Conventional Excavation of Connecting Tunnels in C13 Metro Station (Poland - Warsaw

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The three 40-metre connecting tunnels at Warsaw’s C13 metro station had to pass beneath the busy Wisłostrada road tunnel, close to the Vistula River. Although the original design assumed clay, investigations revealed loose, water-bearing sands above disturbed Pliocene clays, creating difficult excavation conditions and requiring traffic to continue uninterrupted overhead.<br /><br />The first excavation plan used micropile canopies and face reinforcement. During an initial wall drilling, sand and water surged into the station, prompting further ground investigations. A revised approach added jet-grouted columns. However, during excavation of the north tunnel in August 2012, the face failed, releasing a large volume of water, sand and silt and flooding the station. Workers escaped unharmed, but equipment was buried. The incident also left a cavity beneath the Wisłostrada road slab. Concrete and cement grouting helped fill and consolidate the cavity, but further injections were stopped after they lifted the slab by as much as 20 centimetres.<br /><br />The successful third approach used ground freezing. After the station was cleared and a reinforced-concrete wall constructed, horizontal and vertical freezing probes stabilized the ground. The two side tunnels were excavated and lined first; the central tunnel followed, with its steel ribs connected to the side-tunnel linings.<br /><br />The tunnels were completed after a complex 33-month period that included flooding, freezing and construction work. The authors conclude that moving groundwater undermined the earlier improvement methods, while freezing turned the water into an effective temporary support system. The works finished 15 days ahead of the final milestone.

Keywords

Warsaw C13 metro station

connecting tunnels

Wisłostrada road tunnel

Vistula River

water-bearing sands

ground investigations

jet-grouted columns

ground freezing

excavation flooding

metro construction