ABSTRACT
The case studied in this article covers a mined twin track railway tunnel within Scandinavia. Where due to weakness zones and uncertainty on the rock levels, the usually drill and blast solution had to be replaced by soft ground tunneling.
A local lowering in the top of rock was identified in the existing geological information, resulting in very low rock cover (or potentially no rock cover). The soil above the rock consists of high permeable ground with more than 10m of water pressure.
The lowering is located below existing pile founded buildings and under a public road with a range of utilities. This high congestion of both the surface and underground space made it impossible to perform ground investigations prior to construction, to confirm the rock level above the tunnel.
This led to a temporary solution comprising of ground freezing, pipe umbrellas, heavy support frames and a redesigned underpinning of the building immediately above the tunnel.
The current paper summarizes the design process and the chosen design, and the considerations for dealing with uncertain geological conditions within a congested areas with high water pressure.
