Using geophysical data to quantify stress-transmission in gap-graded granular materials

The behaviour of gap-graded granular materials – that is, mixtures of coarse and cohesionless finer grains with measurable differences in particle size – does not always conform to established frameworks of sand behaviour. Prior research has revealed that the role of the finer particles on the stress–strain response, liquefaction resistance and internal stability of non-cohesive gap-graded soils is significant and complex, and highly dependent on both the volumetric proportion of finer particles in the material and the size ratio of coarse particles to finer particles. Quantifying the participation of the finer particles in stress transmission and overall behaviour is central to understanding the behaviour of these materials. No experimental technique for directly quantifying the contribution of finer particles to the overall behaviour, however, has hitherto been proposed. This paper explores to what extent the participation of finer particles can be assessed using laboratory geophysics, recognising that granular materials act as a filter to remove the high-frequency components of applied seismic/sound waves. Discrete-element method simulations are performed to understand the link between particle-scale stress transmission and the overall response observed during shear wave propagation. When the proportion of finer particles is increased systematically, both the shear wave velocity (VS) and low-pass frequency (flp) increase sharply once a significant amount of the applied stress has been transferred by way of the finer particles. This trend is also observed in equivalent laboratory experiments. Consequently, the flp–VS relationship can provide useful insights to assess whether the finer particles contribute to stress transmission and hence the mechanical behaviour of gap-graded materials.