# Study of Static and Dynamic Properties of Sand under Low Stress Compression
## 1 Introduction  
Numerous studies of the acoustic emission (AE) phenomenon make it possible to understand that the AE signals are small-scale events whose properties are similar to the large-scale radiation of an elastic wave caused by earthquakes [1; 2; 3; 4; 5; 6; 7; 8; 9]. It has been shown that the AE time sequence parameters correspond well with the evolution of the rupture events [2; 3; 10; 11]. Presently, AE is the standard method for the assessment of the material stress condition [12; 13; 14]. [PERSON] et al. [15] reviewed six mechanisms of AE, part of which were more or less common for different materials, while others were inherent to some specific materials and conditions. For example, AE produced in soils (when they are dry) can be associated with crack development, grain friction, grain cementation fracture (for cemented soils) and liquid bridge rupture (when the soil is wet or saturated). AE studies during soil loading in laboratory conditions are less common than studies during rock destruction ([16] and references therein). It is known that sand samples under load behave in three regimes [17]--the pre-yield, the elastic and the stiffness regain--while the mechanism of deformation depends on the load level. For example, during the pre-yield regime, the sand stiffness increases due to rolling and sliding between particles, following a more rigid arrangement of particles. [PERSON] et al. [18] studied three types of coarse sand samples with a low height to sample diameter ratio (HD = 0.3). The AE was characterized by counting the number of times when the signal amplitude exceeded a given threshold. Experiments have shown that the obtained AE signals can be used to detect the onset of the elastic and stiffness regain regimes [17]. [PERSON] et al. [19] noted that the generation of lower frequency AE components (below 100 kHz), medium-high frequencyAE components (100-200 kHz) and high-frequency components (200-700 kHz) during sand compression resulted from different modes of micromechanical behaviors of particle readjustment, asperity abrasion and microcracking, respectively. [PERSON] et al. [20] studied sandy soils subjected to triaxial compression. The AE hit number and AE hit rate were used as the AE parameters. A similarity was observed between the relation's stress vs. strain and the AE hit rate vs. strain. It was shown that an increased confining stress or initial relative density caused more AE signals. The authors noted the effect of the failure mechanism under the same values of confining stress: more AE signals were recorded in loose sands that failed due to swelling than in densely compacted sands which failed due to shear banding. Note the discrepancy between the results by [PERSON] et al. [20] and those by [PERSON] et al. [16]. It has also been found that the peak of the AE hit rate was not always synchronous with the peak of the stress but fluctuated around it. [PERSON] et al. [21] studied the AE behavior during soil compression using an oedometer. The sandy samples (Hd\(\approx 1\)) including 12% and 25% water content were studied. A close correspondence was found between the number of AE hits, their energy and the soil strain-stress conditions. It was noted in [22] that AE parameters vary significantly depending on the material properties, the strength of the signal and background noise, among other factors. Thus, it is difficult to select the most valuable parameter (or combination of parameters) for the characterization of a failure process. Alternative non-destructive techniques which are generally utilized for microfracturing characterization in granular media are 3D microscopy and X-ray or neutron tomography imaging [23]. These methods allow for reaching results with high spatial resolutions, but their application is very limited at the geophysical scale due to intrinsic restrictions in the examination depth, their use only in the postpone mode, their high time consumption and their significant cost.