Dewatering, waterproofing and drainage - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 238

In order to be able to counter such a risk, a basic system conforming to minimum require-
ments for the water collection and drainage components should be the starting point of the
design of any tunnel drainage system. Such a basic system consists of two side drainage
pipe runs and a non-central and deeper invert drainage run (Fig. 5-35). The layout of the
drainpipes of the basic system thus corresponds to the guideline detail drawings already
provided in the BMVBS. An additional basis for the design of drainage systems for re-
duced sintering is also provided in Germany by the RI-BWD-TU. This describes various
defined design elements and properties, which - backed up by experimental results - have
proved to result in less sintering. Alternatively, the guideline “Tunnel Drainage” published
by the Austrian Association of Concrete and Construction Technology can also be con-
sulted.
The minimum requirements for a basic system according to all sources are:
1. Shotcrete with low susceptibility to leaching should be used for rock support. This
means the general use of alkali-free accelerator or spraying cement. No aggregate of
limestone or dolomite should be used unless its leaching behaviour has been tested and
approved.
2. All drainpipes , which are laid to collect and drain groundwater, should have a smooth
inner surface, a minimum diameter of 200 mm and any slots should have a width of 5 to
6 mm. Drain pipes should be highly durable and have good abrasion resistance against
high-pressure flushing.
3. For load-bearing and filter layers, the use of 100 kg cement CEM III per m 3 il-
ter material as binder is to be specified as a minimum requirement, and the material,
installation process and the stability of the filter layer should be suitable for the size
of the inlet openings of the drainage elements. Aggregates of limestone or dolomite
should not be used [107]. For further reduction of sintering potential through elution of
calcareous binder contents, filter bodies without binder can also be used in the presence
of aggressive groundwater. This has to be balanced against the risk of damage to pipes
followed by collapse of the filter medium and blockage of the drainpipe.
4. In order to ensure that water passages of surface drainage in the annular gap, a fila-
ment fleece with a weight of at least 1,000 g/m² should be specified and a transmissivity
of 1 × 10 -5 m³/m*s.
The exact layout of the individual water-collecting and -draining components in cases
of high sintering potential depends greatly whether the potential is due to the leaching
of construction materials or the natural lime content in the groundwater. The guidelines
mentioned do not make any statements about this.
Improvements to the basic system. In cases of acute risk of sintering, denoted by a high
sintering potential of the groundwater or derived from other hydrogeological information,
the developed basic system can be improved further. The German guidelines provide in
this case for a bypass construction, which can be used to reduce the sintering exposure of
the side drains in operation. If the sintering potential is forecast as high, bypass construc-
tions according to the following illustration should be included in the design. The decision
whether to design an improved system with a bypass construction is made by the consult-
ant in discussion with the geotechnical consultant and the employer.
In Austria and Switzerland - as already explained - a different approach is followed for
the operational state. This uses so-called hardness stabilisers to reduce the extent of sinter-