Failure Behavior of Granite Affected by Confinement and Water Pressure and Its Influence on the Seepage Behavior by Laboratory Experiments

Abstract

Failure behavior of granite material is paramount for host rock stability of geological repositories for high-level waste (HLW) disposal. Failure behavior also affects the seepage behavior related to transportation of radionuclide. Few of the published studies gave a consistent analysis on how confinement and water pressure affect the failure behavior, which in turn influences the seepage behavior of the rock during the damage process. Based on a series of laboratory experiments on NRG01 granite samples cored from Alxa area, a candidate area for China’s HLW disposal, this paper presents some detailed observations and analyses for a better understanding on the failure mechanism and seepage behavior of the samples under different confinements and water pressure. The main findings of this study are as follows: (1) Strength reduction properties were found for the granite under water pressure. Besides, the complete axial stress–strain curves show more obvious yielding process in the pre-peak region and a more gradual stress drop in the post-peak region; (2) Shear fracturing pattern is more likely to form in the granite samples with the effect of water pressure, even under much lower confinements, than the predictions from the conventional triaxial compressive results; (3) Four stages of inflow rate curves are divided and the seepage behaviors are found to depend on the failure behavior affected by the confinement and water pressure.

Experimental Observation of the Onset of Percolation in Freshwater Granular Ice

“…As in previous studies, our results demonstrate that a modest degree of confinement, here provided by the increased friction along the ice/platen interface, is sufficient to transition the failure mode from axial splitting to Coulombic conjugate shear faulting (Cheng et al, 2017;Renshaw & Schulson, 2001;Schulson & Gratz, 1999;Wachter et al, 2009). And similar to the inferences from the injected dye studies of Stormont and Daemen (1992) and inferred from measured P wave velocity anisotropies (Ahrens et al, 2018), our dye results (Figure 4) also show the importance of secondary cracks aligned with the direction of the most compressive stress in creating a connected pathway for percolation.…”

“…Mitchell and Faulkner (2008) conjectured that the greatest permeability increases during deviatoric compressive loading likely occur in the process zone of a growing fault but did not link the permeability increases in this region to detailed microcrack geometry. Consistent with the predictions of Wachter et al (2009), Cheng et al (2017 observed that only a small degree of effective confinement (confining stress less than~1% of the greatest compressive stress) is required to transition the failure mode from axial splitting to Coulombic shear faulting. Further, Cheng et al (2017) observed that the increase in permeability associated with shear faulting was greater than that associated with axial splitting.…”

“…Consistent with the predictions of Wachter et al (2009), Cheng et al (2017 observed that only a small degree of effective confinement (confining stress less than~1% of the greatest compressive stress) is required to transition the failure mode from axial splitting to Coulombic shear faulting. Further, Cheng et al (2017) observed that the increase in permeability associated with shear faulting was greater than that associated with axial splitting. Based on observations of infiltrated dye in thin sections, Stormont and Daemen (1992) proposed that the onset of dilatancy and increased permeability under deviatoric loading is initially due to the opening of grain boundaries due to sliding and then later enhanced with increasing compression, especially under low confining stress, by the initiation of secondary wing cracks that grow and ultimately intersect to form an interconnected network.…”

“…Although the permeability of individual cracks in rock as a function of stress has been extensively investigated (e.g., Kang et al, 2016;Nemoto et al, 2009;Pyrak-Nolte & Morris, 2000;Pyrak-Nolte & Nolte, 2016;Detwiler, 2010;Renshaw, 1995), there are fewer experimental investigations of the effect of stress on the permeability of crack networks (Ahrens et al, 2018;Benson et al, 2005;Cheng et al, 2017;Mitchell & Faulkner, 2008;Stormont & Daemen, 1992;Zoback & Byerlee, 1975). In general, hydrostatic compressive loading of porous rocks is observed to decrease permeability due to pore closure (Benson et al, 2005;Stormont & Daemen, 1992), while deviatoric compressive loading of both porous and crystalline rock tends to result in dilatency with a corresponding decrease in elastic wave velocity and increase in permeability (Ahrens et al, 2018;Cheng et al, 2017;Mitchell & Faulkner, 2008;Stormont & Daemen, 1992;Zoback & Byerlee, 1975). Mitchell and Faulkner (2008) conjectured that the greatest permeability increases during deviatoric compressive loading likely occur in the process zone of a growing fault but did not link the permeability increases in this region to detailed microcrack geometry.…”

Mobilized Mohr-Coulomb and Hoek-Brown Strength Parameters during Failure of Granite in Alxa Area in China for High-Level Radioactive Waste Disposal

“…What is the relationship between the mobilized Hoek-Brown strength parameters and the rock damage or plastic strain? This has not been investigated in the published researches, and it is required to conduct a detailed study.In the recent several decades, extensive field and laboratory researches have been carried out in the site selection of HLW disposal in China [1,2,4,10,15,20,22,44,45]. Alxa candidate area in Inner Mongolia is one of the three candidate areas with large volume of granitic rock.…”