The undrained dynamic behaviour of gap-graded soils was investigated through a series of strain-controlled cyclic triaxial tests. Specimens with varying fines content ( \(\:{F}_{c}\) ) were subjected to different levels of drained static preshearing prior to undrained cyclic loading. The effects of \(\:{F}_{c}\) , initial static shear stress ratio ( \(\:\alpha\:\) ), and cyclic strain amplitude ( \(\:{\epsilon}_{a,amp}\) ) on key dynamic response parameters, such as liquefaction resistance, strain energy density, secant modulus and damping characteristics, were evaluated. The increase in \(\:\alpha\:\) enhanced peak deviatoric stress, demonstrating increased shear resistance due to stress-induced anisotropy. A gentle improvement in liquefaction resistance was seen as \(\:{F}_{c}\) increased to 10%, followed by a slight decline and stabilisation at higher \(\:{F}_{c}\) , indicating an optimal \(\:{F}_{c}\:\) for cyclic stability. A power-law-based sigmoid function was proposed to describe the evolution of pore pressure ratio ( \(\:{R}_{u}\) ) with cycle number ( \(\:N\) ). Higher \(\:\alpha\:\) delayed pore pressure generation, particularly at low \(\:{\epsilon}_{a,amp}\) . Secant Young’s modulus ( \(\:{E}_{sec}\) ) degraded rapidly at high \(\:{\epsilon}_{a,amp}\) but gradually at low \(\:{\epsilon}_{a,amp}\) , with higher α specimens maintaining greater \(\:{E}_{sec}\) throughout cyclic loading. A transitional threshold in \(\:{E}_{sec}\) was observed at \(\:{F}_{c}\) = 20%, initially decreasing due to fines disrupting sand skeleton’s contact network before increasing as fines contributed to stress transmission.
