Special features in tendering, award and contract - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 345

stated additional works and the alteration of the organisation structure on site, the decision
to use a shielded machine proved correct.
Sieberg Tunnel. The Sieberg Tunnel is part of the high-speed rail line (HLS) Vienna-Salz-
burg. The two-track tunnel has a length of 6.5 km and an excavated diameter of 12.5 m.
The contract was awarded at the end of 1996. The mined sections of the Sieberg Tunnel lie
exclusively in the beds of the molasse zone with intercalated siltstone and fine sandstone
(Fig. 9-4), called Miocene schlier and Oligocene schlier. At the four valley crossings, there
were extensive weathered zones. The hydrology of the Sieberg Tunnel mostly shows two
groundwater tables and a maximum water pressure of 2.9 bar. The water ingress during the
drive was very low and only seldom reached quantities of 5 l/s. The overburden was max.
55 m, although this reduced at the four valleys to less than 10 m in places. The tunnel did
not pass under any buildings on the surface.
Figure 9-4 Geological profile of the Sieberg Tunnel.
The tender documents for shotcrete tunnelling included a tunnelling classification accord-
ing to Austrian Ö-Norm standards into seven classes for the top heading and five classes
for the invert. Classes six and seven required advance support measures (spiles, IBO an-
chors and in some cases pipe screens) for a length of about 3.5 km. One 1.26 km long
section of the tunnel was to be constructed in cut-and-cover. The tender documents did not
include any details of constraints affecting a shield tunnel drive.
Although the tender documents had intended shotcrete tunnelling, an alternative proposal for
the entire length including the cut-and-cover section was submitted. This intended the reuse of
the tunnelling machine from the Adler Tunnel. The flat rock cutterhead was fitted with disc cut-
ters and picks and could be extended on all sides, although retraction of the cutterhead was not
possible. The tender intended final lining of the tunnel with segments. No additional measures
were mentioned in the alternative proposal for the sections with shallow overburden.
The mechanised tunnel drive was rejected because the tender was incomplete. The assur-
ance by the bidder to accept all risks and costs was also regarded as unachievable by the
client.
The difficult geology compared to other projects and the initial experience from the Adler
Tunnel made all parties careful.