The excavation activity in underground always influences the in-situ stress condition and generates the Excavation Damaged Zone (EDZ). New fractures due to underground excavation bring mechanical and hydrological problems around the excavated space, even though intact or less fractured rock masses are selected for underground facilities. During the EDZ formation, re-balancing of “new stress condition” causes unloading fractures and shear fractures (Figure 1.1). It is understandable that the main deformation direction will be tunnel center towards. Thereafter, unloading joints and shear fractures are generated. Especially, shear fractures play an important role in the connection of unloading joints. These fractures or complex fracture zone developed parallel to the tunnel axis can be a transformation path of leaked material from the underground facilities. The worst scenario in EDZ is that the leaked material from the underground facilities (e.g. a radioactive waste disposal facility) transfers to the human environment.
The direction of water flow in EDZ is also important. To assess or predict the leaked material movement through EDZ, It is necessary to investigate the process of new fracture generation and the change of hydraulic conductivity during deformation. Figure 1.2 illustrates the shear fracture (or complex fracture zone) formation and the water flow direction as a major concern in EDZ. The hydraulic conductivity in the direction parallel to the tunnel axis and also in the rock fracture will be the main matter for the safety of human environment. In this point of view, the shear direction and the water flow direction is perpendicular.
The conceptual model of EDZ during and after excavation was suggested by Bossart et al. (2004) in the Mont Terri Rock Laboratory by synthesizing all structural and hydrogeological information. They mapped the fracture frequency in the gallery walls by line counting and interpreted for such zones corresponding to locations where small gypsum spots were observed on the fracture surfaces. Thus, due to this interpretation case, it is needed to observe the process of fracture initiation and propagation for the better understanding regarding with fracture generation in EDZ.
Details of new fracture initiation and propagation which occur in EDZ are necessary to observe because new fractures can connect with previous fractures and increase the possibility of material movement with ground water.
2. Previous Researches
In previous time, shear-flow coupling behavior of fractured rock has been investigated by many researchers such as Yeo et al. (1998) demonstrated that the fracture became more permeable in the direction perpendicular to the shear displacement than in the direction parallel to the displacement with increasing shear displacement, Esaki et al. (1999) and Jiang et al. (2004) with several new apparatuses and techniques developed, Olsson and Barton (2001) introduced a new model developed for the coupling between mechanical and hydraulic aperture change during normal loading and unloading, and suggested the method for the coupling of shear dilation and hydraulic aperture changes, Makurat et al. (1990) (cited from Park, 2008) carried out coupled shear conductivity test on rock joints. They anticipated the gradual blockage of flow paths due to gouge production as a reason of hydraulic conductivity decreasing with shear displacement. All of those researchers used an artificial joint in rock specimens.
Park et al. (2008) developed shear-flow coupling test apparatus and investigated by using intact specimen (without artificial joint). Park et al. demonstrated that the flow rate started to increase from the yielding point of shear stress during shear deformation. The increment of flow rate continued even after the peak point of shear stress.
Difficulty of controlling the test conditions is still the biggest problem that many previous researchers still use single joint specimen.
3. Future Challenges
Not only focus on single joint, but also propagation fractures occurred in rocks, intact soft rocks, is still need to be explored more since the limited research results published. Absolutely, controlling of test condition difficulties is still the biggest problem. However, by modifying and exploring the recent apparatus, indeed using actual technology, it can be achieved.
Another challenge is shear-flow coupling test in forward-reverse direction. Many researches were done this test in forward direction only. However, considering the actual circumstance reveals that it is possible occurred due to the faults, or seismic, the test in two directions, forward and reverse, in one intact or single joint test becomes an interesting topic. Previous researcher (Esaki et al., 1999) explained that hydraulic conductivity of reverse direction test was mostly equal with forward one before the peak point of shear stress in single joint. However, significantly differences were gained after peak point. To conduct this reverse test, modification the existing apparatus (available in Saitama University) must be considered; even it’s not easy and also the possibility to provide a new one. Therefore, research in single joint and intact material need to be considered more. Previous results using intact soft rocks (see Figure 3.2) were focused on forward test only. Therefore, the results haven’t explained entirely about the real condition in nature.
By carrying out experiment with considering more actual condition approach will explain something more that can enrich the knowledge itself.
Bossart, P., Trick, T., Meier P. M., & Mayor, J. C., 2004. Structural and hydrogeological characterization of excavation-disturbed zone in the Opalinus Clay (Mont Terri Project, Switzerland), Applied clay science, 26, pp.429-448.
Esaki T, Du S, Mitani Y, Ikusada K, Jing L., 1999. Development of a shear-flow test apparatus and determination of coupled properties for a single rock joint, Int J Rock Mechanics and Mineral Sciences, 36, pp.641-650.
Jiang Y., Tanabashi Y., Xiao J., & Nagaie K., 2004a, An improved shear-flow test apparatus and its application to deep underground construction, International Journal Rock Mechanics Min. Sci. (SINOROCK2004 Paper 1A28), 41, pp.385-386.
Moeri A., and Bossart P., 1999, Visualisation of Flow Paths (FMBExperiment). In: Thury, M., Bossart, P. (Eds.), Results of the Hydrogeological, Geochemical and Geotechnical Experiments (performed in 1996 and 1997), Geological Report Swiss National Geological and Hydrogeological Survey, Ittegen-Berne, Switzerland, 23, pp. 160– 170.
Olsson R., Barton N., 2001, An improved model for hydromechanical coupling during shearing of rock joints, International Journal Rock Mechanics Min. Sci., 38, pp.317-329.
Pardede, H. P., and Osada, M., 2013, Study on Fluid Containing Micro-bubbles flowing Through Factured Diatomaceous Mudstone Specimens, Department of Civil and Environmental Engineering, Graduate School of Science and Engineering, Saitama University, Japan.
Park, H., Osada, M., Sasaki, T., Takahashi, M., & Kumagai, S., Shear-flow-visualization coupling test and triaxial shear-flow coupling test for soft sedimentary rock, Department of Civil and Environmental Engineering, Graduate School of Science and Engineering, Saitama University, Japan.
Yeo IW, de Freitas MH, Zimmerman RW. Effect of shear displacement on the aperture and permeability of rock. Int J Rock Mech Min Sci 1998; 35: pp. 1051-1070.