The LT 31 contract section of the railway link between the Western, Southern and Donaulände
Railway Line in Vienna (Lainz Tunnel) com-
prises a double tracked tun-
nel with a length of 3050 m
in closed coverage type. The tunnel runs partly below an operating railway track and partly below
city struc-
tures. The excavation star-
ted from two vertical shafts, in each case in both direc-
tions.
The overburden varies from 6 m to 26 m. The structure is situated in Miocene sedi-
ments of the
Vienna Basin. The material can be descri-
bed as a succession of sand, locally also gravel,
silt-clay, as well as zones of sandstone and conglome-
rate.
Ground water is found in several bearing layers at a pressure level of up to 10 m above tunnel crown.
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BGG Consult worked for the project in the fields of geology, geotechnics and geohydrology immediately
after the time the route was chosen until the completion of construction. Based on subsoil explorations,
geo-
technical and geohydrological expert's reports have been compiled.
During construction, consul-
ting with regard to geology, geohydrology and geotech-
nics was carried out.
Moreo-
ver, the implementation and evaluation of inclinometer measurements as well as the supervision
of ground water lowering measures was within the scope of work. Furthermore, BGG Consult was
commissioned for the geological documentation of the shafts and the tunnel.
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Ground Water Lowering Measures:
In order to grant a stable subsoil situation in the vicinity of the tunnel and the shafts, the water
bearing layers had to be dewatered and the pressure relieved respectively. The ground water drawdown
was achieved mostly by vertical wells from above ground. For the design of the wells, i.e. the
determination of the location of the well screen, the slot width and the well system (gravitative or
vacuum), the stratification of the subsoil and the different water bearing layers had to be scrutinized.
A total of 120 vertical wells with depths of up to 50 m were built.
Residual water and local water bearing layers were captured by drainage pipes (driven or placed in
bore-
holes, with or without vacu-
um) during excavation and tunnel advance respective-
ly.
The monitored data (flow rates, water level in ground water gauges) were read automatically and could
be accessed online in real-time.
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