Analysis and interpretation of acoustic emissions in sands subjected to compression processes

Abstract

In this research, a series of compression tests have been carried out, under oedometric conditions, on sand samples prepared with different densities and moisture contents. In these tests, in addition to the usual measurements of the vertical effective stress and deformation of the sample, a series of acoustic emission (AE) sensors were used to monitor the acoustic signal parameters coming from inside the samples. A rather novel technique with great potential, but sometimes difficult to tackle due to the large amount of AE data generated. The Acoustic Emission Technique (AET), or simply Acoustic Emission (AE), is a non-destructive materials inspection method based on the transient elastic waves produced by the sudden release of mechanical energy in a material or structure. This method generates multiple AE events (hits) during the tests, making it unavoidable to use a series of acoustic parameters that allow us to describe the characteristics of the AE signals. This gives rise to a group of AE signal analysis techniques called Parameter Analysis (or parametric analysis). Before addressing the central objective of this research, which is none other than to determine, analyse and interpret the correlations that may exist between the state parameters (geotechnical variables) of the soil and the parameters characterising the AE coming from inside the samples (acoustic variables) when compressive stresses are applied to them, this work makes a small contribution to the field of knowledge of parametric analysis by proposing some novel acoustic parameters that provide an accurate description of the waveforms of the AE signals: earliness, transitoriness, and early transitoriness. The main advantage of these new parameters is that they are dimensionless, thus having less dependence on the type of sensors and equipment used during the tests than the traditional (basic or fundamental) parameters, and they describe relevant characteristics of the waveforms of the AE, such as how transient or continuous they are. In this thesis, an exhaustive correlation and regression analysis has been carried out to quantify the correlations between the geotechnical variables of the sands and the parameters that characterise the acoustic emissions generated when compressive stresses are applied to them. The results presented open an interesting horizon of possibilities since, as has been concluded, it is possible to infer the values of the geotechnical properties from the acoustic variables, by means of the regression functions obtained for each type of soil. In any case, the correlation and regression study carried out in this thesis provides, at the very least, a complementary tool in the determination of the mechanical properties of soils subjected to compression, although it may also be useful in those situations in which field monitoring of the geotechnical variables describing the stress-strain behaviour of the soil may be difficult, if not impossible.