Experimental study of elastic properties of different constituents of partially saturated argillite using nano-indentation tests

• A reduction in the unloading modulus occurs under increasingly saturated conditions.
• The unloading modulus of inclusions can only be measured through tests on desaturated material.
• At 50% humidity, the measured mean deferred unloading modulus of the clay matrix is 16 GPa.
• At 95% humidity, the measured mean deferred unloading modulus of the clay matrix is less than 5 GPa.

Callovo-Oxfordian argillite, obtained from the ANDRA underground research laboratory in Meuse/Haute-Marne (France), is characterized as a multiphase material. The argillite is composed of carbonate inclusions (10–50 μm) embedded in an argillaceous matrix (representing expanding clay minerals, such as smectites). The matrix itself is also multiphase, composed of clay aggregates of 1 μm-size carbonate and quartz inclusions. The high sensitivity of the mechanical behaviour of argillite to saturation is an important characteristic of this material as it has been previously demonstrated in macro-scale mechanical experiments performed under varying degrees of humidity.The study presented here consists of grids of nano-indentation tests performed under controlled saturation conditions. The influence of the load hold time before unloading has been studied. Series of indentations were performed without or with load hold, which led to consider instantaneous or deferred unloading moduli respectively. The experimental procedure employed allows the micro-mechanical properties of the different phases (matrix and inclusions) to be determined under controlled hydration and therefore under partially saturated conditions. Several series of measurements were performed at relative humidity levels of 50%, 85%, 90% and 95%, and at a constant temperature of 20 °C. A statistical analysis enabled to discriminate the deferred unloading modulus of the different phases. At 50% humidity, we measured a mean deferred unloading modulus of 16 GPa for the clay matrix (a mean instantaneous unloading modulus of 13 GPa was observed for the clay matrix and moduli higher than 70 GPa were measured for the carbonate macro-inclusions). The mean deferred unloading modulus of the matrix appears to decrease with increasing saturation; at 95% humidity (near-saturation) it is less than 5 GPa. However, it was impossible to verify the instantaneous unloading modulus of the carbonate macro-inclusions at this high-level of saturation.

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