What is the significance of pore pressure in a saturated shale layer?

The central aim of the paper is to discuss the applicability of the effective stress principle as defined by Terzaghi (total stress minus pore water pressure) to predict the behaviour of expansive clay aggregates. Phenomena occurring between individual clay minerals are reviewed first at the molecular level obtained in the colloid science research. In particular, it is noted that, for inter-particle distances higher than approx. 1.5 nm, the pore water pressure in bulk equilibrium solution forms an additive component of the inter-particle disjoining pressure. It is concluded that for these distances the Terzaghi's effective stress principle should be adequate to describe the clay behaviour. To support the developments, an extensive experimental database of nine different sodium and calcium bentonites available in publishhed literature was analysed. With the aid of double structure constitutive modelling, procedures were developed to extract the information on the behaviour of clay aggregates from the experimental measurements. It was then shown that unconfined water retention curves, swelling pressure tests, swelling under constant load tests and mechanical unloading tests are all uniquely related in terms of the dependency of dry density (or void ratio) of clay aggregate vs. mean effective stress. By considering reversibility of aggregate behaviour and full saturation of aggregates, this implies that the effective stress principle is valid in predicting expansive clay aggregate volumetric deformation.

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“…to u w . Similar reasoning regarding the influence of pore pressure in shale has been presented by Gonçalvès et al (2010).…”

Section: Proposed Interpretation Of the Effective Stress In Clay Aggrsupporting

confidence: 51%

Swelling phenomena and effective stress in compacted expansive clays

Maš́ın

Khalili

2016

Can. Geotech. J.

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“…Some of them could be the relative electrochemical effect of the other clays (smectite, chlorite, and illite) presented in OPA (Behnsen & Faulkner, 2012;Morrow et al, 2000), disjoining pressures (Gonçalvès et al, 2010), saturation of contact area (Ikari et al, 2009), pressure solution (Yasuhara et al, 2005;Zhang, Spiers, et al, 2010), thermal expansion by local temperature increase (even though room temperature was constant), a combination of two or more of the previous mechanism, etc. Some of them could be the relative electrochemical effect of the other clays (smectite, chlorite, and illite) presented in OPA (Behnsen & Faulkner, 2012;Morrow et al, 2000), disjoining pressures (Gonçalvès et al, 2010), saturation of contact area (Ikari et al, 2009), pressure solution (Yasuhara et al, 2005;Zhang, Spiers, et al, 2010), thermal expansion by local temperature increase (even though room temperature was constant), a combination of two or more of the previous mechanism, etc.…”

Section: 1002/2017jb014931mentioning

confidence: 99%

Frictional Properties of Opalinus Clay: Implications for Nuclear Waste Storage

et al. 2018

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The kaolinite‐bearing Opalinus Clay (OPA) is the host rock proposed in Switzerland for disposal of radioactive waste. However, the presence of tectonic faults intersecting the OPA formation put the long‐term safety performance of the underground repository into question due to the possibility of earthquakes triggered by fault instability. In this paper, we study the frictional properties of the OPA shale. To do that, we have carried out biaxial direct shear experiments under conditions typical of nuclear waste storage. We have performed velocity steps (1–300 μm/s) and slide‐hold‐slide tests (1–3,000 s) on simulated fault gouge at different normal stresses (4–30 MPa). To establish the deformation mechanisms, we have analyzed the microstructures of the sheared samples through scanning electron microscopy. Our results show that peak (μpeak) and steady state friction (μss) range from 0.21 to 0.52 and 0.14 to 0.39, respectively, thus suggesting that OPA fault gouges are weak. The velocity dependence of friction indicates a velocity strengthening regime, with the friction rate parameter (a − b) that decreases with normal stress. Finally, the zero healing values imply a lack of restrengthening during interseismic periods. Taken together, if OPA fault reactivates, our experimental evidence favors an aseismic slip behavior, making the nucleation of earthquakes difficult, and long‐term weakness, resulting in stable fault creeping over geological times. Based on the results, our study confirms the seismic safety of the OPA formation for a nuclear waste repository.

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“…Moreover, in this particular case, if the salt concentration c f is used as a state variable (assuming that P depends only on c f for a given material), the formulation of the chemomechanical specific storage coefficient obtained by Gonçalvès et al (2010), S 0chem S , is retrieved:…”

Section: Implication Of the Disjunction Pressure Term For The Storatimentioning

confidence: 99%

“…An algebraic treatment, which is detailed in Gonçalvès et al (2010) is used to obtain the following relationships for the components p N and p T of a generalized "pressure" tensor p normal and tangential to the surface, respectively:…”

Section: Surface Forces and Disjunction Pressurementioning

confidence: 99%