Tunneling through a graphite bearing fracture zone during the expansion of the Stockholm metro to Nacka

This paper presents a case study of tunneling through a graphite bearing geological structure during the excavation of an access tunnel for the expansion of the blue metro line to Nacka. A straight section of the access tunnel and the graphite bearing zone had almost the same orientation and geographical location which made the excavation works particularly challenging. The graphite formation had not been identified during the preinvestigation and it followed the tunnel heading during the excavation of over 170 m. Parts of the fracture system was connected to an overlying aquifer resulting in ground water inflow in those sections. The rock conditions were handled through a combination of spiling bolts and reduced round lengths. Sections with higher water inflows were challenging due to reduced adhesion of shotcrete, leaking bolts and bolt holes. A close cooperation between the designer, site geologist, client's supervisor and contractor enabled a successful passage of the zone area.