3rd Croatian Conference on Earthquake Engineering 3CroCEE

The dynamic behaviour of sand and sand mixtures containing low-plasticity fines at low confining stresses is of crucial importance for geotechnical engineering, especially in regions prone to seismic activity. Such materials can be found in nature or produced in the laboratory. For these mixtures, the accurate assessment and representation of the initial void ratio, defined by the phase relationship or intergranular void ratio, is essential for understanding their cyclic behaviour. The initial void ratio has a direct influence on the response of sand-fine aggregate mixtures under cyclic loading, especially when fine aggregate fills the voids between the sand grains.
Although the cyclic shear threshold for pore water pressure tends to increase with higher plasticity indices, the behaviour of sand mixtures with low plasticity fines remains highly susceptible to liquefaction at low confining pressures. These mixtures exhibit a rapid build-up of pore water pressure and undergo faster cyclic degradation under seismic loads. The problem becomes particularly apparent when the void ratio is calculated using standard phase relationships, which may not fully capture the complex interactions between sand and fines.
This paper presents preliminary findings on the role of fines in filling the voids between sand grains and investigates the behaviour of sand-kaolin powder mixturesunder dynamic conditions. A series of cyclic triaxial tests were conducted under undrained conditions using both strain and stress-controlled approaches as well as resonant column tests. The results highlight the critical role of intergranular void ratio and effective confining stress in determining the susceptibility to liquefaction and overall cyclic behaviour of sand and sand-kaolin mixtures under low confining stresses. These results provide important insights into the factors influencing soil stability in seismic environments and emphasise the need for further research on the interaction of sand and low-plasticity fines.