Civil Engineering Reference
In-Depth Information
The entire length of the Hetzendorf Tunnel runs through the sediments of the Vienna ba-
sin. These are very variable and extremely heterogeneous beds of gravelly and silty clay.
The maximum water pressure is 1.8 bar and high water inflow has to be expected. In the
Vienna tegel (similar to marl), only pore water pressure has to be expected due to the low
water permeability. The local conditions are characterised by the route of the tunnel below
residential areas, which restricts the access and launching possibilities. The overburden is
also shallow, so the requirements of permissible surface settlement were increased.
For the Lainzer Tunnel, most of the tunnel alignment passed through moderately hard
solid rock of 80 % Tonstein (claystone) as well as siltstone and marlstone, with 20 % of the
alignment in “Klippenstein”, sandstone and limestone rocks. Water only occurs locally,
but can reach quantities of up to 10 l/s and the maximum water pressure is 6.5 bar.
The soil section (Hetzendorf Tunnel) was tendered for shotcrete tunnelling with side head-
ings. The aquiferous beds were to be dewatered from the surface using deep wells. Grout-
ing of the soil was intended to reduce settlement.
The rock section (Lainzer Tunnel) was also tendered for shotcrete tunnelling. This sec-
tion was to be excavated using a roadheader or tunnel excavator in the partial sections top
heading, bench and invert. In some areas, rock bolts, spiles and pipe screens were planned.
As an alternative to shotcrete tunnelling, investigations were also undertaken for both
sections into the use of a shield machine as a single-track or two-track solution. The re-
quirements for the tunnelling process in the soil section would best be fulfilled by a shield
machine with slurry-supported face, with a single-pass segment lining for the permanent
support.
In the rock section, the use of a shielded TBM is possible, although it should be possible
to close the excavation chamber to overcome the water pressure. In accordance with the
Swiss tunnelling method, a two pass lining with umbrella waterproofing can be used for
the permanent support.
The example of the Lainzer Tunnel clearly shows the influence of local conditions; the
launching area for the shield machine and thus the entire tunnel route cannot be optimally
adapted to suit the local conditions.
The construction of switch niches after the shield drive worsened the preconditions for
mechanised tunnelling, although the escape niches could be replaced by box niches inside
the shield-driven cross-section according to the design handed in for approval.
The use of a tunnelling machine is possible in principle under the existing geological
and hydrological conditions. The construction of the required external rescue niches and
switch niches is time-consuming and expensive, so shotcrete tunnelling was estimated to
cost less and was used for construction.
Figs. 9-8 to 9-10 show some examples of the different details for the construction of safety
niches. Fig. 9-8 shows a cross-section with safety niches in conventional tunnelling.
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