Significance of Fracture Patterns in a Rock Mass during Excavation by Blasting in Bandhagen, Sweden

Sammanfattning: When excavating a rock wall by blasting, pre-existing structures in the rock has a strong impact on the stability of the wall. For excavation in Bandhagen in Stockholm, the nature and orientation of the pre-existing geological features, namely fractures, were not taken into consideration before excavation begun. Geological field studies were carried out in order to investigate the possibility of a more favorable outcome than in the Bandhagen case. Mapping conducted in March 2015 was focused on fracture distribution and the results showed two sets of open shear fractures with fresh surfaces. The first set of the fractures cross-cuts the wall with a strike of NNW-SSE and dips between 70°±30°. The second set of fractures strikes WSW-ENE and are almost parallel to the wall (which strikes roughly 65° E) with a dip towards it, ranging between 55°±35°. The two set of fractures intersect with an acute angle of around 80° and due to their orientation, and that one of the sets dip towards the free face of the wall, they create an unfavorable fracture pattern that makes the wall, at parts, very instable in regard to rockfall and rockslide.  Fractures within the respective sets dip towards opposite cardinal points and making an acute angle of 50° for set 2 and 70° for set 1 fractures within the own set are interpreted to conjugate. In addition, there is a well-developed folded foliation that change in strike from parallel to perpendicular to the excavated wall. The foliation is aligned to the strike of both of the fracture sets. This has enabled fractures to open parallel to the mica-rich layers in the gneiss which further adds to the unfavorable pattern of fractures that creates rhombohedral unstable blocks in the rock mass. Due to the fracture pattern, sliding and rockfall have been frequent and safety measures such as rock bolts and a wire mesh have been installed to increase the security factor for the wall. During mapping, a general fracture pattern was possible to deduce from mapping of solely an unexcavated, vegetated part of the area. It became clearer though together with the excavated surface of the wall. Several pre-blast measures could have been preformed to limit rockfall and sliding of rock after excavation. Line-drilling could have been used when blasting near the contour of the wall to decrease the blast-induced fractures by the more effective venting of the excess explosion gas. This could have decreased the closely spaced blast-induced fractures that have been mapped on both the crest and the body of the wall. Another measure would have been to install pre-blast reinforcements on the crest of the wall, which could have prevented at least two large rockfalls that have occurred.  If the fracture pattern were known before excavation begun and the aforementioned measures would have been considered, the stability of the wall and the first excavation would undoubtedly have been more successful. A proposal for future open face excavations is to thoroughly assess the geological features to, in a preliminary stage of the planning, eliminate the risk for this outcome to occur in the future.