The hydro-mechanical processes of a reconstituted clay shale (Opalinus Clay) were studied and implications for constitutive modelling of swellable clay shales derived. The study aims to provide further insights into these coupled processes. For that, oedometric swelling tests with varying boundary conditions and loading paths, oedometric compression tests under dry and saturated conditions, and permeability tests were performed. The results showed a unique preloading line for the dry state when plotting the void ratio against the vertical stress. For saturated states, this line was shifted to the left (to smaller stresses) which revealed a severe reduction of preloading stress due to saturation. All loading paths converged in a normal compression line (NCL) regardless of the mechanical boundary conditions during saturation. A trend line affine to the NCL was established for swelling pressures at different initial void ratios. The influence of saturation on the mechanical properties became further evident, as the un-/reloading stiffness was significantly reduced. A linear relationship between volumetric deformation and saturation was found. A severe swelling induced reduction of permeability in the saturated state was demonstrated and confirmed by the analysis of consolidation processes during oedometric loading. The experimental findings were used to derive fundamental assumptions for constitutive modelling and were discussed within a numerical framework for expansive soils.
The mechanical behavior and the influence of compaction banding on the hydraulic properties in soft porous rocks were studied. The tested rock was Calcarenite Tuffeau de Maastricht. In the frame of experimental investigations, triaxial and oedometric tests were conducted under dry and drained conditions. The results demonstrated that the rock is forming discrete compaction bands under high confining stresses and steep angle shear bands under low confining stresses. Permeability measurements during the oedometric and triaxial compression tests under drained conditions demonstrated that the axial permeability decreases with increasing axial strain. The maximum permeability decrease was three orders of magnitude for 40% of axial strain.
In this paper the performance of a constitutive model for the description of the hydro mechanical behaviour of soft rock is evaluated with respect to the experimentally observed behaviour of Maastricht Calcarenite under different stress states that is presented in the companion paper. The mechanical model is elasto-plastic and consists of an associated yield surface, internal variables for the description of the hardening and softening behaviour and a non-local extension for the simulation of strain localization in form of shear bands and compaction bands. The model is implemented in the software ABAQUS and the laboratory results from the tests under dry condition with Maastricht Calcarenite are used for the calibration. The good agreement of the numerical results with the laboratory results is shown and the suitability of the model is discussed. To describe the effect of compaction bands on the permeability of soft rocks a simple analytical model based on the Kozeny–Carman equation is proposed and calibrated with the experimental results from drained tests under different stress states for Maastricht Calcarenite rock material. As the results are in good accordance with the experimental results, the model is implemented in the software ABAQUS and the numerical results are presented and discussed. Finally the performance of the model is evaluated and possible improvements are suggested.
<p><span>The safe disposal of nuclear and radioactive waste is one of the most challenging tasks in current and future environmental geosciences. In so-called deep geotechnical repositories nuclear waste barrels are either directly embedded in argillaceous buffers located in deep bedrock formations or the buffer is placed at distance to seal the deposit tunnel cavity. Due to their swelling capacity and low hydraulic permeability bentonite- or clay-based materials are widely regarded as suitable buffer materials. </span></p><p><span>The design of these deep geotechnical repositories is not a simple task and its evaluation and improvement is still subject of current research. During the design, finite element models can be used to simulate the behavior of the buffer and the bedrock subjected to hydro-mechanical loading. In order to achieve realistic predictions, these models have to meet several requirements: coupled hydro-mechanical simulation techniques are needed to capture the dependence of the swelling process of the buffer (or the bedrock) on the amount of supplied mountain water. Further, the swelling induced changes in the hydraulic permeability, strain and stress should be addressed in the simulations. The swelling process, however, is a path-dependent process which should also be taken into account by the numerical model. </span></p><p><span>Although several models capable of predicting the swelling process already exist, a hydro-mechanical model, which incorporates the capability of modelling swelling of an initially fully saturated material depending on its loading history, still lacks. </span></p><p>&#160;</p><p><span>The proposed constitutive model is aimed to be suitable for application in both the dry and the swollen state. The swelling process is activated by a change in volumetric water content. We therefore introduce a swelling water content which defines how much of the pore water contributes to the swelling of the porous medium. </span><span>The swelling water is assumed to be attached to the material particles and cannot be reduced by mechanical processes. Thus, the swelling process is regarded irreversible. To incorporate the path dependency of the swelling process, the evolution of the swelling water content depends on the effective stress state. Irreversible changes of the material due to swelling, e.g. a reduction of the stiffness, are modelled by introducing a swelling degree, which allows the transformation of material properties from the dry material to those of the swollen material.</span></p><p><span>The experimental studies which form the basis of the proposed constitutive model include oedometric swelling tests, standard oedometer tests and measurements for the determination of the suction-saturation relation and the hydraulic permeability. All tests are carried out on reconstituted samples of opalinus clay.</span></p><p><span>The proposed hydro-mechanical model is implemented using the finite element method and validated by numerical simulations. First simulations are in good agreement with the experimental results. </span></p>
<p>In this work, the mechanically induced compaction process in highly porous rocks is studied with experimental investigations and constitutive modeling. The focus of the study is on the influence of the inherent anisotropy on the mechanical properties. From a practical point of view, such behavior is of particular interest when considering reservoirs in soft, porous rocks. The reduction in pore pressure, which is linked to the production, leads to the possibility of compaction in the vicinity of the borehole. One effect is the risk of the loss of stability or of increased sand production. Another is the reduction of the permeability locally. The probability of such occurrences and the magnitude of such effects is currently under debate.</p><p>Although the formation of compaction bands in porous rocks has already been investigated in several studies, both in the laboratory and in situ, the extent data about the influence of the inherent anisotropy on the mechanical properties of porous rocks is limited. Baud et al. [1] documented an influence of the orientation of the bedding plane on the mechanical behavior of Diemelstadt sandstone and Louis et al. &#160;[2] documented an influence of the bedding plane on the formation of discrete and continuous compaction bands in Rothbach Sandstone.</p><p>On the basis of an extensive experimental program of triaxial and isotropic compression, triaxial extension tests as well as investigations with ultrasonic pulse method, the mechanical behavior of a highly porous rock (Maastricht Calcarenite) is analyzed with a special focus on the formation of compaction bands. The test program is performed with samples cored under different inclinations to the bedding plane to study the influence of the inherent anisotropy on the mechanical properties.</p><p>Based on the experimental results, the applicability of a constitutive model for the description of the mechanical properties is tested. Furthermore it is examined how the inherent anisotropy may be considered in the constitutive model and different approaches are discussed.</p><p>For the numerical simulation a nonlocal model is suggested to simulate the formation of compaction bands. Finally, conclusions are drawn and an outlook on experimental investigations of the influence of compaction banding on the hydraulically properties is given.</p><p>&#160;</p><p>[1]P. Baud, P. Meredith und E. Townend, &#8222;Permeability evolution during triaxial compaction of an anisotropic porous sandstone,&#8220; Journal of Geophysical Research, May 2012.</p><p>[2]L. Louis, P. Baud und T.-f. Wong, &#8222;Microstructural Inhomogeneity and Mechanical Anisotropy Associated with Bedding in Rothbach Sandstone,&#8220; Pure and Applied Geophysics, July 2009.</p><p>&#160;</p>
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