Examination of the Excavation Damaged Zone in the TASS tunnel, Aespoe HRL

The question of an existing continuous Excavation Damage Zone (EDZ) is very important for SKB. Is it possible to use drilling and blasting in the planned repository for spent nuclear fuel? Could fractures from blasting form a continuous EDZ? In order to increase the understanding of the EDZ and the possibility of an existing continuous EDZ along the deposition tunnel, SKB decided to examine the fracturing in a selected area of the TASS tunnel and to create a 3D model of the fractures in the investigated area. It was of special interest to study the transition zones between the blast rounds to examine if the EDZ from the bottom charges could form a continuous EDZ from one round to another. The TASS-tunnel is situated at the 450-m level in the Aespoe Hard Rock Laboratory. The tunnel, with a cross-section area of 20 m2, was planned to be 90 m long. In a subproject called Excavation the purpose was to test different plans for drilling, charging and initiation in order to give recommendations on how the final repository of spent fuel should be excavated. The test methodology used in this investigation comprised the following steps: selecting test area, drilling and wire sawing of blocks, surveying the blocks, removal and transportation of the blocks to the surface, cutting the blocks into slabs, fracture identification with penetrants, positioning and photographing the slabs, digitizing and 3D modelling of the fractures. The test area for EDZ consisted of an 8 m long and 1.5 m high section in excavation sequence no 4. The selected section covered the end of round 9, the entire round 10 and the start of round 11. In the contour and the helpers small diameter charges for smooth blasting were used (decoupled charges). These charges also have a relatively low detonation velocity (VOD) and this, together with the decoupling, gives short fracture lengths i.e. a small EDZ. The contour holes and the helpers were initiated with electronic detonators to achieve a simultaneous initiation, which also results in shorter fracture lengths in the remaining wall after blasting. Eight adjacent blocks were excavated from the tunnel wall using wire sawing. The blocks were 1 m wide, 1.5 m high and c. 0.7 m deep. The blocks were transported to the surface for investigation, surveying and wire sawing into slabs. Of the 8 blocks, 5 were sawed into 9 slabs and 3 were sawed into 10 slabs. This makes a total of 75 slabs. One side of each slab was surveyed, cleaned and examined using penetrant fluid, which enabled detection of fractures with an aperture down to 20 μm. After this each slab side was photographed from a fixed position and the sawing of the next slab could start. The digitalization of the fracture traces was done on screen in Quantum GIS and the fractures were classified into three different types of fractures: direct blast fractures, blast induced fractures and natural fractures. Direct blast fractures are fractures formed by the blasting process and these fractures originate from the borehole. Blast induced fractures are also caused by the blasting although they do not originate from the borehole itself. Natural fractures are fractures that existed in the rock before the blasting. They could be completely closed and/or filled, wide open or partly open. All visible fracture traces were digitized, except traces within crush zones. A total of 2,509 fracture traces were identified in the eight blocks. After digitalization the 3D modelling was done in SKB's Rock Visualization System. A model volume was setup, covering the investigated area. In RVS the fracture traces were connected to form fracture planes in 3D space. All modelled fractures were assigned one of the three types Blast Fracture (direct), Blast Induced Fracture or Natural Fracture. All modelled fractures were confidence classed regarding geometry. Using this function a fracture model of the investigated volume was created. Fracture traces that could not be connected to any other fracture trace were modelled as small, planar fractures with unknown orientation. In t he model there are a total of 1,218 modelled fractures. There are 773 modelled natural fractures, 260 blast fractures and 185 blast induced fractures. To verify the model, six areas were selected for a detailed investigation. The lesson learned from the detailed investigation was that the modelling was quite successful for the blast fractures and that no major revision of the model was necessary. The following conclusions regarding the EDZ are drawn from the main investigation. - No evidence is found of a continuous EDZ in the investigated area. No evidence is found that blasting fractures from different rounds are connected. - Blasting fractures are strongly influenced by the presence of natural fractures as they are drawn towards the natural fractures. - Since the blasting fractures do not form a continuous network, the capacity of the longer natural fractures is the limit of potential water flow in the rock mass.