The purpose of this paper is to (i) review field data on stress‐induced permeability changes in fractured rock; (ii) describe estimation of fractured rock stress‐permeability relationships through model calibration against such field data; and (iii) discuss observations of temperature and chemically mediated fracture closure and its effect on fractured rock permeability. The field data that are reviewed include in situ block experiments, excavation‐induced changes in permeability around tunnels, borehole injection experiments, depth (and stress) dependent permeability, and permeability changes associated with a large‐scale rock‐mass heating experiment. Data show how the stress‐permeability relationship of fractured rock very much depends on local in situ conditions, such as fracture shear offset and fracture infilling by mineral precipitation. Field and laboratory experiments involving temperature have shown significant temperature‐driven fracture closure even under constant stress. Such temperature‐driven fracture closure has been described as thermal overclosure and relates to better fitting of opposing fracture surfaces at high temperatures, or is attributed to chemically mediated fracture closure related to pressure solution (and compaction) of stressed fracture surface asperities. Back‐calculated stress‐permeability relationships from field data may implicitly account for such effects, but the relative contribution of purely thermal‐mechanical and chemically mediated changes is difficult to isolate. Therefore, it is concluded that further laboratory and in situ experiments are needed to increase the knowledge of the true mechanisms behind thermally driven fracture closure, and to further assess the importance of chemical‐mechanical coupling for the long‐term evolution of fractured rock permeability. This paper reviews stress‐induced permeability changes in fractured rock observed from field data, including effects of temperature and chemically mediated fracture closure. While the stress‐permeability relationship of a rock mass might be bounded from site specific field investigations, it is concluded that further laboratory and in situ experiments are needed to increase the knowledge of the true mechanisms underlying thermally driven fracture closure, and to further assess chemical‐mechanical coupling effects on the long‐term evolution of fractured rock permeability.
Summary Scanning electron microscopy/energy dispersive X‐ray spectrometry (SEM/EDS) is a widely applied elemental microanalysis method capable of identifying and quantifying all elements in the periodic table except H, He, and Li. By following the “k‐ratio” (unknown/standard) measurement protocol development for electron‐excited wavelength dispersive spectrometry (WDS), SEM/EDS can achieve accuracy and precision equivalent to WDS and at substantially lower electron dose, even when severe X‐ray peak overlaps occur, provided sufficient counts are recorded. Achieving this level of performance is now much more practical with the advent of the high‐throughput silicon drift detector energy dispersive X‐ray spectrometer (SDD‐EDS). However, three measurement issues continue to diminish the impact of SEM/EDS: (1) In the qualitative analysis (i.e., element identification) that must precede quantitative analysis, at least some current and many legacy software systems are vulnerable to occasional misidentification of major constituent peaks, with the frequency of misidentifications rising significantly for minor and trace constituents. (2) The use of standardless analysis, which is subject to much broader systematic errors, leads to quantitative results that, while useful, do not have sufficient accuracy to solve critical problems, e.g. determining the formula of a compound. (3) EDS spectrometers have such a large volume of acceptance that apparently credible spectra can be obtained from specimens with complex topography that introduce uncontrolled geometric factors that modify X‐ray generation and propagation, resulting in very large systematic errors, often a factor of ten or more. SCANNING 35: 141‐168, 2013. Published 2012 Wiley Periodicals, Inc.