Crack Classification During Tunneling in Scale Model Testing Using Acoustic Emission

Abstract

ABSTRACT Detecting the damaged precursor of deep tunnel excavations using crack classification is essential for ensuring structural safety and durability since the cracking mode of a crack (tensile or shear) is connected to a rockburst. Acoustic emission (AE) is related to the mechanisms of spalling (or slabbing) and rockburst phenomena. Acoustic emission is a passive nondestructive technique that allows for classifying cracking modes by analyzing AE signals using mostly empirical relationships between rise time of amplitude (RA) and average frequency (AF). Using AF-RA plots for different rocks, transition lines could be set up to show the difference between tensile and shear cracks. The main objective of this study is to investigate the impact of excavation on tunnel spalling observed in a scale model test on a brittle synthetic. The scale model test used a cubical specimen 300 mm × 300 mm × 300 mm in size with a 51-mm diameter tunnel drilled through one of the specimen faces. The tunnel length is approximately 150 mm or half of the specimen length. Six AE WSα sensors were employed during the excavation to record the accompanying AE signals during and after tunnel excavation. The locations of micro-or mesoscale fracturing were identified, and a comprehensive analysis of the intensity, temporal-spatial distribution, and quantitative investigation of the crack classification mechanisms was conducted. A frequency–amplitude relationship of AE signals in an excavation at each stage was proposed. The lower (60-120 kHz) and the higher (120-290 kHz) frequency ranges were revealed by the fast Fourier transformation (FFT) analysis of the AE signals. The AE signals have a high frequency and low amplitude when the specimens are under relatively low loading. But, as the load increases, the AE signals shift more towards a signature of high amplitude. It is observed that there are much higher amplitude and lower frequency events close to the specimen spalling failure. Hence, it directly reflects the lower frequency range of the shear cracks, and the higher frequency range is the tensile cracks that occurred. It was found that more than 74% of the AE signals were found below the transition line, and shear cracks dominated the surface during excavation.