In-situ Experiments on Excavation Disturbance in JNC s Geoscientific Research Programme

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1 In-situ Experiments on Excavation Disturbance in JNC s Geoscientific Research Programme H. Matsui, K. Sugihara and T. Sato Japan Nuclear Cycle Development Institute, Japan Summary The HLW disposal program in Japan is currently in the implementation phase. In order to contribute to the formation of a firm scientific basis for the R&D activities, the JNC is conducting an extensive geoscientific research program. As a part of the rock mechanics research in this program, in-situ experiments on excavation disturbance have been performed at the Tono Mine for soft sedimentary rocks and at the Kamaishi Mine for hard crystalline rocks, both of which are common in Japan. These experiments have revealed the development of excavation disturbed zones (EDZs) at the study sites. It is suggested that mechanical excavation and controlled excavation have reduced excavation damage in the rock mass around drifts, although some improvements in the currently available methods for measuring and simulating the EDZ are essential to understand excavation disturbance in more detail. Figure 1. Location map of the Tono Mine and the Kamaishi Mine, including the Underground Research Laboratory sites. 1. Introduction In Japan, as outlined in the overall High-level Radioactive Waste (HLW) Management Program defined by the Atomic Energy Commission of Japan (AEC,1994), the HLW separated from spent nuclear fuel at reprocessing plants will be immobilized in a glass matrix and stored for a period of 30 to 50 years to allow cooling. It will then be disposed of in a deep geological formation (geological disposal). Pursuant to the overall HLW management program, Nuclear Waste Management Organization of Japan (NUMO), the organization with responsibility for implementing HLW disposal, was established in However, as some important R&D issues still remain, Japan Nuclear Cycle Development Institute (JNC) has been assigned the role as the leading organization responsible for R&D activities. The JNC is now promoting two projects to build underground research facilities in Japan. One is the Mizunami Underground Research Laboratory (MIU) project for granitic rocks and the other is the project in Horonobe for sedimentary rock (Fig.1).

2 2. Conceptual model of EDZ We define the EDZ as the rock zone adjacent to a drift where rock properties and conditions have changed due to the processes induced by excavation, such as fracturing, stress redistribution and desaturation (Fig.2). Rock properties, such as deformability, seismic velocity and permeability around underground openings, are expected to change due to these phenomena. The excavation damage (fracturing) is considered to be dependent on the excavation method. The EDZ is considered to be physically less stable and could form a continuous and high-permeable pathway for groundwater flow, so the EDZ has the potential of affecting the safety of the underground openings and repository performance. Figure 2. Conceptual model of the EDZ. Based on the conceptual model of EDZ formation, the in-situ experiment on EDZ in Tono and Kamaishi Mines were planned and carried out. 3. In-situ experiment on EDZ in Kamaishi Mine 3.1 Site description and experimental setting The geology in the area consists of Paleozoic and Cretaceous sedimentary rocks and two igneous complexes, Kurihashi granodiorite and Ganidake granodiorite (Fig.3). The in-situ experiments on EDZ were carried out at the northern end of the EL.250m level drift about 730m below the ground surface. The average elastic modulus and uniaxial compressive strength are 58.2GPa and 144.4MPa, respectively. The direction of maximum principal stress was almost N-S and horizontal based on stress measurements using overcoring. The average magnitudes of maximum, intermediate and minimum principal stresses were 43.8, 26.8 and 17.8 MPa, respectively. Three fracture sets, oriented N25E80NW, N85E85SW and N20W75NE-75SW occur. The average fracture frequency is about 3.1/m. Figure 4 shows the experimental layout and measurements items at EL. 250m drift. Two new drifts, measurement drift and test drift, were excavated without support. Both normal blasting (NB) and smooth blasting (SB) techniques were Figure 3. Geology of experiment site at the Kamaishi mine. Figure 4. Layout of the drifts and boreholes for Excavation Disturbance Experiment at Kamaishi mine used to assess the influence of different excavation methods on excavation disturbance.

3 3.2 Results and discussion a) Processes related to the changes in rock properties of the EDZ An example of a measured shock wave at blasting is shown in Fig.5. Figure 6 shows the distribution of the acoustic emission (AE) events, together with the computed source mechanisms and energy level. The area of tension cracks induced by blasting is in good agreement with the expected damaged zone created by the blasting induced compressional wave. The shear-wave with the highest amplitude was also measured during vibration measurement and AE events show both tension and shear movements. Figure 5. An example of the measured shock wave at blasting Small rock bursts occurred at the upper, right-hand corners of both drifts of the EL. 250m drift (as seen looking toward the east) during excavation, where numerical modeling work suggested that the stress concentrations should occur and where a notch can be seen in a portion of the roof. These results indicated that rock failure was caused by stress concentration. An ultrasonic tomography survey and velocity measurements made under hydrostatic pressure conditions on core samples suggested fracturing related to stress concentration. Figure 6. Source locations of AE events at blasting In conclusion, the damage due to excavation was mainly induced by P- and S-waves generated by blasting, but a failure zone caused by stress concentration is also indicated.

4 b) The spatial extent of the EDZ and the degree change of rock properties due to excavation Figure.7 shows distributions of the seismic velocities around the test drift by seismic refraction survey. The extent of the low velocity zone, which is about 20-50cm from the drift wall, is very similar for both the SB and NB sections. The average velocities found for the NB section ( 1 st layer: 2.5km/sec, 2 nd layer: 4.0km/sec) are smaller than the velocities for those sections excavated by SB( 1 st layer: 2.6km/sec, 2 nd layer: 4.3 km/sec). Total thickness of the overbreak of drift and low velocity zone agreed well with the thickness estimated by vibration measurements. This means that the thickness of the zone can be predicted if the vibration due to blasting is known. Figure 7. P-wave velocity distribution around test drift The elastic modulus of the low velocity zone detected by the seismic refraction survey was also measured directly with a borehole expansion test. In the zone, the elastic modulus was reduced by a maximum 50% compared to that of host rock. The results of connected permeability tests and hydraulic tests in short sections (10cm) in the low velocity zone of the drift floor show that the measured hydraulic conductivities were about two orders of magnitude higher than in the undamaged part. In the zone of stress redistribution, the measured permeability after excavation of test drift decreased about one order of the magnitude compared to that before excavation. 4. In-situ experiment on EDZ in Tono mine 4.1 Site description and experimental setting The regional geology around the Tono mine consists of Tertiary and Quaternary sedimentary sequences unconformably overlying Cretaceous granitic basement (Figure.8). The in-situ EDZ experiments have been carried out in the northern part of the Figure 8. Schematic figure of geology and mine drift in the Tono mine. horizontal NATM drift, about 135m below the

5 ground surface in the Tertiary sedimentary sequence. The average elastic modulus and uniaxial compressive strength are 2.8GPa and 6.6 MPa, respectively. Figure.9 shows the layout of the in-situ experiment and measurement items for mechanical excavation experiment. Similar layout was also adapted for blasting excavation experiment. New test drifts were excavated using either blasting or mechanical methods to estimate the effects of different excavation methods on excavation disturbance. The test drifts have a horseshoe shaped cross-section and were supported by rock bolts and shotcrete. with a width and a height of 2.4m. Test Drift-2 was excavated by boom header. Before excavation of Test Drift-2 and Test Drift-2M, a 3-m long section of Test-Drift-2M was excavated by drill and blast (Test-Drift 2D) for monitoring vibration induced by blasting. Figure 9. Configuration of drifts and boreholes in mechanical excavation experiment. 4.2 Results and discussion a) Processes related to changes in rock properties within the EDZ Figure.11 shows the model for numerical simulation of the EDZ considered, with properties based on the results of the seismic refraction survey (Fig.10). The widths of the EDZ were assumed to be 0.3m in the case of mechanical excavation, and 0.8m in the case of blasting. The trends of the measured displacements were almost the same as the results of the numerical analysis assuming both with and without an EDZ. In the case of blasting, the calculated displacement without an EDZ is small compared to the measured results (Figure.12). All simulation results suggested that there are no yield zones around the NATM drift. Therefore, an EDZ may be caused by mechanical damage during excavation. Figure 10. Distribution of P-wave velocity determined by seismic refraction survey in the NATM drift.

6 b) The spatial extent of the EDZ and the degree of change in rock properties due to the excavation Table-1 summarizes the rock properties and spatial extent of the EDZ detected in the NATM drift. Mechanical excavation is effective to reduce excavation disturbance and maintain the desired rock properties around underground openings. The EDZ induced by blasting excavation has a width of about 0.8m and a P-wave velocity of 50-60% of that of the intact rock. Hydraulic conductivity is increased by more than one order of magnitude within at most 1.4m from the drift wall. Figure 11. Mesh layout and input data of numerical analysis for rock mass displacement. Table 1 Extent and properties of the EDZ detected in the Tono Mine Figure 12. Rock mass displacements measured by extensometer compared to the numerical analysis using FEM 5. Conclusions The in-situ experiments on EDZ in the Tono Mine and the Kamaishi Mine indicate that the main process related to change in rock properties within the EDZ is mainly fracturing by the shock wave during blasting. It caused large changes in rock properties in both sedimentary and granitic rocks. Stress concentration in the vicinity of drift walls is also a possible cause of damage in the deep underground like the Kamaishi Mine. The spatial extent of the EDZ is around 1m from drift wall in the case of blasting in both rocks. The degree of the permeability change in the damaged zone is one to two orders of magnitude larger than of host rock. In the stress redistributed zone, the permeability change affect to performance assessment were not detected in both experiments.

7 6. Acknowlegements The authors thank Dr. Glen McClank of Glenelg Geoscience for valuable comments and manuscript revision. Reference T. Sato, T. Kikuchi and K. Sugihara (2000): In-situ experiments on an excavation disturbed zone induced by mechanical excavation in Neogene sedimentary rock at Tono mine, central Japan, Engineering Geology Vol. 56 pp K. Sugihara, H. Matsui and T. Sato (1999): In-situ Experiments on Rock Stress Condition and Excavation Disturbance in JNC s Geoscientific Research Program in Japan A Rock Mechanical Basis of Underground Research Laboratory Project-, Proceedings of the international workshop on the Rock Mechanics of Nuclear Waste Repositories pp