Deformation Characteristics and a Physical Model for Porous Rocks under Air-Dried and Water-Saturated Conditions

Abstract

Most rocks indicate some extent of visco-elastic properties or time dependency. For example, strength and Young's modulus increase with loading rate in uniaxial compression test. In creep test, strain increases with time though stress is maintained to be a predetermined value. This visco-elastic behaviour is especially notable in porous rocks such as tuff or weathered rocks.
In this study, visco-elastic properties of porous rocks are shortly reviewed and a new physical model or a constitutive equation is proposed. The model consists of a spring and a dashpot. It is assumed that the constitutive equation formerly proposed by these authors can be applied to the spring. Viscosity of the dashpot is quite low before loading, and is increasing gradually with progress of loading. In creep testing at low stress level, strain of the dashpot corresponds to creep strain because the spring constant does not decrease meaningfully at the low stress level.
The experimental results of muddy sandstone, Oya tuff, Tage tuff and Kawazu tuff were compared with the calculated results. It is found that stress-strain curve shows good coincidence between experimental and calculated results. Increase of strength and Young's modulus with loading rate is well simulated by the model. Most important results are that, even at low stress, strain of the dashpot is considerably larger than formerly considered. By this model, the difference of Young's moduli between air-dried and water saturated conditions is well simulated where the difference in Young's modulus is assumed to be the deference in strain of the dashpot. In water-saturated condition, strain of the dashpot increases more rapidly and then Young's modulus is relatively small.