Seepage tests under constant triaxial compression stresses were conducted on single granite fractures selected from Beishan with the groundwater in Beishan site as the seepage fluid. Ion concentrations of Al3+ and SiO2 in the exit solution were measured during the seepage tests, and the results were compared with those in the groundwater before test. The mineral morphologies of fracture surfaces before and after test were also examined through SEM scanning. Test results show that there are no obvious variations of SiO2 and Al3+ in the outlet solution during the process of seepage test, while the concentrations of K+ and Na+ increase dramatically and Ca2+ decreases evidently. SEM scanning of the fracture surface also shows that superficial calcification and precipitation occur after the seepage of groundwater. Comprehensive analyses of tests results show that for permanent HLW repository, the long-term effects of groundwater on rock fracture permeability can't be ignored.
Repositories for high-level radioactive waste (HLW) are located deep underground. The surrounding rockmasses are exposed to the coupled circumstances of crustal stress, seepage of groundwater and chemical dissolution. Long-term reaction between ions in the groundwater and the active minerals in rocks can lead to mineral dissolution, precipitation and chemical element migration. On the other side, the above geochemical behavior and solute migration processwould affect the seepage paths and result in redistribution of stress. Therefore, the long-term permeability evolution of fractured surrounding rockmasses under the seepage of groundwater is one of the key issues to long-term safety prediction of the HLW repository.
long-term safety prediction of the HLW repository. Chemical effect of groundwater has important influences on permeability evolution of fractured rockmass. In recent years, more and more scholars have begun to pay attention to the issue of water-rock reactions. Singurindy & Berkowitz (2003, 2005) conducted a series of laboratory experiments and showed that mineral dissolution and precipitation in carbonate rocks under acid solution seepage would change fracture roughness and affect fracture permeability. Polak et al. (2003, 2004) carried out seepage tests on both limestone and novaculite fractures and verified that the dissolution of minerals at contact points leads to decrease in permeability of novaculite fracture; while for limestone, the intense dissolution process leads to extra flow paths and correspondingly greater permeability in the final stage. Moore et al. (1994) and Tenthorey et al. (2003) conducted experimental studies on sandstone and granite fractures respectively and came to the similar conclusion that under acid solution seepage, fracture aperture decreases due to dissolution of minerals at contact points. However, Liu et al. (1997) and Dijk et al. (2000, 2002) found out that apertures of fractures in carbonate reservoir increase under acid solution seepage. On the basis of previous work of Polak et al., Yasuhara et al. (2004, 2006) and Yasuhara & Elsworth (2008) carried out a series of laboratory tests and came to the conclusion that the competition between dissolution at fracture contact points and the free surface dissolution dominates the fracture aperture variation. Experiments of single granite fracture under constant triaxial stress and chemical solution seepage were conducted by Shen et al. (2010) andYang et al. (2011). The seepage solution is selected as Na2SO4 solution with different pH value to simulate the mineral composition of Beishan groundwater. A preliminary analysis on fracture aperture evolution was carried out with the theory of pressure dissolution and mineral precipitation.