Deformation Behavior of Sandstones during Multilevel Cyclic Loading and Unloading under Varying Lower Stress Limits: Effect of Stress Amplitude and Underlying Mechanism

The purpose of this study was to explore the influence and action mechanism of the stress amplitude on the evolution of the deformation behavior of sandstones during multilevel triaxial cyclic loading and unloading (CLU) under varying lower limits of stress (VLLS). To realize this purpose, a conventional triaxial compression test under a confining pressure of 5 MPa and multilevel triaxial CLU tests under VLLS and stress amplitudes of 5, 10, and 15 MPa were independently carried out. By summarizing and analyzing the change patterns of the deformation parameters of sandstones under the different stress amplitudes, the influence law and potential action mechanism of the stress amplitude on sandstones were illustrated. The test results indicated that the increase in stress amplitude provided more opportunity for grain slippage and crack formation in sandstones, causing further softening. As a result, the ratio of the stress at the point when the bearing capacity of the tested sandstones was lost at the postpeak stage to the peak stress decreased, and the elastic modulus decreased while the Poisson's ratio increased. Based on an analysis of the evolution trend of the incremental irreversible strain, the aforementioned phenomena occurred because the change curve of the incremental irreversible strain gradually steepened with increasing stress amplitude, transitioning from a horizontal linear curve to a U-shaped curve. However, during the observed volumetric strain change, stress amplitude increase caused a reduction in cohesion in the sandstones, thus shortening the transition period from expansion to contraction. The sandstones were directly damaged without a transition period when the stress amplitude reached a certain level. Additionally, by generalizing and determining the typical deformation parameters, the process before rock failure was divided into four stages, each with a threshold. When reaching the different thresholds, hardening and softening mechanisms influenced the loading and unloading process in different ways. Combined with fracture analysis, the influence mechanism of the stress amplitude on the deformation behavior of the sandstones entailed that with increasing stress amplitude, the stress threshold was eventually reached. Therefore, the accumulated degradation rate of the mechanical properties attributed to the softening mechanism varied, further changing the microscopic evolution characteristics of sandstone deformation. The research results provide construction guidance in geotechnical engineering.