Cyclic Shear-Induced Degradation in Saturated Uniform Sands at Small Strains

Understanding the behavior of sands under cyclic loading is crucial for seismic safety, particularly in regions with high water tables or those located near coastal areas. This study investigates the dynamic behavior and degradation characteristics of uniformly graded Drava River Sand (DrOS018) under undrained cyclic loading conditions. A series of strain-controlled cyclic triaxial tests were conducted at relative densities of 33%, 50%, and 80% under confining pressures of 100 kPa, 200 kPa, and 400 kPa. Utilizing sinusoidal loading frequencies of 0.1 Hz and 0.05 Hz, the experiments provided significant insights into the behavior of sand across a wide range of axial cyclic strains. The results indicate that at cyclic shear strains slightly below and above 0.01%, Drava River sand exhibits an initial hardening phase, characterized by a degradation index above 1 and an increase in pore pressure of up to 35%. This phenomenon, attributed to microstructural grain contact, represents a notable deviation from the traditional view of uniform strength degradation with increasing pore pressure. Beyond this threshold strain, the material enters a phase of evident strength degradation, typically at cyclic shear strains ten times the threshold. At higher effective stresses and relative densities, the sand exhibits increased resistance and can withstand up to 20 load cycles at a cyclic shear strain of 0.2 before complete degradation. Conversely, a rapid loss of strength is observed at lower relative densities (e.g., 33%). The study also confirms the increasing trend of the equivalent viscous damping ratio, consistent with existing literature. Furthermore, the results confirm that isotropic consolidation, while differing from natural anisotropic conditions, yields trends comparable to those documented for similar sands. This research highlights the critical role of effective stress and relative density in controlling sand behavior under cyclic loading and emphasizes the initial consolidation phase as a key factor in seismic design. The findings improve predictive modeling of liquefaction potential and site-specific responses. Moreover, the identified trends provide a solid foundation for future investigations into the micromechanical behavior of sands under dynamic loading conditions.