Abstract
The dynamic failure behaviour of tunneling rock in the cold region where freezing-thawing frequently occurs is unclear. This study aimed to test and understand the damage characteristics of tunneling sandstone samples in the cold region via triaxial unloading test and acoustic emission (AE) technique. The sandstone samples were first subject to different cycles of freezing-thawing. Their stress-strain curves, deformation modulus, and the AE characteristics were then measured under triaxial unloading conditions and through the AE test. The results showed that the freezing-thawing treatment with less than 60 freezing-thawing cycles caused rather less damage compared to the triaxial unloading condition. For the samples subject to more severe freezing-thawing treatment, more cracks were produced. These cracks were not closed under small confining pressure during the triaxial test, causing weaker mechanical properties of samples. We also found that the freezing-thawing treatment had a significant deterioration on the mechanical properties of the sandstone samples when the number of freezing-thawing cycles exceeded a certain threshold (between 60 and 80 in this study). As the AE characteristics matched well with the key stages of the measured axial stress-strain curves and the deformation modulus that varied with the decreasing confining pressure, the AE characteristics can be potentially used to quantify the released energy of rock cracking and identify the critical damage phases during the tunneling engineering process.
Highlights
During the excavation of highly stressed rock mass, the rock mass is often in a state of unilateral or double-side unloading. e unloading condition causes the stress redistribution and disturbance, energy release, and rock damage in the forms of spalling and strain burst [1,2,3]. e stability of rock structures and the mechanical properties of rock masses could be impacted and altered [4]
In order to understand the damage characteristics of sandstone subjected to stress and energy release during the excavation in cold region, this paper first investigates the mechanical and acoustic emission (AE) characteristics of representative sandstone samples that were collected from the field and subjected to different freezing-thawing cycles and triaxial unloading test and AE test
After the freezing-thawing treatments, the cracking effect on the samples due to the increase of axial compression value could outweigh the resistance effect caused by the decrease of confining pressure
Summary
During the excavation of highly stressed rock mass, the rock mass is often in a state of unilateral or double-side unloading. e unloading condition causes the stress redistribution and disturbance, energy release, and rock damage in the forms of spalling and strain burst [1,2,3]. e stability of rock structures and the mechanical properties of rock masses could be impacted and altered [4]. The damage characteristics of rock (e.g., cracking and failure modes) in the excavation zone induced by the tunnel boring machine or blasting have been extensively investigated through experimental and numerical studies [5,6,7,8,9]. The rock is under freeze-thaw cycles, and its internal pores are subject to high pore pressure, causing damage on the microstructure of pores and on the whole mechanical properties of rocks [10,11,12]. The mechanical properties of the rocks under unloading state are rather different from those under loading state [2, 13, 14], and it is necessary to study the physical characteristics of the rock under unloading condition [3], those in the cold region. The interface contact between the loading platens and the rock sample was assumed to be frictionless, and the end friction effect on the damage behaviors was ignored
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