Tian, H. (RWTH Aachen University) | Ziegler, M. (RWTH Aachen University) | Kempka, T. (Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences) | Xu, N.-X. (China University of Geosciences)
Rocks often experience high temperatures (several hundred degrees Celsius) due to underground operations, such as deep geological disposal of nuclear waste, geothermal heat extraction, CO2 geological storage and underground coal gasification as well as deep mining. Laboratory studies have shown that mechanical properties such as compressive strength, tensile strength, elastic modulus, etc. of rocks such as granite, marble and sandstone are temperature and temperature-history dependent. Therefore, the conventional failure criteria may not be suitable enough under high temperature conditions. In the present study, a thermo-mechanical modified Mohr-Coulomb failure criterion is proposed based on the extensive review and interpretation of mechanical properties of granites exposed to high temperatures. The deduced criterion takes into consideration the effects of thermal damage and confining conditions. The numerical study indicates that the proposed criterion provides a higher quality depicting rock strength under high temperatures compared with the conventional Mohr-Coulomb criterion. Moreover, according to analyses of the behavior of other rock materials exposed to high temperatures, this criterion is also suitable for other rocks.
Rock-mechanical engineering in high temperature environments is of universal interests and a challenge to scientists and engineers of different disciplines. Rock mass may undergo high temperatures (several hundred degrees Celsius) in modern projects, such as deep underground nuclear waste disposal (Bergman 1980, Rutqvist et al. 2008), geothermal heat extraction (Zhao 2000, Zhao 2002), geological CO2 storage (Roddy & Younger 2010) and underground coal gasification (Burton et al. 2007, Kühnel et al. 1993), as well as deep mining (Zhou et al. 2005, He 2009). Under the action of high temperature, the micro-structures of rocks change significantly (Dwivedi et al. 2008), new micro-cracks are developed, and pre-existing ones are extended/widened (Den''gina et al. 1994). Meanwhile, various physical and mineralogical changes take place within these rocks.
This paper introduces the use of 3D Light Detection and Ranging (LiDAR) for measuring rock mass discontinuities and tunnel excavation profile details, based on a case study of the Raabstollen tunnel in eastern Styria, Austria. The basic survey procedure involves: (1) creating a comprehensive 3D LiDAR point cloud model (PCM); (2) forming detailed triangulated surface model (TSM) from the PCM; and (3) mapping of fracture network characteristics (discontinuity orientation, size, intersection and termination) using advanced digital processing techniques. The result is an actual discrete fracture network being mapped directly on the excavation surface, which facilitates evaluation of over- and underbreaks and provides a permanent digital archive for further analysis and evaluation. This case study shows that LiDAR surveys can provide high quality data for both geological documentation (especially rock mass structure) and excavation geometry.
Geo-spatial data representations of rock mass conditions encountered during tunnel construction are becoming increasingly common, as they have been found to facilitate technical and economical project success. However, the immediate installation of ground support at the working face gives the engineering geologist/ tunnel engineer limited opportunity to inspect and document the ground conditions, and rock mass conditions exposed in the crown area are often not directly accessible for close inspection and measurements. The increased application of remote characterization methods has greatly enhanced tunneling documentation. Over the last 10 years, digital photogrammetry soft-ware has evolved into useful mapping tools for underground excavation (e.g. Gaich et al 1998, 2005, Birch 2008). More recently, terrestrial Light Detection and Ranging (LiDAR), also referred to as 3D terrestrial laser scanning (TLS), has seen increased application in rock mass characterization studies (e.g., Kemeny & Turner 2008, Ferre-ro et al 2009, Lato 2010, Sturzenegger 2010, Liu & Kieffer 2011).