## 3 Test Stand and General Experimental Methodology
### General Methodology for Conducting the Experiment  
The soil sample of the SHPB test is cylindrical in shape and is placed in a rigid casing made of different materials depending on the experimental setup, e.g., Plexiglas, duralumin, or steel. The dimensions and material characteristics of the casing ensure that the uniaxial deformation state of the tested soil occurs when the specimen is pressed from both sides by the initiating and transmitting bars. According to the markings in Figure 6 and the methodology for conducting the SHPB experiment, the bar projectile load is propelled inside the bar projectile guide by rapidly released air pressure from a pneumatic launcher. The laser timing system takes a reading of the time it takes for the bar projectile to move a known distance. Knowing this time allows the impact velocity to be determined. When the bar projectile strikes the initiating bar, a compressive elastic wave is generated in the bar (the incident wave generates deformations in the initiating bar \(\varepsilon^{1}\)), propagating along the bar toward the soil sample surrounded by the casing element (rigid ring). On reaching the end of the initiating bar, the wave partly continues to propagate (the transmitted wave generates deformations in the transmitting bar \(\varepsilon^{T}\)) through the sample towards the transmitting bar and partly is reflected and starts to return to the beginning of the initiating bar (the reflected wave generates deformations in the initiating bar \(\varepsilon^{R}\)). The wave that has passed through the sample and reached the transmitting bar at its end is finally attenuated in the damping element. The signals from the set of measuring strain gauges are recorded by a digital recorder and transmitted to a computer. The investigation in the laser particle size analyzer (Figure 7) begins by flushing the unit system and pumping a new portion of the liquid from the distilled water tank into the circulation tank. Once the soil sample has been transferred to the circulation tank, the water-soil mixture flows through the circulation and dispersion system. During the measurement, the particles dispersed in the selected medium are pumped through a set of two measuring chambers within the main core of the analyzer. In the first measuring chamber, short-wave laser light (532 nm) passes through the sample and is scattered according to the particle size. The 96 detectors detect optical signals over an angle range of 0.02-165\({}^{\circ}\). Meanwhile, in the second measurement cell, CCD cameras (with 0.5\(\times\) and 10\(\times\) magnification) continuously take images to provide particle image analysis in the range of 2 m to 3500 m. An additional lens between the laser and the sample cell (lens 2) converts the diverging laser beam into a parallel beam and enables the detection of back-scattered light. The evaluation of the fine particle size distribution is based on the Fraunhofer or Mie scattering theory, which depends on the particle size and optical properties. Mie scattering theory enables an accurate  
Figure 7: View of the Bettersizer S3 Plus laser particle size analyzer test stand located in the Department of Military Engineering and Infrastructure of the MUT, with markings of crucial components. description of the laser scattering law of particles--it is a widely accepted theory in modern laser particle size analysis. Fraunhofer diffraction theory is a simplified version of Mie theory--relatively rarely used, mainly applied to describe the laser scattering law of relatively coarse particles. Mie theory is adopted for all bulk materials over a wide range of grain diameters. The Fraunhofer method is more dedicated to the analysis of coarse-grained powders. In the present work, Mie theory is adopted for further analysis. The methodology described is in accordance with the ISO standards on the subject [52; 53; 54].