Effects of cyclic frequency on the dynamic response and liquefaction resistance of silica sands with different grain size distributions

This study investigates the effect of loading frequency on the liquefaction resistance of reconstituted sandy soils with different grain size distributions. Three sand samples (SAND1, SAND2, and SAND3) were tested under undrained cyclic loading using the cyclic direct simple shear (CDSS) test. The experimental program was performed at multiple cyclic stress ratios (CSRs) and loading frequencies ranging from 0.03 Hz to 0.5 Hz. The number of cycles to liquefaction (N_cyc-liq), excess pore water pressure generation, and cyclic resistance ratio at 15 cycles (CRR₁₅) were evaluated to assess the frequency-dependent liquefaction behavior. The results reveal a consistent trend across all three sand types, characterized by two distinct behavioral zones. In the low-frequency range (0.03–0.10 Hz), the influence of frequency on N_cyc-liq and CRR₁₅ is minimal. However, when the frequency exceeds 0.10 Hz, both parameters exhibit significant increases, indicating enhanced liquefaction resistance. This transition at 0.10 Hz was observed consistently, regardless of CSR level or soil type, and is proposed as a threshold frequency distinguishing a Stable/Minor-Effect Zone from an Increasing Resistance Zone. The findings underscore the importance of incorporating loading frequency into liquefaction assessments. The threshold frequency offers a useful reference for engineering calculations and seismic stability assessments of sandy soils, as it marks the boundary beyond which frequency effects must be carefully considered in liquefaction evaluation and design.