Experimental Study and Modeling of the Mechanical Behavior of Cross-Anisotropic Sandstone
In natural soils cross-anisotropic fabric is common due to deposition in a gravitational field. The behavior and properties of sandstone are different from sand due to cementation. The focus of this study is to describe the features connected with cross-anisotropy and cementation. Furthermore, a model capable of predicting the behavior of cross-anisotropic sandstone is developed based on The Single Hardening Model. First the behavior of cross-anisotropic sandstone is examined in a literature review of cemented geological materials. The behavior is further explored by triaxial and torsion shear tests on artificially cemented sandstone. A review of The Single Hardening Model revealed minor possible improvements, which are addressed by developing an improved plastic potential function and a new softening function. The triaxial tests showed that cross-anisotropic sandstone has an initial cementation yield surface. Hollow cylinder torsion shear tests confirmed the behavior inside the cementation yield surface to be elastic. The elastic behavior inside the cementation yield surface was found to be stiffer in vertical direction than in horizontal direction. The failure surface at high confining pressures corresponds to the critical state line. At intermediate confining pressures the failure surface is curved and at low confining pressures the failure surface is affected by the initial cementation yield surface similar to overconsolidated soil, increasing the strength further. The cross-anisotropy was found to decrease as the confining pressure increased and the cementation started to break. The modeling of the cementation is captured by translation of the stress space. As the cementation breaks down, the stress space moves back towards the origin, corresponding to uncemented sand. The rate of decementation is controlled by the initial cementation yield surface and a second yield surface corresponding to zero tensile strength. The cross-anisotropy is modeled using a microstructural tensor describing the anisotropy in the material. The cross-anisotropic model requires parameters corresponding to vertically cored specimens and horizontally cored specimens. The model then averages the behavior depending on the loading direction. To successfully model the artificial cross-anisotropic sandstone tested here, it was found sufficient to apply the modifications to the failure surface and the yield surface.
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