Abstract
Underground rock masses have shown a general trend of natural balance over billions of years of ground movement. Nonetheless, man-made underground constructions disturb this balance and cause rock stability failure. Fractured rock masses are frequently encountered in underground constructions, and this study aims to restore the strength of rock masses that have experienced considerable fracturing under uniaxial compression. Coal and sandstone from a deep-buried coal mine were chosen as experimental subjects; they were crushed by uniaxial compression and then carefully restored by a chemical adhesive called MEYCO 364 with an innovative self-made device. Finally, the restored specimens were crushed once again by uniaxial compression. Axial stress, axial strain, circumferential strain, and volumetric strain data for the entire process were fully captured and are discussed here. An acoustic emission (AE) testing system was adopted to cooperate with the uniaxial compression system to provide better definitions for crack closure thresholds, crack initiation thresholds, crack damage thresholds, and three-dimensional damage source locations in intact and restored specimens. Several remarkable findings were obtained. The restoration effects of coal are considerably better than those of sandstone because the strength recovery coefficient of the former is 1.20, whereas that of the latter is 0.33, which indicates that MEYCO 364 is particularly valid for fractured rocks whose initial intact peak stress is less than that of MEYCO 364. Secondary cracked traces of restored sandstone almost follow the cracked traces of the initial intact sandstone, and the final failure is mainly caused by decoupling between the adhesive and the rock mass. However, cracked traces of restored coal only partially follow the traces of intact coal, with the final failure of the restored coal being caused by both bonding interface decoupling and self-breakage in coal. Three-dimensional damage source locations manifest such that AE events are highly correlated with a strength recovery coefficient; the AE events show a decreasing tendency when the coefficient is larger than 1, and vice versa. This study provides a feasible scheme for the reinforcement of fractured rock masses in underground constructions and reveals an internal mechanism of the crushing process for restored rock masses, which has certain instructive significance.
Highlights
Thanks to continuous sedimentation effects and tectonic movements that have occurred on the scale of billions of years, the Earth’s strata show a general trend of stability, even though seismic activities and volcanism are dispersed worldwide
Different acoustic emission (AE) parameters can be used to evaluate the propagation from inherent microscopic fissures to macroscopic cracks in rock masses, such as the AE count, AE energy, amplitude, rise time, and duration time
Rock masses “talk” when they experience stress, and humans can “listen” to the sounds of fissures growing with the aid of AE equipment
Summary
Thanks to continuous sedimentation effects and tectonic movements that have occurred on the scale of billions of years, the Earth’s strata show a general trend of stability, even though seismic activities and volcanism are dispersed worldwide. Artificial underground excavations within the earth significantly redistribute the strata’s stresses, and these excavations can vary according to their different construction purposes, such as radioactive waste disposal, underground hydraulic projects, tunnels, subways and shallow-burial underground facilities. The stability of tunnels, subways and shallow-burial underground facilities should be sustained for multiple generations, and dynamic monitoring for more than a century for a nuclear waste repository is fairly normal [1,2,3]. Leaving the free deformation of the surrounding rock alone for a certain amount of time after a specified space is shaped by an integrated excavation is recommended. The initially high pressure in the peripheral strata is released first; subsequently, the support scheme is used to restrain further deformation of the rock mass. The support structures generally comprise bolts, cables, U-shaped supports, steel nets, or different combinations
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