Tunnel Design for the Vienna Underground Construction Lot U1/9

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The construction lot U1/9 with “Altes Landgut” station is part of the extension of underground line U1 to the South from Reumannplatz station to Oberlaa station. The U1 will grow by 4.6 kilometres and five stations in forthcoming years. By 2017, it will be the longest underground line in Vienna (19.2 km). The Construction lot U1/9 consists of two app. 25 m and 30 m deep cut & cover shafts, two mined app. 120 m long platform tunnels, four mined single track running tunnels, several cross-passages and one emergency shaft. The tunnels are located below residential buildings, main roads and the very critical “Laaerberg Tunnel” of the A23 freeway. 

Main focus of the design of the sequential excavation method (NATM) for the tunnels was the minimisation of settlements and achieving high safety level. Prior tunnelling the buildings at surface and the Laaerberg Tunnel were improved partly regarding their structural capacity. The tunnels run mainly in the Miocene silt/clay layers which have a relatively homogeneous structure. They basically comprise of slightly sandy, grey, dark grey to grey-blue silts/clays, turning semi-solid to solid with increasing depth. A thick mixed layer of fine sand or sandy silt layers, 5 m thick on average, runs through the construction lot from approx. 20 m below ground level. It was necessary to pre-drain this layer by filter wells from surface. Extensive monitoring was foreseen to control the performance of the excavation works. The monitoring devices comprise bolt gauges for 3-D trigonometric shotcrete shell displacement measurements, surface settlement points, liquid levelling system on A23 tunnel, horizontal automatic inclinometers above the tunnel tubes underneath the A23 and in above the platform tunnel in places. The observed deformations during construction were within the predicted range.

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By Atzl Georg at ITA WTC 2015 Congress and 41st General Assembly