Scale-model investigation of brittle tunnel failure using a true-triaxial device
Brittle instabilities in tunnel excavation have led to severe consequences such as spalling, violent rock ejection or rock burst, and tunnel collapse. Detached rocks from the tunnel boundary not only compromise the structural integrity of tunnels but also pose significant safety risks to personnel and construction equipment, disrupting the operations. Over the past decades, in-situ studies and laboratory tests have contributed to understanding the brittle failure mechanisms. However, the factors contributing to the progression and extent of brittle failure damage remain not fully understood, limiting the predictive and preventive capabilities. This dissertation aims to improve the understanding of tunnel brittle instabilities through laboratory experiments. The research focused on (1) developing an analog brittle rock model, (2) performing a series of tunnel model tests using a true-triaxial cell, a mock miniature tunnel boring machine (TBM), and acoustic emission (AE) monitoring, and (3) proposing more robust analytical methods and predictive models for brittle failures. The analog rock was designed to mimic the brittle behavior of sedimentary rock while maintaining a low uniaxial compressive strength (UCS). This combination of weak and brittle response was vital, as it allowed for using a lower-capacity true-triaxial loading apparatus and enabled large specimens, making it accessible to observe and document the progressive failure inside the excavated tunnel. The true-triaxial setup and mock miniature TBM allowed for realistic tunneling simulations, capturing unloading stress paths and face support behavior. Damage progression observation and post-mortem investigation provide detailed identification of damage mechanisms around the tunnel boundary, such as surficial spalling and damage zone development, as a V-shaped notch formed from progressive shear fracturing and resembled brittle-to-ductile transition. Real-time data on microcracking progression was captured using acoustic emission (AE) sensors during the loading stage, revealing potential precursors on brittle failure onset. Additionally, triaxial extension (TE) tests were introduced to improve the accuracy of spalling predictions. These tests provided a more representative biaxial stress state and unloading conditions in spalling that proved more robust than conventional triaxial compression (TC) tests, providing factors such as the entry angles and damage depth. In this study, the thin spalling with a steep angle measured from the minor principal stress direction in the tunnel model provided evidence of shear-driven fractures rather than extensional fractures. While steep angles may resemble extensional fractures, appearing perpendicular to minor principal stress, this likely results from shear-induced dilation along the fracture surface, triggering the mobilization of friction angle. Another novel contribution of this work is the development of quadratic Bézier curves in capturing the successive progression of fracturing based on damage factors and breakout width. This method offers a more robust representation of tunnel damage under anisotropic stress conditions and improves the theoretical logarithmic spiral plastic slip lines that rely solely on friction angle. In conclusion, this research improves the understanding of brittle failure mechanisms in tunneling, contributing to safer and more reliable tunnel designs in brittle rock environments. The developed experimental methods and predictive models offer valuable tools for mitigating the risks of spalling and rockburst during underground excavations.
brittle-to-ductile transition, geotechnical hazard, predictive models for brittle failure, rock mechanics, true-triaxial testing, tunnel brittle instabilities
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Colorado School of Mines. Arthur Lakes Library