Mestrostav is using two Rocket Boomer L2 C rigs from Atlas Copco to drive almost two-thirds of the 3km long Spelc twin tube tunnel in Prague. The bores will pass below the Vltava river, driven north to south as part of the 6.4km long Blanka section of the city’s inner ring road.
The faces are being advanced by heading (65m2), bench (35m2) and invert (25m2) excavations. Primary support is 300mm thick SCL with lattice girders, mesh and expanded friction bolts.
By last month the north tube had advanced 1,180m, or almost three-quarters, and the south bore 870m, or nearly half way, respectively. The best advance was 147m in a month using a four-shift sequence.
Geology along the ‘S’-shaped alignment was established by a pilot tunnel and comprises sandy shales, clayey shales and fine grained quartzite. Pressure grouting was undertaken along stretches from the pilot bore.
Cover to Spelc tunnel is up to 14.5m, though a 160m long section of thin cover overlain with water bearing gravels that required ground treatment by pressure and jet grouting.
The completed tunnel – to be opened in 2011 – will have an elliptical cross-section for two-lane stretches (123.7m2) and three lanes (172.6m2). A PE waterproofing membrane is being used for the 400mm thick final, PP fibre reinforced lining.
Czech contractor Metrostav is driving twin tunnels in Prague for part of the inner ring road
