Rail conquers the Alps

gotthard base tunnel | switzerland
Italian frontier at Chiasso.
Construction adits for workers,
materials and equipment were built
at Amsteg, Faido and Bodio, and two
800 m deep access shafts were cut at
Sedrun. This enabled tunnelling to
take place simultaneously at several
faces, so shortening the construction
time and keeping costs in check. In all,
no less than 28 million tonnes of rock
were removed to create the two bores.
Zwischenangriff Sedrun intermediate Sedrun
access
Multifunktionsstelle Multi-function stationSedrun
mit Nothaltestellen
Portal Bodio Portal Bodio
Multifunktionsstelle Multi-function stationFaido
mit Nothaltestellen
Faldo Zwischenangriff intermediate Faido
access
Geological obstacles
Querschlag Cross-passage
Construction of the two running
tunnels provided the opportunity to
make a very detailed assessment of
the geology in this part of the Alps.
Despite a few unpleasant surprises,
the geologists who predicted what the
tunnellers would find proved to be
largely correct in their assessment.
From the Erstfeld portal the alignment
passes through the Erstfelder
gneiss, a rock forming the northern
half of the Aar massif. This material
offered generally good conditions for
construction.
In the Amsteg section the tunnellers
encountered a mix of rocks that
included 50% old crystalline rock,
41% plutonite and 9% volcanic rock
from the Permo-carboniferous period,
which was close to the geologists’
predictions. However, beneath
the Maderaner valley the tunnel teams
unexpectedly struck a 600 m long
section of medium-grained pale-coloured
hard granite. This did not outcrop
on the surface and was therefore
a surprise.
The next obstacle was a 60 m section
of gneiss that was disintegrating
through exposure to heat and water.
Found level with a line drawn from
the Chrüzli Pass to the Etzli mountain
hut, this material was at first sight solid,
but it proved to be so leached out
and damaged by hot water that fragments
could be broken off by hand
and rubbed into grains of sand. In the
west running tunnel an inflow of water
made matters worse, and this led to
a collapse. The TBM was brought to a
halt and it could only be restarted after
major work to stabilise the surrounding
rock.
Between the Aar massif and the
Tavetsch intermediate massif there is
a 400 m length of rock known as the
Clavaniev zone. The rocks here are under
heavy tectonic stress causing frequent
faulting. This was a major problem
for the construction teams who
had to apply much technical ingenuity
to ensure the passage of the TBMs.
Amsteg Zwischenangriff intermediate Amsteg
Portal Erstfeld Erstfeld Portal access
In the Sedrun section the alignment
reaches the northern part of the
Tavetsch intermediate massif, passing
through it for 800 m. Created at a very
late stage during the formation of the
Alps, this area is marked by numerous
faults. The southern side of this massif
is 2·3 km long, the dominant rocks being
gneisses and schist-gneisses. This
section is less subject to stress and
the firmer rock was better suited for
tunnelling.
South of the Tavetsch massif
the alignment passes through the
Urseren-Garvera zone which features
metamorphosed sedimentary rocks
such as limestone, schists and dolomite.
This was much more favourable
for the construction teams, as predicted
on the basis of knowledge gained
during construction of the Gotthard
motorway tunnel.
The Sedrun and Faido sections cut
through the Gotthard massif itself,
which consists mainly of different
types of gneiss and schist. A few faults
exist between the strata which in most
cases did not cause too many problems.
However, another unexpected
fault known as ‘50b’ proved to be 160 m
long, causing serious difficulties.
The southern part of the Gotthard
massif is formed of Medelser granite,
the characteristics of which were hard
to predict, and the tunnellers found
that this rock stretched twice as far
north as expected.
The Piora zone
Long before work began, the project
team was aware of the existence of the
Querschlag Cross passage
ca. 40 40 m m
Piora zone, where tunnelling conditions
in what was termed a ‘sugar dolomite’
were expected to be execrable,
so much so that the entire project appeared
at one time to be in jeopardy.
Fortunately, reason prevailed, and
test bores were made to determine if
there was a problem and how serious
it would be. The tests in fact revealed
that the tunnel alignment would pass
through dry dolomite and anhydrite
— a particularly attractive rock exhibiting
white and blue-grey colours.
The predicted problems did not materialise,
and in the end it turned out
that the rocks in the Piora zone were
well suited for tunnelling.
South of the Piora zone the alignment
reaches the Pennine gneiss zone
where there is also a rock known as
the Lucomagno gneiss. Although
this is relatively good for tunnelling,
the overburden of more than 1 500 m
led to technical problems. The ‘MFS’
fault, 30 m to 40 m deep, proved to be
nearly 600 m in length. The fault zone
also included the transition to the adjacent
Leventina gneiss lying to the
south, which with the 1 300 m overburden
led to numerous rock bursts,
causing serious construction problems.
The original plan had envisaged
that the Faido multi-function station
would be built at this location, but the
construction difficulties led to it being
resited further south.
By contrast, the Leventina gneiss
is good tunnelling rock, and the only
problem occurred when the miners
encountered the ‘2705’ fault shortly
after work started on the Bodio section.
This fault affected the alignment
Fig 1. The Base
Tunnel consists
of two singletrack
running
tunnels linked by
cross-passages at
325 m intervals.
‘Multi-function
stations’ with
crossovers and
emergency
facilities are
located at Faido
and Sedrun.
Railway Gazette International | July 2011 37