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A new railway tunnel c. 30 m beneath central Stockholm is planned, to relieve the congestion on the existing railway network. The bedrock along the planned path has been subjected to detailed geological mapping, fracture analysis, and engineering rock characterization. 70 drillcores with a total length of c. 3000 m, 1850 m of existing tunnels, and 1000 m of rock outcrop was mapped. The c. 5 km stretch from Tomteboda in the north to Riddarholmen in the south consists of coarse, strongly migmatitic, mainly metasedimentary gneisses and mica schists cut by numerous pegmatites of variable size. Fine-medium-grained, grey granite ("Stockholm granite") forms an extensive network of 0.1 to 1 m wide dykes and bodies (100 m) that cut the gneisses sharply, and carry abundant gneiss xenoliths. In the south, weakly gneissic tonalites occur, cut by minor pegmatites, and "Stockholm granite" that carry partly assimilated, dark enclaves of tonalite.
Almost 8000 fractures was mapped and characterized according to ISRM recommendations. Structural analysis of fracture orientations allowed subdivision of the investigated stretch into 17 preliminary brittle structural domains, characterized by distinctive combinations of fracture sets. The direction, density, and quality of these sets, relative to the tunnel direction are of major importance for the construction. The entire area is dominated by fractures in three main directions: 1) subhorizontal, 2) steep, striking NE, and 3) WNW-NW-striking with variable dips to the NE. The two first dominate the northern and southern, mostly granitoid parts of the area, while the NW striking fractures are most prominent in the central, gneiss-dominated area. However, in both cases the direction of the brittle fractures appears to be largely controlled by the foliation in the gneiss. A significant part of the subhorizontal fractures may be related to unloading after isostatic rebound.
Rock quality classes in the City Line project have been subdivided into Rocktypes (Bergtyp) based on the RMR system; Rocktype A: RMR≥70, Rocktype B: 50RMR70, Rocktype C: 30RMR50, and Rocktype D: RMR30. Q-system Ja values are mostly 2 as calcite and minor chlorite are the main fracture minerals. Clay and graphite (only in the gneiss) occur as infill materials locally in Rocktype B, C, and D. The gneisses in general yield slightly lower RMR and Q values. Hydrothermal fluid flow has locally turned the rock (mostly the granite) red and oxidized, around laumontite-bearing fracture zones, resulting in slightly weaker rock. Most of the rock mass along the stretch is forcast to fall within Rocktype A (c. 70 %), c. 25 % into Rocktype B, and 5 % into Rocktype C. 13 weakness zones was identified along the route; the most prominent cross Citybanan at Kungsgatan and södra Riddarholmen, and strike WNW. The first is c. 20 m wide zone of brecciated, crushed, chlorite and clay-rich rock. The second a 5 m wide, strongly crushed, chloritized and clay-rich zone.
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