The influence of water on the méchanical behaviour of argillaceous rocks

Abstract : This thesis was done in collaboration with the French national radioactive waste agency ANDRA. The aim is to contribute to a better understanding of the mechanical behaviour of argillaceous rocks under the influence of aqueous solutions, as they are considered to be a possible host rock for a nuclear waste disposal site.
Shale, can depending on the composition, change its mechanical and petrophysical properties in a wide range due to fluid-clay mineral interactions. One and the same shale can be soft or hard, ductile or rigid, permeable or sealing - depending on the environmental conditions the sample is exposed to. Shale can be very sensitive to a change of conditions such as humidity, stress state, temperature, and chemical potential gradients.
One of the most obvious shale reactions is the swelling and shrinking as a function of it's saturation and the chemical potential gradient in the fluid system. Theses volume changes also sometimes alter other properties. Depending on the nature and the concentration of a liquid, a shale can either shrink or swell when in contact with a liquid. The material may sometimes also disperse in the liquid and deteriorate completely.
An understandable description of the clay-minerals from the site of Est is given. The physicochemical micro mechanism of the fluid solid interaction such as adsorption, absorption, capillarity and osmosis are also presented. The possible consequences of these mechanisms on the macro mechanical behaviour, such as swelling and shrinking, crack induction and others was analysed.
X-ray microfocus technology was introduced and used to analyse shale under different environmental conditions. The advantage of this technology is it's non-destructive character. The preparation of classical thin cross section is a rather inappropriate method, concerning micro cracks and deterioration in shale, because of the grinding of the material.
A solution test was conducted to qualitatively observe the real-time reaction of the shale, from the site of Est, when in contact with different aqueous solutions and under uniaxial load. The cracking as well as the strain and the general behaviour of the material was investigated by using the X-ray μRG technology while the material was immersed in a solution. It could be shown, that the deterioration, the development of micro cracks and the strain are partially governed by the ion concentration and the ion species of the solution.
In the kinetic test arrangement the shale was exposed to different relative humidity and uniaxial load conditions. A computer controlled air conditioning system, with online data transfer of the axial strain and the environmental parameters, was used. The relation between environmental humidity, axial strain, axial load condition and micro cracks was analyzed. X-ray microfocus technology was used to take images of the specimen before and after the test. The material was tested in a range of relative humidity from 5% to 99%, in order to evaluate the influence of the humidity on the crack and strain behaviour. A cyclical test was conducted to asses the crack sensitivity in a range from 35% RH to 75% RH.
Finally a combined test was realized, to determine whether the material reacts to pure water in a different way after being saturated in a humid environment, compared to its reaction to water in its initial saturation state. The material was initially exposed to 5% and 99% relative humidity and subsequently immersed into pure water. A hydration phenomenon was discovered through this test, which is believed to be of importance with reference to commonly applied testing procedures.
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Harald Freissmuth. The influence of water on the méchanical behaviour of argillaceous rocks. Applied geology. École Nationale Supérieure des Mines de Paris, 2002. English. ⟨NNT : 2002ENMP1104⟩. ⟨pastel-00579409⟩

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