# Dynamic Characterization of Cohesive Material Based on Wave Velocity Measurements
## 2 Methods and Materials
### Methods of Interpretation of BE Results  
The BE technique has many advantages, which have been mentioned so far, but the interpretation of BE results can often be questionable [18; 22; 27]. A number of methods for the analysis of bender element results are commonly used in the time or frequency domain [17; 28]. The time domain methods are classified as direct measurements, which use plots of electrical signals _versus_ time [19; 29]. These methods employ a time-based axis in order to identify the propagation time, hence [6]. In frequency domain approaches, on the other hand, the spectral breakdown of the signals is analyzed, and the shift in the phase angle between the trigger and response signal is calculated [19; 30]. The time domain techniques are generally simpler and more straightforward, as the travel time can be directly defined by studying the time interval between characteristic points in the transmitted and received wave traces. However, the frequency domain methods appear to be more detailed, since they employ the support of signal processing and spectrum analysis tools. Additionally, they enable automated data acquisition and processing [18]. It is important to note that no method of interpreting the BE test results is yet proven superior to the others [6; 19; 31]. Reliable determination of travel time is very significant, due to BEs being installed in laboratory equipment and their application for studying relatively small specimens, which means that the travel distance is quite small. Below, the authors briefly present the methods of determining the arrival time of the shear wave that were employed in the paper. These are further illustrated in Figure 5.  
The peak-to-peak method [19; 28] is based on the assumption that the received signal bears a high resemblance to the transmitted one. The travel time may be treated as the time elapsed between any two corresponding characteristic points in the signals. By doing this, the near field problems should be  
Figure 4: Non-desirable orientation of the top cap and base pedestal element. Figure 5: Typical identification techniques of travel time in the time domain method. reduced. The characteristic points that are most commonly used to identify the wave travel time are the first positive peak in the input of S-wave signals and in the output [32]. In some circumstances, however, properly defining the first major peak becomes quite difficult, as the received signal has several consecutive peaks, which differ slightly in amplitude. The reliability of choosing the right first peak as the first major one can be influenced by sample geometry and size or, for example, by the energy-absorbing nature of the soil. The arriving signal is then distorted to various extents due to the increase of damping with distance. The quality of received signals significantly affects this technique [28]. The principle of the start-to-start method, also known as the visual picking method, is the acceptance of the moment, when the first major deflection of the received signal occurs, as the shear wave arrival time [19; 33]. The presented method is characterized by simplicity, which is the main reason behind its popularity. Depending on the installation and polarity of the bender elements, the first important deviation from zero amplitude may be positive or negative [28]. In many cases, it is simply a single sine pulse. This represents the start of energy transfer from the source to the soil. However, in case of the output wave signal, it is the moment when the investigated receiver begins exhibiting motion. Said moment represents the instant of the energy transfer from the soil to the receiving BE [32]. During the bender element test, shear wave velocity is calculated from the simple measurement of propagation distance (\(\Delta\)s) and propagation time (\(\Delta t\)). Based on a number of previous works, it is generally accepted that the travel distance is the distance between the tips of two BEs [11].