The features of the V-shaped behavior of the AE parameters can be understood by considering an example of three vibration diagrams (Fig. 5a-c) that show the changes of vibration parameters during dynamic loading of dune sand sample (Fig. 5a shows the displacement chart while Figs. 5b and 5c show the charts of vibration velocity and acceleration, respectively). The liquefaction state is created 10 seconds after the start of oscillations (”0” on the X-axis – like in Figs. 3,4). The displacement diagram can be characterized by three key features as follows: a. The amplitude of the displacement increases until a liquefaction condition is created while the slope of the graph remains unchanged, b. The slope of the displacement curve changes when the liquefaction conditions are created, c. The displacement amplitude remains quite unchangeable until the end of the test. As for the velocity and acceleration charts, their amplitude reaches its maximum value when the liquefaction state is created and does not change significantly after the liquefaction point, as does the displacement behavior. Note the similarity between the presented above parameters and the values of PGD, PGV, and PGA (3.7*10− 4-1.0*10− 7, 1.6*10− 3-8.3*10− 7, 4.4*10− 2-1.2*10− 5, respectively) presented in the Earthquake Catalog28.
A comparison of three stages of displacement, velocity, and acceleration charts (Fig. 5a, b, c, respectively) with the three-phase behavior of AE parameters (Figs. 3,4) portrays that the first stage in Fig. 5 (the stage of amplitude increase in displacement, velocity, and acceleration corresponds to phase A - the phase of increase in AE parameters value - Figs. 3,4). The zone before the breaking point in graphs of displacement, velocity, and acceleration (Fig. 5) nearly corresponds to Phase B where the AE is not caused (so-called the zone of silence, Fig. 4b). Stage 3 in Fig. 5 (the stage of the unchangeable amplitude of displacement, velocity, and acceleration as well as the stage characterized by the slope change in displacement curve that is much bigger than in Stage 1 and the slope of velocity and acceleration curves quite close to 0) corresponds to Phase C where when the magnitude of AE parameters increases again.
It can be assumed that such behavior of AE in phase A is due to microfractures/displacements between sand grains caused by an increase in pore pressure. Phase B reflects the equality between pore pressure and cell pressure. The behavior of AE in phase C can be explained by intense friction between sand grains during their movement caused by liquefaction. Note that, despite the obvious signs of the V-shaped behavior of the AE excitation relative to the liquefaction point, changes in the AE parameters depend on the composition of the sand. This feature can be explained by the difference in friction between sand grains of varied sizes, which is manifested in the unequal behavior of three geo-mechanical parameters. As can be seen (Fig. 2), the behavior of the B factor, as well as the values of the effective stress and shear, are qualitatively similar. The values of the B factor increase to a value of 1, while the values of the effective stress and shear stress decrease significantly when the state of liquefaction is reached. The value of the rate of three parameters increases with approaching the liquefaction state and decreases after the liquefaction point. However, the rate of change of these three parameters is different for each type of sand and is determined by its granular composition.
The absolute value of the rate increases with decreasing grain size, while their highest value is noted for dune sands, which are a mixture of the three studied fractions.
The noted above similarity in behavior of AE parameters (asymmetrical "V" type) implies that the change in the behavior of AE can indeed be an indicator of the approach to the liquefaction state.
Our previous studies24,25 showed that most AE parameters are interdependent apart from the lack of the inter-correlation between two parameters: the number of AE hits and the values of absolute energy of AE hits. Such a similar character of the behavior of the studied AE parameters during liquefaction is not unexpected, but, on the contrary, indicates the consistency of the present study with the previous ones24,25 conducted during the static loading.
The results of our vibration experiments with four different sand types show that if the pore pressure exceeds 80% of the value of effective stress, a liquefaction state will inevitably be reached, and the number of vibrations required to reach this state (or in other words, the time required to reach the liquefaction state) is highly dependent on the granular composition of the sand. For example, the time necessary to create a liquefaction state in the dune sand is only 10 seconds. This finding means that a magnitude 6–7 earthquake will cause liquefaction of dune sand. The above results are consistent with the previous ones1. The changes in the sand composition from the poorly graded dune sand to "extremely poorly graded sand" (consisting of a very thin range of grain sizes e.g., 2.36–0.6 mm, 0.6 − 0.3 mm, and 0.3-0.075 mm) significantly increases the time for the creation of liquefaction state. Moreover, the larger the grain size, the longer the time needed to reach the liquefaction state. Since the pore diameter is often estimated to be 20% of the D10 size (the grain size corresponding to 10% sieve passing) and since the soil hydraulic conductivity is frequently considered to be related to the value of D10, the increase in D10 value means the increase in soil hydraulic conductivity and hence the longer time required to reach significant pore pressure in the coarser-grained sand. Since the time to reach the liquefaction state is related to the duration of the vibrations caused by the earthquake and the composition of the soil at the base of the structure, the above conclusion has the potential to increase the warning time by increasing the time to create the liquefaction state and therefore creating safer conditions for structures built in the marine environment. Changes in the AE parameters observed during the study indicate the possibility of developing an early warning system for the creation of liquefaction conditions at the base of structures in the marine environment, as well as applying the above parameters to assess the integrity of offshore structures due to soil liquefaction.
The circumstances of the influence of the composition of the sand on the symmetry/asymmetry of the letter "V", as well as its amplitude and aperture, are currently not clear and will be studied in further studies.