ABSTRACT
As a part of the ongoing extension of the Metro System in Stockholm several critical passages have been identified whereas the passage under Ladugårdslandsviken is one of the most critical. The passage comprises a stretch of underwater tunnels with a limited rock cover estimated to about 8 meters in combination with an identified fault zone crossing the tunnels. The geometry and ground conditions led to technical challenges in the design of rock support and grouting. Erosive fill material in joints could in worst case lead to uncontrolled high water ingress during excavation with unforeseeable consequences.
Additional investigations consisting of core drilling running parallel to planned tunnels were performed during the design phase from an existing access tunnel. Indication of cavities and sand and clay filled joints in combination with poor rock quality only 5m above the planned tunnels required complementary analysis and additional reviewing of presented technical solution.
The original design consisted of a cast in place water-tight concrete lining as the permanent rock support. Design limits were established for assessing if a cast in place concrete lining was required or not. The limits were mostly based on the actual rock conditions and hydrogeological parameters as conductivity and water ingress. Detailed analysis of available data resulted in a customized temporary reinforcement with a specific excavation strategy and comprehensive grouting.
To minimize remaining risks identified during design a routine was established during construction where representatives from client, designer and contractor formed a workgroup with frequent follow up. This routine allowed continuous adaption of the technical design mostly by analysing observed conditions and verification based on design criteria. It was possible to specifically determine the actual need for pre-bolting, fibre reinforced shotcrete and extent of grouting.
After completion of the tunnels underneath the Ladugårdslandsviken and verification of design criteria a concrete lining could be replaced with mesh reinforced shotcrete as the permanent rock support.
