Transition of gap-graded soil fabric – shear wave measurements and dispersion relation

The dynamic response of gap-graded soils is complex; it requires massive experimental and theoretical investigation in order for a sound understanding to be developed. The complexity of the behaviour of such soils arises partially due to the presence of many influential parameters, such as the finer sand/silt content (F_s), size ratio (R_d), and void ratio (e). In this contribution, experiments are conducted on a series of gap-graded soil mixtures, where the coarser sand is mixed with four different finer sands or non-plastic silt with different sizes, namely various R_d, and one well-graded material is also tested. The S-wave velocity (V_s) and low-pass threshold frequency (f_lp) are measured in the isotropic stress state using a triaxial apparatus equipped with planar piezoelectric transducers. The experimental results reveal that, for a given e or relative density (D_r), both f_lp and V_s decrease at a low F_s, but increase at a high F_s. The slope of the linear regression between f_lp and V_s appears to capture this transitional behaviour, depending on F_s and R_d, which can then be used to correlate the physical state of the gap-graded soil fabric. The transitional behaviour is more noticeable at a larger R_d, which is in line with the existing literature. To understand the effect of the R_d on the shear wave propagation at a low F_s, the particle-scale dispersion relation for binary chains is newly introduced and correlated with accompanying experimental observations.