## 3 Test Stand and General Experimental Methodology  
### Description and Components of the Test Stand  
Once the soil sample has reached the laboratory, the testing analysis process is carried out on various test stands, including macroscopic evaluation of the physical properties of the soil or determination of the optimum moisture content (Proctor apparatus). In accordance with the subject of this article, the main test stands used in the experiment include a split Hopkinson pressure bar (SHPB) and a laser particle size analyzer.  
The SHPB test stand takes into account the known density, moisture content, and strain rate of the granular structure of the soil sample in order to determine the dynamic properties of the soil being analyzed and to understand the stress-strain relationship. The main essence of the Hopkinson technique is that it is possible to analyze the response of different materials (including soil samples) subjected to dynamic action. The dynamic tests, the effect of which is the main focus of this work, were carried out based on the SHPB test stand located in the Department of Military Engineering and Infrastructure in the Faculty of Civil Engineering and Geodesy, MUT. A schematic diagram of the SHPB is shown in Figure 5, and an actual view of the test stand is in Figure 6.  
Figure 4: Comparison of optimum moisture content w\({}_{\text{opt}}\) for samples: (a) \(f_{S|i+C|,1}=15.14\%\) and (b)/\(f_{S|i+C|,2}=20.48\%\) (the blue line is the volumetric density curve of the soil in relation to its moisture content, the red line is the horizontal drop from the Y axis of the maximum volumetric density value of the soil and the vertical drop to the X axis (moisture content) to determine the optimum moisture content, the green color indicates the point of inflection of the curve of the graph).  
Figure 5: SHPB test stand diagram with designations of important components \(\blacklozenge\) (reprinted from Ref. [50]).  
The following components of the SHPB test stand can be distinguished as important due to the specifics of the dynamic experiment methodology:  
\(\bullet\)-pneumatic bar projectile launcher [51];  
\(\bullet\)-design to ensure that the bars are axially aligned with each other;  
\(\bullet\)-bar projectile guide (launcher barrel);  
\(\bullet\)-bar projectile;  
\(\bullet\)-initiating bar;  
\(\bullet\)-set of measuring strain gauges;  
\(\bullet\)-soil sample casing (rigid ring);  
\(\bullet\)-soil sample under test;  
\(\bullet\)-transmitting bar;  
\(\bullet\)-damper (impact-absorbing element);  
\(\bullet\)-bar projectile speed measurement kit \(v_{0}\);  
\(\bullet\)-digital recorder with computer.  
The Bettersizer S3 Plus laser particle size analyzer was used to investigate the granulometric composition for each soil sample variant. The instrument combines the measurement methods of static light scattering and dynamic image analysis, thus offering the versatile ability to characterize particle size and shape in the range from 0.01 \(\upmu\)m to 3500 \(\upmu\)m. Analyses of changes in the granulometric composition of silty sand samples (as a result of dynamic bar projectile interaction within the SHPB stand experiments) were carried out based on the wet version of the grain size curve study in the Bettersizer S3 Plus laser particle size analyzer--the device is shown in Figure 7. It is located in the Department of Engineering and Military Infrastructure in the Faculty of Civil Engineering and Geodesy, MUT.  
Important components of the laser particle size analyzer include:  
\(\dagger\)-main analyzer core;  
\(\dagger\)-measuring chamber system (water-soil mixture input-output);  
\(\dagger\)-circulation tank;  
\(\dagger\)-circulation and dispersion system of the water-soil mixture;  
\(\dagger\)-distilled water tank;  
\(\dagger\)-computer with dedicated software.  
Figure 6: View of the SHPB test stand located in the Department of Military Engineering and Infrastructure of the MUT, with markings of crucial components \(\blacklozenge\).