Degradation of fully saturated uniform sand subjected to small-strain undrained cyclic shearing

In soil dynamics, cyclic tests on sands have been extensively studied over the past several decades. Among the natural materials most susceptible to strength loss due to earthquakes, sands are commonly tested under varying loading, frequency, and drainage conditions. Traditionally, it has been assumed that pore pressure increases with constant strength loss once the threshold for pore pressure build-up is reached. However, recent studies have revealed that at small strains, the material initially hardens despite the generation of pore pressure. This paper presents the response and degradation of uniformly graded Drava River sand (DrOS018), similar to well-known sands such as Toyoura, Nevada or Ottawa sands, and the initial hardening phenomena that occur around threshold strains. Tests were conducted using a triaxial cyclic device at three relative densities and cell pressures (100 kPa, 200 kPa, and 400 kPa) under undrained conditions. Strain-controlled tests were conducted at 0.1 Hz and 0.05 Hz using sinusoidal loading, with samples prepared by under-compaction. After crossing the threshold, the sand initially shows hardening (degradation index greater than 1) with up to a 35% increase in pore pressure, followed by strength degradation at higher strains. This study is critical for seismic design and safety, particularly for fully saturated sands in coastal and high water table areas. The findings enhance our understanding of liquefaction potential and site response, aiding more informed engineering practices by contributing to enhanced knowledge in soil dynamics and improved predictive models. The results support effective mitigation strategies and infrastructure resilience in earthquake-prone regions such as Croatia.