The two 1720m- and 1729m-long tunnels are part of the new construction section of the A71 motorway between Erfurt and Sangerhausen. In total, the section is 7.5km long and is being constructed by a consortium consisting of building contractors Baresel, Herman Kirchner Hoch- und Ingenieurbau and Alfred Kunz Untertagebau. The construction costs for the project are approximately euro100 million (£76.6 million).

Four sprayed concrete machines speed up construction

The prevailing ground conditions meant that a full outbreak would not have been controllable for a tunnel excavation between 105 and 147m^sup 3^. 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 sprayed concrete machines for the work. Two additional, identically constructed spraying machines were on standby over the period.

The consortium has been on schedule since the tunnel operation started in December 2005. At the end of November 2006 the semi-circular crown profile had already been completely excavated in both tunnels. Different section lengths between 0.8 and 2m 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.

Water level lowered with 35 wells

The Schmucke mountain has considerable water resources. The ground at 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 Schmucke with depths 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 reinstated.

Up to 300mm-thick sprayed concrete shell

As soon as the first cross heading was reached after 140m in the crown drive, the consortium began the side wall and base drives. The cut sections also varied significantly from each other here, with lengths ranging between 2 and 12.6m as a result of geological conditions (including shell limestone and various types of mottled sandstone). After an initial application of sprayed concrete to stabilise the rock, a second application was applied as a sealant carrier. Due to the very smooth surface (80% of the aggregate granulation measures up to 4mm, 20% up to 8mm), the sprayed concrete shell can be lined directly with fleece and plastic sealing sheeting. On average, the wall thickness of the outer shell is between 200 and 300mm. Approximately 75,000m^sup 3^ of sprayed concrete is required altogether for the two tunnels and the four entrances of the Schmucke tunnel.

Moving mountains

Of course this enormous demand also subjects the six spraying machines to natural wear, for example of the spray nozzles. Downtime, however, was reduced to a minimum because two of the six sprayed concrete manipulators were kept available as reserves during the service intervals. Foreman Rüdiger Appel says, "After experiencing co-ordination problems at the start, co-operation between the construction site and machine manufacturer has run very efficiently. If we now need parts for one of our Sika-PM500 machines urgently, the order is placed via the Putzmeister branch in Gera that's not far away. And the lads there move mountains for us!"

Four BSA pumps concrete the inner shell

Before the turn of the year from 2006/07, the placement of concrete began on the north and south side of the Schmücke tunnel for the base and the arch of the inner lining. When doing this, the consortium used formwork carriages and up to four stationary Putzmeister concrete pumps (two BSA 1408 E pumps as well as a BSA 1405 E and BSA 1005 E pump).

Before concreting the arch, a section of the base measuring around 60m in length had to be concreted to support the arch formwork carriage. The BSA pumps follow directly behind the formwork carriages and are pulled along section by section. The delivery lines are deliberately kept short here (maximum 10m). The 12m-long inner shell blocks made from cast-in-situ concrete (C30/37) are constructed in thicknesses ranging from 0.4 to 0.6m. The time which the concreting crew requires for placing approximately 135m^sup 3^ of concrete for each arch section is around 5.5 hours and they require around 3-3.5 hours to place approximately 75m^sup 3^ of concrete for the base sections. By varying the proportion of cement, the consortium reached an optimum relationship between processability and hardening temperature. The aim is to concrete one base and one arch block per day, per tunnel section. This requires striking times of less than 12 hours in the arch.

Concluding remarks

Each of the tunnels is concreted in 144 blocks and each block is approved by the site management. The total requirement of concrete for the inner shell is approximately 65,000m^sup 3^.

The Schmücke tunnel consortium procures both sprayed and cast-in-situ concrete from several plants in the region. Work is usually conducted in two 12-hour shifts, seven days a week. This is the only way that the schedule can be met. The headwork was concluded in May 2007 and the inner shell was due to be concreted by the end of 2007.