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When the 1994 Winter Olympics open in Lillehammer, Norway, on 12 February,
the first world record of the games will already have been set. But the
record breaker has never made the sports pages: it is a $25 million hole
in the ground.

Three years ago, Norwegian engineers blasted into Hovdetoppen hillside
in Gjovik, some 25 kilometres south of Lillehammer, to create the world’s
largest cavern designed for public use. Next week the first rounds of the
Olympic ice hockey tournament will be played there, in a purpose-built,
5300-seater stadium. The athletes will be competing 25 to 50 metres underground
– perhaps not in the bowels of the Earth, but certainly under its skin.

Building the Gjovik cavern was a world-class achievement – a geotechnical
decathlon. Engineers used 170 tonnes of explosives to shatter 140 000 cubic
metres of rock. Japanese engineers creating another huge underground hole,
for the Super-Kamiokande experiment to capture the elusive neutrino particle,
are blasting out little more than half this volume of rock. A combination
of advanced tunnelling technology, a computer modelling system and data
from painstaking geological studies made sure that the roof or walls of
the Gjovik cavern did not collapse.

The Olympic cavern is 91 metres long, but what makes it such an achievement
is its 62-metre span. Its closest rival is a civil defence chamber in China,
which spans a mere 38 metres. Another subterranean contender is an underground
hockey rink in Finland, which consists of two separate caverns, each spanning
36 metres. However, nature can still put the Gjovik cavern to shame. The
Sarawak cave in Borneo, the largest yet discovered, measures 396 metres
across, and the labyrinth of Carlsbad Caverns National Park in New Mexico
covers almost 19 000 hectares. At Carlsbad, 14 football fields would fit
inside the Big Room alone; the Gjovik cavern would hold just one.

There are good reasons why Norway has built the biggest cavern of this
type. With so much water in its glaciers, rivers and lakes, the country
gets all of its electricity from hydropower, and has almost 200 underground
hydropower caverns, nearly half the world’s total. Such underground structures
are popular for their energy efficiency and low maintenance costs in the
cold of the Norwegian winter. The Gjovik cavern will require around 30 per
cent less energy than a comparable rink above ground. ‘There’s a great tradition
in building this sort of thing,’ says Nick Barton of the Norwegian Geotechnical
Institute (NGI) in Oslo, who led the design investigations at Gjovik.

Some of that tradition was already in evidence before the Olympic planners
arrived in Gjovik. Several smaller caverns had already been dug out of the
bed rock near the town to house a swimming pool, telecommunications centre
and a civil defence centre. And after finishing the main Olympic hall, engineers
carved out three smaller caverns beside it for the Norwegian postal service.

Norwegian know-how

The idea of building an extra-large cavern in Norway goes back to the
early 1970s, when the Norwegian State Power Board commissioned the NGI to
study the feasibility of putting nuclear power plants underground. Large
facilities are often located underground ‘in countries like Switzerland,
Sweden, and Norway, where you have locally dense populations combined with
a poor climate’, says Herbert Einstein, professor of civil engineering at
the Massachusetts Institute of Technology. As part of its investigations
into how to accommodate a reactor – it would have required a cavern arch
at least 50 metres across – the NGI ran the first computer models of stress
levels in the rock surrounding large caverns.

Since then, the NGI has refined its models and combined them with the
latest methods of tunnelling and geological classification. It is this parallel
advance in several technologies – in particular, computer modelling combined
with a certain method of rock classification – that has made possible the
building of the Gjovik cavern. The Olympic Games provided the impetus for
the project.

Before deciding exactly where to blast the hole, the engineers had to
make careful measurements of the stresses and other properties of the rock
under Gjovik. In 1991, the NGI – in cooperation with Norwegian companies
NOTEBY and SINTEF and the cavern designers Fortifikasjon – started to investigate
the geology of the area, especially the mechanical properties of the rock.
The cavern site lies under a smoothly sloping hillside, and because it is
not far below ground, the largest stresses are horizontal, which is a big
advantage when building a 62-metre span.

The Gjovik rock is gneiss, a red and grey metamorphic rock, which is
at least 600 million years old. The gneiss is extensively fractured by rough,
irregular joints that occur between one and three times per metre. The more
joints there are, the harder it is to keep the roof up. Parts of the gneiss
are fractured into a network of tiny cracks with clay fillings, the result
of folding and faulting within the rock. Norway has its share of seismic
activity, but the Gjovik cavern should be strong enough to withstand an
earthquake. Underground caverns survive quakes relatively well because they
can’t tilt, sway, and amplify waves generated by ground movement in the
way a skyscraper might.

Much of the strength and stability of the cavern comes from its method
of construction. The engineers used the ‘Norwegian method of tunnelling’,
or NMT, which reinforces the rock by inserting bolts and spraying concrete
reinforced with fibres, rather than supporting it with cast concrete or
lattice girders, forming a horizontal grid of steel supports. The NGI developed
this method 15 years ago specifically for the hard, jointed rock that is
common in Norway. Another popular way to tunnel is the ‘new Austrian tunnelling
method’ (NATM) which works best in ground that is soft enough to be excavated
rather than having to be blasted. This is still the method of choice for
road and railway tunnelling projects worldwide, including the tunnel for
the recently abandoned Superconducting Supercollider in Waxahachie, Texas,
though not for the recently completed Channel Tunnel between England and
France. Engineers using the NATM method place temporary supports against
the roof and walls as they go along, monitor the rock deformation, then
put in permanent supports of lattice girders and spray mesh-reinforced concrete.
This stepwise method allows them to correct problems as they arise.

In the Norwegian method, by contrast, the geology of the area is studied
as fully as possible before construction begins. It relies on the ‘Q-system’
of rock classification, developed in 1974, which gives engineers an idea
of the rock support needed to keep a tunnel stable. The Q-system calculates
a value for a variable, Q, which represents the rock quality, according
to six parameters, including the spacing of joints and their roughness.
The more joints there are, the weaker the rock is likely to be, but rough
joints are less likely to slip than smooth ones, and so weaken the rock
less. The value of Q ranges from 0.001 for exceptionally poor rock to 1000
for very good quality rock that is practically unjointed.

The Q-system has been used in hydroelectric projects in Turkey, India,
China, Taiwan and elsewhere. The NGI and the engineering consultancy W.
S. Atkins are using an updated Q-system in a study of the feasibility of
building caverns for the storage of radioactive waste 800 metres below the
surface near Sellafield in Cumbria.

In 1991, NOTEBY engineers began the Gjovik project by drilling four
cores from the surface into the rock where the cavern would be. From these
cores, and measurements made on existing cavern walls nearby, they measured
the six parameters needed to derive a Q value for the rock. In the rock
around the Gjovik cavern the mean value of Q was 9.4, with a typical range
from 1 to 30, which corresponds to a ‘fair’ rock quality. During construction,
however, the contractors found the rock quality in the cavern to be slightly
worse than expected, probably because the blasting opened up artificial
and healed joints.

Classy joint

Like the Austrian method, NMT involves spraying the sides of the tunnel
with concrete, but reinforces it with steel fibres in place of mesh reinforcement.
The value of Q tells engineers roughly how much concrete they need to spray
and how many bolts they need to put into the rock permanently. The Gjovik
cavern is lined with 1300 cubic metres of concrete, 10 centimetres thick,
and is reinforced with some 3000 bolts and several hundred cables, both
10 to 12 metres long.

Once they had an idea of the Q value of the rock, the engineers double-checked
the stability of the cavern using a number of different methods. The NGI
bounced seismic waves between the boreholes to check the joint frequency
against the predictions made with the Q-system. SINTEF measured the rock
stress, coming up with a high horizontal stress of 3.5 to 4 megapascals,
a figure that the NGI later confirmed and that helped to make the 62-metre
span of the cavern possible. Finally, the NGI ran several computer models,
based on data from the core drilling, stress measurements and cross-hole
seismic tomography. From these data, the NGI model calculated how much the
rock would deform once excavation began.

These comprehensive preliminary studies made for less work once the
blasting started. Veidekke-Selmer, a joint venture between Norway’s two
main tunnelling firms, finished excavating the cavern 200 days after construction
began in April 1991. With all the rock was blasted out, the domed ceiling
of the cavern settled a mere 6 to 8 millimetres, just as the numerical
models predicted.

The inside of the cavern was completed last spring with the installation
of floors, seats, ventilation and lighting. Since then, several events have
been warming up the hall prior to the Olympic events. Barton was at a concert
held there in May: ‘I sat way in the back, and the acoustics were terrific,’
he reported. A skating exhibition from the cavern has gone out on national
television, giving Norwegians a flavour of things to come.

The finals of the Olympic ice hockey tournament will be held at Lillehammer.
But in the opening rounds, as the grunting and sweating ice hockey teams
battle it out in the Gjovik cavern, the engineers who built it will be thinking
that their own gold medal has already been won. Alexandra Witze is a science
writer based in California.

Alexandra Witze is a science writer based in California.