Putz: holding up tunnel walls

Up to four shotcrete machines from the Sika/Putzmeister alliance have been used by the Schm?cke consortium in Germany to safeguard tunnel excavation work.

Four shotcrete Sika/Putzmeister machines work on two tunnels being built in Germany.
Four shotcrete Sika/Putzmeister machines work on two tunnels being built in Germany.

Up to four shotcrete machines from the Sika/Putzmeister alliance have been used by the Schmücke consortium in Germany to safeguard tunnel excavation work.

The two 1,720m and 1,729m-long tunnels are part of a new section of the A71 motorway between Erfurt and Sangerhausen. In total, the tunnel is 7.5km long and is being constructed by a consortium of Baresel, Herman Kirchner Hochund Ingenieurbau and Alfred Kunz Untertagebau. The construction costs for the project are approximately US $140 million (EUR100 million).

The prevailing ground conditions mean that a full outbreak would not have been controllable for a tunnel excavation between 105m3 and 147m3. The consortium therefore decided to excavate the east and west tunnel using separate crown/side wall blasting and crown/side wall/base blasting drive methods, respectively. As the tunnelling work in both tunnels progressed simultaneously from the north and south side, the consortium used four independently operating Sika-PM500 shotcrete machines for the concrete spraying work. Two additional, identically constructed spraying machines were on standby during the period.

The consortium has been well on schedule since the tunnel operation started in December 2005. At the end of November 2006 the semicircular crown profile had already been completely excavated in both tunnels. Section lengths between 0.8m to 2.0m were necessary for the headwork as a result of the strongly fluctuating load bearing behaviour of the surrounding mountains. For safety reasons a total of five emergency galleries, which run in parallel at a distance of 20m, connect the two tunnels.

The Schmücke mountain has considerable water resources. The ground in the central area of the two tunnels was so saturated with water that excavation with the chosen drive method would not have been possible. Therefore a gallery consisting of 35 wells had to be sunk in a depression on the rear of the Schmücke at depths of up to 90m for drainage purposes. When work is completed the wells will be restored to their previous state so that natural groundwater conditions can be reproduced in the Schmücke mountain.

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