The thermo-mechanical behaviour of the Callovo-Oxfordian claystone

Abstract

International audienceThe Callovo-Oxfordian (COx) claystone is considered as a potential host rock in the French concept of high level radioactive waste disposal at great depth. To better understand and to complement existing published data on the thermo-hydro-mechanical behaviour of the COx claystone, an experimental program was carried out using a hollow cylinder triaxial device specially developed for low permeability materials. Special care was devoted to the saturation of the specimens that was made under stress conditions close to in-situ, and to conditions ensuring full drainage thanks to a reduced drainage length and low shear rate. Tests were carried out under in-situ, half to in-situ and twice the Terzaghi mean effective in-situ stress at 25 °C and 80 °C to investigate the effects in the close field of the temperature elevation due to the exothermic nature of the waste. Some radial permeability tests were also conducted at various temperatures. The data obtained showed that there is little effect of temperature on the elastic parameters determined, whereas a tendency to a decrease in shear strength was noted, in accordance with the few available published data. Temperature also appeared to have little effect on the intrinsic permeability, with higher flows mainly due to the decrease in water viscosity. & 2015 Published by Elsevier Ltd

Drained Triaxial Tests in Low-Permeability Shales: Application to the Callovo-Oxfordian Claystone

“…This small drainage length, four times smaller than standard triaxial drainage length (38 mm), reduces the pore pressure diffusion rate by a factor of 16 compared to standard triaxial specimens (Monfared et al, 2011). As shown by Monfared et al (2011), Mohajerani et al (2013), Menaceur et al (2015) and Menaceur et al (2016a) with hollow cylinder specimen and by Hu et al (2014) with small-sized specimen, the 10 mm drainage length made it possible to achieve satisfactory saturation within 3 to 4 days, compared to a period of around one month in standard triaxial tests. Reasonable results of fully saturated and drained isotropic compression tests have also been obtained for the COx claystone with a 10 mm high specimen drained at the bottom, also resulting in a 10 mm drainage length (Mohajerani et al 2012;Belmokhtar et al 2017aBelmokhtar et al , 2017b.…”

“…Swelling claystones are quite sensitive to changes in water content, as observed by Chiarelli et al (2003), Valès et al (2004), Zhang and Rothfuchs (2004), Pham et al (2007) and Zhang et al (2012). As mentioned by Menaceur et al (2015), besides drainage issues, the differences in testing procedures with respect to the partially saturated states of the specimens are one of the reasons, beside natural variability, of the differences observed in the shear strength properties of claystones. Partial saturation and poor drainage may lead to biased assessment of the mechanical properties of the tested specimens, over-estimating the shear strength with partially saturated specimens, and over-estimating the Young's modulus by running partially drained tests.…”

“…Mohajerani et al, 2012;Delage et al, 2014) and interesting self-sealing capabilities (e.g. Davy et al, 2007;Zhang, 2011;Menaceur et al 2015). The clay and carbonate contents of the COx claystone vary with depth, with total porosity between 14% in carbonated levels and 19.5% in the most argillaceous levels (Yven et al, 2007) in which the URL has been excavated.…”

“…The in-situ state of stress in the Bure URL has been investigated in detail by , who estimated a vertical lithostatic total stress v = 12.7 MPa , a major horizontal stress H = 14.8-16 MPa, a minor horizontal stress h = 12.4 MPa and a hydrostatic pore pressure pw = 4.9 MPa. Typical characteristics of the COx claystone are a very low permeability (in order of 10 -20 m 2 , Escoffier, 2002;Davy et al, 2007;Zhang, 2011;Menaceur et al, 2015Menaceur et al, , 2016, a good ability to retain radionuclides and a low diffusion coefficient, resulting in very slow solute transfer.…”

“…Similarly, Islam and Skalle (2013) adopted rates of 1-2×10 -7 s -1 in undrained tests and 2×10 -8 s -1 in drained test on Pierre I shale, with a drainage length h = 19 mm (lateral drainage on 38 mm standard triaxial device). These values are to be compared with the values adopted on the COx claystone mentioned above: 6.5×10 -6 s -1 by Zhang and Rothfuchs (2004) on 40 mm diameter and 80 mm high specimens, 1.0×10 -7 s -1 and 0.9×10 -7 s -1 by Hu et al (2014) and Menaceur et al (2015), respectively, with 10 mm drainage length.…”