The Mechanical Response of Wet Volcanic Sand to Impact Loading, Effects of Water Content and Initial Compaction

Abstract

The effects of water content and initial compaction on the dynamic response of volcanic sand from Mount Etna were investigated by a series of experiments on a long Split Hopkinson Pressure Bar apparatus capable of generating stress pulses of duration exceeding one millisecond. The dynamic stress–strain characteristics were determined until large final compressive strains were achieved. An experimental protocol for the preparation of samples characterised by different initial porosity and moisture content was defined in order to reproduce, in a laboratory environment, granular volcanic aggregates representative of naturally occurring soils in different initial density and water content states. It was found that, for limited amounts of water content, the dynamic response of the investigated volcanic wet sand is more compliant than in dry conditions. Conversely, highly saturated samples exhibit a steep increase in stiffness occurring at strains when the dynamic compressive behaviour becomes dominated by the response of the nearly incompressible water. The presence of water has negligible effect on the mechanical behaviour when the samples are loaded at quasi static strain rates. The grain size distribution and morphology of samples tested in different conditions were evaluated and compared by means of edge detection analysis techniques applied to high contrast images.

Experimental Study on Dynamic Compressive Behaviors of Sand under Passive Confining Pressure

“…Deformability increases with the water content on a macroscopic scale. However, the dynamic stress of low-saturated sand reported in [32,[49][50][51] tended to decrease compared with dry sand. No obvious decreasing trend was observed in the current study, which may have been related to the SHPB instrument used in the experiment.…”

“…For relatively low values of moisture content (w < 8%), the main functions hindering sand deformation were the friction and occlusion among particles. It is known that water occupies most of the contact surface of sand particles [49]. The lubricating effect of water reduces the interactions among the particles and the compression resistance.…”

Strength Characterization of Soils’ Properties at High Strain Rates Using the Hopkinson Technique—A Review of Experimental Testing

“…There is a greater tendency to conduct research in non-cohesive types of soil (mainly sandy) than in cohesive soils: Quartz sand—a particularly good analysis was performed in [ 75 ], where the conclusion is valuable that before full water-saturated (below 7.5%), additional water did not result in the stiffness of the loose soil. However, after full water-saturated (above 7.5%), additional water in the soil pores increased the stiffness of the loose soil; Calcareous sand—a very good analysis was performed in [ 78 ], where a valuable note is that after exceeding a certain limit value of the dynamic load, the influence of the initial pressure on the dynamic mechanical properties of the calcareous sand sample was reduced; Carbonate sand—a particularly good analysis was performed in [ 79 ], where the important conclusion is that the fracture mechanism depends on the level of stress values—the mechanism takes the form of attrition and abrasion for low stress values, but the mechanism for high stress values is fracture; Volcanic sand—a particularly valuable analysis was performed in [ 81 ], with a conclusion that a sample with a high percentage of water in the pores (water-saturated sample) has a significant dynamic reaction—water behaves like an incompressible material at the moment of dynamic impact, and there is a visible increase in the stiffness phenomenon at the strain of volcanic sand sample; Coral sand—a particularly good analysis was performed in [ 82 ], where an important observation is that the susceptibility of a given type of coral sand to the strain rate is significantly dependent and related to the internal structure of grains, soil pores and the phenomenon of inter-particle friction; Silty sand—a particularly valuable analysis was performed in [ 71 ], where a unique element of the work was that it carried out and showed the results through a particle size distribution test curve for two different types of silty sand samples subjected to the same bar-projectile impact. …”

“…Volcanic sand—a particularly valuable analysis was performed in [ 81 ], with a conclusion that a sample with a high percentage of water in the pores (water-saturated sample) has a significant dynamic reaction—water behaves like an incompressible material at the moment of dynamic impact, and there is a visible increase in the stiffness phenomenon at the strain of volcanic sand sample;…”

“…; Zhang, L.; Pellegrino, A. Order of Cited Paper: [ 81 ] Highlights/Abstract: Based on the split Hopkinson pressure bar test stand in the variant with an elongated bar, the soil was analyzed on the example of volcanic sand from Mount Etna in order to test the dependence of sample water moisture and the initial compaction coefficient to maintain dynamic soil subjected to a load by a projectile impact. The result of the experiment was the determination of, e.g., a graph of the dynamic dependence of stress–strains ultimately ending with a significant value of compressive strains.…”