Experimental study on slabbing process of hard rock in deep tunnels

Using the perforated granodiorite specimens, the slabbing of hard rock in deep tunnels is successfully reproduced. The real-time monitoring system, optical microscope and acoustic emission (AE) system are used to investigate the slabbing process, V-shaped slabbing band, micro-fracturing mechanism, characteristic stresses, fragments and crack characteristics. The results indicate that the slabbing can be reproduced by the continuously loaded stress paths. The slabbing is mainly inoculated in the region of compressive stress concentration. From a microscopic perspective, the extensile and dilatant cracks, generated in the form of trans-granular cracking, continue to interact and coalesce and then intersect with the free surface at the angle of 33.42°~41.91°, forming a series of thin plates with the thickness between 101.38~120.9 μm near the potential V-shaped pits. Subsequently, the plates continue to bulge outward, buckle and fracture, and finally form an approximately symmetrical V-shaped band. In the early stage, the dominant frequency of AE is mainly concentrated in 260 ~ 340 kHz, and the amplitude is mainly concentrated in 40~60 dB. This stage is mainly characterized by the cracks with high frequency, low amplitude and small rupture. In the later stage, the AE with the low frequency of 25~75 kHz begins to appear, resulting in a signal distribution from 25 to 350 kHz, and the AE with high amplitude larger than 80 dB appears. This stage is characterized by the appearance of cracks with low frequency and high amplitude and the coexistence of cracks with low, medium and high frequencies and low, medium and high amplitudes. In the medium stage, the cracks with the frequency and amplitude between the two are produced.