Special features in tendering, award and contract - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 346

The experience with the tendering and award of the Sieberg Tunnel shows clearly that the
client should include the criteria and requirements for mechanised tunnelling in the call for
tenders. This would require at least a preliminary design to enable competition.
Airport Tunnel, Stuttgart. The airport tunnel is part of the S-Bahn rapid transit network
in the metropolitan area of Stuttgart. Two single-track rail tunnels were constructed, each
with a length of 2.2 km and an excavated diameter of about 8.5 m. The S-Bahn tunnels
run through the Jurassic clay-siltstone with intercalated limestone and sandstone banks of
variable frequency (Fig. 9-5). Swelling and increased horizontal stresses also had to be
expected along the alignment.
Figure 9-5 Geological profile of the airport tunnel, Stuttgart.
The clay-siltstones are generally of low permeability, so little water ingress had to be
expected in these sections. The limestone and sandstone banks, on the other hand, are
highly permeable and increased water ingress had to be expected in them. The maximum
forecast water pressure along the tunnel was about 2.5 bar. Both the permeable limestone
and sandstone banks and the clay-siltstones are suitable for grouting. The environmental
requirements are influenced by the route of the tunnel passing under many roads and
buildings and also the runways of the airport with stringent settlement requirements. The
minimum radius for the tunnel was R = 300 m.
The tender design intended the use of shotcrete tunnelling. This was to be carried out without
groundwater lowering as a low-settlement tunnel drive. The tunnel cross-section was circular.
Nevertheless, an alternative proposal was tendered for a shield drive equipped for com-
pressed air working where required, to provide active face support. The closed cutting
wheel of the full-face machine was equipped with discs and scrapers and an articulated
joint was provided in the shield to cope with the tight radii. All the required equipment was
of the latest state of technology for the reduction of settlement. The lining of the tunnel
was to consist of segments, as is usual in shield tunnelling.