Analysis of the Influence of Silty Sands Moisture Content and Impact Velocity in SHPB Testing on Their Compactability and Change in Granulometric Composition

Abstract

This paper presents the results of a test cycle of two types of silty sand (siSa) with different contents of fine fractions. Fine fractions are understood as soil grains with a grain diameter of less than 63 µm (as the sum of silt and clay fractions). The soils tested had a content of fine fractions of fSi+Cl,1 = 15.14% and fSi+Cl,2 = 20.48%, respectively, before the study. Changes in the content of these fractions after the experiments were analyzed. These experiments consisted of dynamic bar projectile impact loading, and a split Hopkinson pressure bar (SHPB) test stand was used in the study. Changes in the granulometric composition of the silty sands studied were carried out in a laser particle size analyzer, allowing measurement of fractional content in the grain size range from 0.01 µm to 3500 µm. As a result, a summary of changes in soil grain size curves in the range of fine fractions was compiled. Repeated trends were observed in the changes in the granulometric composition of the soil samples as a function of the moisture content of the soil sample (w1 = 0%, w2 = 5%, w3 = 10%, and w4 = 15%) and the impact velocity of the loading bar projectile for SHPB pneumatic launcher pressures (p1 = 1.2 bar → v1 = 12.76 m/s, p2 = 1.8 bar → v2 = 17.69 m/s and p3 = 2.4 bar → v3 = 21.32 m/s). The influence of the initial moisture content of the investigated soil on the value of the optimum moisture content obtained during its dynamic compaction was discussed. The trend in the behavior of the change in the granulometric composition of the tested samples was determined, taking the value of the initial moisture content of the soil in relation to the optimum moisture content of the reference sample as a reference. The largest percentage change in granulometric composition through an increase in the value of the silt and clay fraction relative to the reference sample fSi+Cl for both types of silty sand tested occurs for the same moisture content variant w2 = 5%–for soil fSi+Cl,1 = 15.14% there is an increase in the fine fraction of 11.08% and for soil fSi+Cl,2 = 20.48% there is an increase in the fine fraction of 15.17%. In general, it can be seen that more silty soil is more strongly susceptible to the phenomenon of grain crushing for moisture content w1 = 0% and w2 = 5% less than its optimum moisture content wopt,1 = 8.70%. In contrast, less silty soil is more susceptible to the phenomenon of grain crushing for moisture contents w3 = 10% and w4 = 15% greater than its optimum moisture content wopt,2 = 9.20%. The presented dynamic physical phenomenon of soil behavior is crucial during explosive and impact impacts on structures made of soil, e.g., as ground protection layers.