The Snake: a rock tunnel for storing combined sewage water, Stockholm
The fullface-driven rock tunnel “The Snake” has been bored in a geological environment consisting of hard rocks, such as granite and gneiss, beneath the city of Stockholm. Previous lowering of groundwater levels have recurrently caused structural damage to old buildings and other structures on the surface. To minimise this damage, the Snake Project was designed with a very high limitation on the acceptable inflow of groundwater, allowing only 2 litres per minute for every 100 metres of tunnel length (9.6 m2). The importance of a professional and competent engineering geological approach to the lay-out and planning of the tunnel route is described, in addition to the in situ control programmes and documentation routines practised. Settlement surveys in the project area and control of other ongoing underground projects are mentioned as tools to understand the hydrogeological situation. Grouting routines and mechanical equipment for driving TBM tunnels are described, and the importance of a control programme is underlined, together with a presentation of the volume of grout consumed and the different qualities of grout used.
The conclusion that can be derived from the Snake Projects is the significance and necessity of an engineering-geological approach to tunnelling in urban areas and the indisputable advantages of using a TBM in combination with pregrouting equipment directly at the face.
KeywordsClay Dust Depression Transportation Sandstone
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