To explore the failure precursors of hard rock, a series of triaxial loading and unloading experiments were carried out on sandstone\nsample using the acoustic emission systems.Theextreme-point symmetricmode decomposition method (ESMDmethod) was used\nto denoise and reconstruct the AE data.The AE quiet period in Scheme I becomes much more obvious with the confining pressure\nincreasing, which can be regarded as the precursor information of the sample failure under conventional triaxial compression.\nUnlike Scheme I, there are no obvious precursory characteristics before failure in Schemes II and III, and the count rate reaches\nthe maximum at the peak point. When the stress ratio ranges from 0.8 to 1.0, the fractal values of acoustic emission can be used\nto investigate the failure precursors of samples at a lower confining pressure.When the time ratio is greater than 0.8 under higher\nconfining pressures, the fractal values of sandstone samples under unloading paths are rapidly reduced,which can be used to predict\nrock failure at higher confining pressures.
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