Real-time porosity inversion and compaction-damage characterization of siltstone under cyclic triaxial complete loading and unloading tests

The initial cracks and new cracks are primary factors on the mechanical properties of building materials. To quantitatively reveal the compaction effect on initial cracks and the “pseudo closure” phenomenon on new cracks, triaxial compression tests (TC) and cyclic triaxial complete loading and unloading tests (CTCLU) with real-time ultrasonic velocity monitoring were performed on siltstone under different confining pressure. The variation of volume compression strain and damage stress with confining pressure is significant, indicating the triaxial compression mechanical behavior of rocks shows strong compaction and damage effects. Under triaxial compression, the “pseudo closure” phenomenon of new cracks in the post-peak stage is the reason that ultrasound velocity does not decrease significantly. However, the real-time ultrasonic velocity law under CTCLU tests can truly reflect the compaction and damage behavior of materials. The test results indicate that the open and closure of initial cracks, new cracks, and pores occur at different loading and unloading stages of confining pressure and deviatoric stress, which determine the variation of real-time ultrasonic velocity under CTCLU tests. The ultrasonic velocity increment models for confining pressure and deviatoric stress loading and the velocity models after complete stress unloaded were established. In these models, initial crack plastic compaction limit, pore elastic compaction limit, and new crack damage limit were defined, to illustrate the changing characteristics of the micro structure on materials. The Raymer porosity inversion model was improved based on real-time ultrasonic velocity model, the calculation results show that the porosity of the initial cracks accounts for 30–40 % of the total porosity. Finally, based on the real-time porosity inversion results the effective compressibility coefficient of initial cracks and the cumulative crack density of new cracks were calculated, and the comparison coefficient and damage variable of rocks were established, respectively.