Strainburst Damage Assessment of Kannur Limestone Using Piezo Transducers

Strainburst is one of the most serious geological hazards in underground rock excavations, which results from concentration of excavation-induced tangential stress at the periphery of the opening. The rockburst tendency and intensity may differ for the complex geological features, geometry of opening and method of excavation. This paper attempts to simplify and investigate the complex strainburst evolution process using piezo transducers. For this to achieve, Kannur limestone is loaded in obedience to the predetermined true-triaxial stress path, at a displacement rate of 0.02 mm/s. The evolution process during the test is monitored using the transient acoustic waves produced by the initiated and propagating cracks from the progressive vertically loaded rock specimens. Acoustic emission signals are acquired by simple, cost-effective and reusable AE technique comprised of thin piezoceramic patches which record signals up to 24 kHz. The lead zirconate titanate patches used have high dielectric constant and low mechanical quality factor, which is apt for the monitoring. However, due to such design and placement of the piezo elements results in quite a bit of noise and resonance, which needs to be filtered later on in post-processing. AE hit rate, cumulative AE hit counts and AE energy provide a cognizance of the four stages of rupture, namely, calm period, random macrocracking, precursor event and complete failure of the specimen. Comparing the interpreted AE signals at different stages of loading can serve as an efficient method of early detection of strainburst. Based on the rockburst potential, strain energy exchange and acoustic emission, the research aims to provide easily understandable and adoptable guidelines for the design of early warning system near the periphery of an underground opening.