Special features in tendering, award and contract - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 347

The consultant and specialist consultants declined the alternative proposal despite its low-
er cost.
The stated grounds were:
- The ability of the machine to cut the limestone banks was questioned, although posi-
tive experience was already available from the Adler Tunnel and the cutting wheel was
equipped with discs and scrapers,
- The possible jamming of the shield skin was seen as a problem; this was dealt with by
the provision of copy cutters.
- The waterproofing and the durability of the segment lining were doubted. Single-layer
segment linings are, however, now the state of technology.
- It was feared that the application of shield tunnelling could not comply with the permis-
sible settlements that had been specified. On the other hand, it should be mentioned that
no blasting is necessary in shield tunnelling and synchronous grouting equipment with
the latest control systems had been tendered.
Controlled overcutting with rapid ring closure also spoke for the use of a shield machine.
These are both advantages compared to shotcrete tunnelling.
The shield tunnel drive was rejected although extensive experience was available under
similar conditions (see Adler Tunnel) and the solution was cheaper and technically better.
Figure 9-6 Geological profile of the Rennsteig Tunnel.
Rennsteig Tunnel. The Rennsteig Tunnel is 7.9 km long and when completed will be the
longest road tunnel in Germany. The four lanes of the autobahns A71/A73 will run through
two tunnel bores. The crossing of the ridge of the Thüringer Wald hills will be followed by the
Alte Burg, Hochwald and Berg Block Tunnels, a total tunnel length of 12.6 km. The excavated
cross-section of the tunnels is about 11 m. The geological conditions for the Rennsteig Tun-
nels are characterised by Permian porphyries, which are mostly very compact and hard (Fig.
9-6). Intercalated in and below this igneous rock are further strata of sandstone, clay stone and
Keuper clay. The groundwater table alternates from below to above the vertical alignment. The
maximum water pressure is about 8.5 bar and the water ingress is forecast up to 5 l/s per 100 m