitation can be explained using physical evidence. The first outcome is that it is possible for a high-capacity tensiometer to function to its full range with a large water reservoir. Then, the analysis of high-speed photographs reveals that the bubbles triggering cavitation are nucleated in the ceramic and make their way to the water reservoir. Cavitation occurs only when the air phase reaches the water reservoir. Key words: cavitation, tensiometer, high-speed cameras, suction, bubble. Le tensiometre a haute capacite, developpe par Ridley et Burland en 1993, a ete une avancee importante pour l'experimentation dans le domaine de la mecanique des sols non satures. Cet appareil, qui considere le developpement de la tension dans une masse d'eau confinee, permet de mesurer directement le potentiel negatif de l'eau. Plusieurs tensiometres ont ete fabriques depuis 1993; tous etant bases sur le meme principe malgre que le design puisse etre different de l'original. La caracteristique commune est un reservoir d'eau tres petit qui serait l'endroit ou les bulles se forment, un phenomene appele cavitation, qui empeche la sonde de fonctionner adequatement. Plusieurs chercheurs ont etudie la cavitation et differentes explications ou mecanismes de cavitation ont ete proposes. Cependant, toutes les considerations apportees ont ete derivees sans etre dans une position pour reellement voir ce qui se passe a l'interieur du reservoir d'eau lors de la cavitation. Le nouveau tensiometre le permet puisqu'il est specialement concu pour [much less than] voir ≫ l'interieur du reservoir d'eau lors des mesures de succion. Pour la premiere fois, la cavitation a ete enregistree a l'aide de la photographie a haute vitesse, et les mecanismes de cavitation peuvent etre expliques selon des evidences physiques. Le premier resultat est qu'il est possible qu'un tensiometre a haute capacite puisse fonctionner a sa pleine capacite avec un grand reservoir d'eau. Ensuite, l'analyse des photographies a haute vitesse a revele que les bulles qui declenchent la cavitation proviennent de la ceramique et se font un chemin jusqu'au reservoir d'eau. La cavitation se produit seulement lorsque la phase gazeuse atteint le reservoir d'eau. [Traduit par la Redaction] Mots-cles: cavitation, tensiometre, cameras a haute vitesse, succion, bulle., Introduction Water potential, commonly called suction when in the negative range, is known to significantly affect the mechanical response and hydraulic behaviour of soils. This is one reason why measurement [...]

Accurate measurement of PEG concentration is primordial when using the osmotic method to control soil suction. Because PEG is an inert molecule, common chemistry methods are not adequate to measure its concentration and researchers resort to the measurement of Brix Index. Such measurement can be manual, which is not very accurate and time consuming but cheap. On the other hand, more advanced sensors do exist but they tend to be quite expensive. This paper presents a new sensor for the indirect measurement of PEG concentration based on a tuneable diode laser and fibre optics. The laser signal is shone through the PEG and the reduction in signal voltage is correlated to the PEG concentration. The system has been assessed for temperature stability and accuracy. Such sensor offers the possibility of multipoint measurement, which significantly reduces the capital cost of the equipment.

Natural discontinuities are known to play a key role in the stability of rock masses. However, it is a non-trivial task to estimate the shear strength of large discontinuities. Because of the inherent complexity to access to the full surface of the large in situ discontinuities, researchers or engineers tend to work on small-scale specimens. As a consequence, the results are often plagued by the well-known scale effect. A new approach is here proposed to predict shear strength of discontinuities. This approach has the potential to avoid the scale effect. The rationale of the approach is as follows: a major parameter that governs the shear strength of a discontinuity within a rock mass is roughness, which can be accounted for by surveying the discontinuity surface. However, this is typically not possible for discontinuities contained within the rock mass where only traces are visible. For natural surfaces, it can be assumed that traces are, to some extent, representative of the surface. It is here proposed to use the available 2D information (from a visible trace, referred to as a seed trace) and a random field model to create a large number of synthetic surfaces (3D data sets). The shear strength of each synthetic surface can then be estimated using a semi-analytical model. By using a large number of synthetic surfaces and a Monte Carlo strategy, a meaningful shear strength distribution can be obtained. This paper presents the validation of the semi-analytical mechanistic model required to support the new approach for prediction of discontinuity shear strength. The model can predict both peak and residual shear strength. The second part of the paper lays the foundation of a random field model to support the creation of synthetic surfaces having statistical properties in line with those of the data of the seed trace. The paper concludes that it is possible to obtain a reasonable estimate of peak and residual shear strength of the discontinuities tested from the information from a single trace, without having access to the whole surface. [ABSTRACT FROM AUTHOR]