Anisotropy of Consolidation Yield Stress and Comparison with Anisotropy of Elastic Wave Velocity and Tensile Strength in Sedimentary Soft Rocks

Sedimentary soft rocks are one of the foundations of large and important constructions. Because the mechanical properties of soft rocks control the behavior of foundations, it is necessary to understand the consolidation characteristics of sedimentary soft rocks. Particularly, the anisotropy in mechanical properties is important; therefore, the relationship between these properties to bedding planes and the stress environment should be considered. In this study, consolidation tests were conducted in three directions using four block samples of Neogene to Quaternary sedimentary soft rocks from the western part of the Boso Peninsula, central Japan: orthogonal to the bedding plane (σ_3direction) and two directions within the bedding plane defined by paleo-stresses (σ_1 and σ_2). In comparison with other fundamental physical properties, elastic wave velocity measurements and tensile strength tests were conducted to investigate anisotropy. The results show that the consolidation yield stresses in the three directions are almost the same or greater in the past principal stress direction than in the other directions, suggesting that the rocks reflect the tectonic stress history experienced in the past. In addition, the anisotropic degree of consolidation yield stress is almost the same as that of tensile strength and higher than that of P-wave velocity. As the degree of anisotropy of consolidation yield stress increases, tensile strength increases negatively. Anisotropy of the internal structure due to lateral compressive stress increases with the action of lateral compressive stress, while anisotropy of the internal structure due to the sedimentation process decreases.