Behavior of bonded geomaterials such as soft rocks and sensitive clay is characterized by the yielding and structural degradation. Leroueil and Vaughan(1990) pointed out the congruent effects in two materials, soft rocks and sensitive structured clay based on the experimental laboratory test results. Kimoto and Oka(2005) developed an elasto-viscoplastic model for soft clay considering rate dependency and degradation due to straining. Then the model can describe the strain softening and rate dependency due to the structural change and degradation and it has been successfully applied to the analysis of soft clayey ground such as consolidation and excavation. In this paper considering the congruent behavior of soft rocks and structured clay, we have applied the elasto-viscoplastic constitutive model for clay to the triaxial compression behaviors of sedimentary soft rock called Ohya stone in Japan with different confining pressures, which exhibits both distinctive strain-softening under the low confining pressures (Adachi and Oka, 1995). Simulation results are in good agreement with the experimental results. For example, Fig. 1 shows the simulated and experimental results for Ohya stone under the confining pressure of 5 kPa. We have found the model can applicable to both soft clay and soft rocks. The model is numerically stable, and is thus highly suitable for computational analysis and design tools.
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