## 3 Test Stand and General Experimental Methodology
### Process of Preparing a Soil Sample for Testing  
In the process of preparing a non-cohesive soil sample for dynamic testing using the SHPB test stand, an important step is to ensure the initial compaction state of the soil. In particular, this is an important step of the preparation process for dry and low moisture content samples. Such a sample is in a two-phase or three-phase state--it has a gas phase (usually in the form of air). Failure to carry out the compaction process will result in incorrect experimental results-some of the impact energy of the bar projectile will be used to compact the air voids. The soil sample is cylindrical in shape, and its dimensions must comply with the condition [56] in Equation (1):  
\[L=d\cdot\sqrt{\frac{3\cdot\
u}{4}}, \tag{1}\]  
where \(L\)--length of soil sample tested;  
\(d\)--diameter of soil sample tested;  
\(\
u\)--Poisson's ratio of soil sample tested. Figure 8: Detailed diagram of a soil sample test procedure on the Bettersizer S3 Plus laser particle size analyzer located in the Department of Military Engineering and Infrastructure of the MUT (reprinted from Ref. [55]). Using approximations of the calculated values, it is possible to determine the general approximation condition [57] in Equation (2):  
\[L\approx 0.5\cdot d. \tag{2}\]  
The diameter of the soil sample corresponds to the diameter of the bar projectile and the measuring bars (initiating and transmitting). The indicated elements of the SHPB test stand in the Department of Military Engineering and Infrastructure of the MUT have a diameter of 20 mm--based on Equation (2), the length of the soil sample is approximately 10 mm. In the experimental methodology of SHPB testing, an important step is the selection of a suitable impulse shaper to ensure dynamic stress equilibrium. The introduction of an additional impulse shaper into the SHPB test stand system between the bar projectile and the initiating bar allows for a longer time of rising impulse, which enables the effect of better impedance matching and homogeneous deformation of the test material. The pulse shaper is usually a small and thin disc made of soft material--it can be made of aluminum, brass, rubber, nylon, copper, or stainless steel, about 0.1-2 mm thick [58]. During testing, the bar projectile strikes the impulse shaper in front of the initiating bar, thus generating a non-dissipative impulse propagating through the initiating bar. This pulse, with its slowly rising front, facilitates dynamic balancing of forces on the sample--a slowly rising pulse is the preferred loading pulse to minimize the effects of dissipation and inertia, thereby facilitating dynamic stress balance in the sample. The task of the pulse shaper is to ensure a constant strain rate during loading and to maintain force balance throughout the specimen. By changing the geometry of the pulse shaper, a variety of different pulses incident on the specimen can be produced. Depending on the materials being tested, different loading pulses are needed, which can be achieved by designing the shaper accordingly. The dynamic stress equilibrium condition is shown in Equation (3):  
\[\varepsilon^{I}(t)+\varepsilon^{R}(t)\cong\varepsilon^{T}(t); \tag{3}\]  
where \(\varepsilon^{I}\)--initiated wave (incident impulse);  
\(\varepsilon^{T}\)--transmitted wave (transmitted impulse);  
\(\varepsilon^{R}\)--reflected wave (reflected impulse). Taking into account the addition and negative signs of the \(\varepsilon^{I}\), \(\varepsilon^{T}\), and \(\varepsilon^{R}\) graphs, it is possible to check the fulfillment of the dynamic stress equilibrium condition according to Equation (3) by comparing the changes in the course of the graphs. An example of such a graph is shown in Figure 9--it is possible to compare the correspondence of the course of the blue graph \(\varepsilon^{I}\) and the black graph with the dashed line \(\varepsilon^{T}-(-\varepsilon^{R})\), where final form is \(\varepsilon^{T}+\varepsilon^{R}\).