Volume 7, no. 13 - Colbond Geosynthetics

High-pressure cascade
Tunnel shells alone, of course,
cannot evenly distribute that kind of
pressure, and would actually be
destroyed by it. The recurring
problem, until now, has been how to
ensure that this high-pressure
cascade finds an outlet well away
from the inner tunnel walls.
Traditionally, tunnel-builders use a
waterproof liner between the rough
cement-covered rock walls and the
inner shell. But it is very important
that the liner remains intact in order
to keep water off of the shell itself.
That is where geotextiles come in.
“Geotextiles are not only used to
protect the liner from perforations,”
Zwicky says, “but also create a
hollow drainage space between the
shotcrete and the liner. That serves a
very important function in diverting
water pressure from the inner shell.
The problem is that the pressure from
the shuttered concrete shell on the
inside and the pressure coming from
the outside can press normal
drainage material flat, so the water
cannot escape. With all the problems
that would entail. Even under
pressure, though, geotextiles like
special Enkadrain tunnel types
provide a transmissivity of almost 200
times that of normal drainage
material.”
All well and good, of course, but at
temperatures like those found in the
Gotthard base tunnel, the use of
thermoplastic geotextiles in
combination with a thermoplastic
waterproof liner would only make the
problem worse. This problem was
therefore one of those recently
addressed during the specifying
rounds for the more westerly
Lötschberg base tunnel.
Cover story
There, AlpTransit’s preliminary testing
indicated that not only pressure, but
also the combination of heat and the
caustic chemical makeup of the rock
itself - and therefore of the water
seeping through it - called for
construction materials unlike those
commonly applied in tunnel
construction.
Water from the rock
“Rock water,” Zwicky says, “can have
properties that affect the resistance of
geotextiles. These can include an
extremely high pH, or the presence of
carbonic acid, microbes, etc. The
geotextiles used must therefore be
resistant to all these things.”
The first round of testing in the year
2000 perfectly illustrated his point.
The standard materials submitted by a
host of manufacturers, proved, without
exception, incapable of surviving the
hostile climate to which they would
later have to be submitted. Despite
the expense involved in a second
round, a number of manufacturers
returned to their designers with an
even stricter set of parameters.
In the case of Colbond Geosynthetics,
this meant taking a good hard look at
items like chemical resistance, fire
resistance and oxidation - the latter as
a result of the abnormally high
temperatures measured in the
Lötschberg base tunnel.
“The Enkamat material we had tested
again in the second round,” says
Colbond Geosynthetics Sales
Manager Karl Wohlfahrt, “was based
on a core and filter made from
polypropylene. This material actually
exceeded the drainage demands
placed by the specifiers, and also
significantly reduced the accelerated
aging of the material at higher
Colbond Geosynthetics News 13 - 2002
7