An underground structure is nominated as one of the most significant works of architecture created so far in the 21st Century by a list of eminent international architects.

The largest Atlas caverns

The Large Hadron Collider at CERN on the French-Swiss border near Geneva was selected by two of 52 award winning architects asked by Vanity Fair magazine to list, in their opinion, the most important buildings completed in the last 30 years and so far since the turn of the Century. The results published in the August issue of the magazine has, as the clear winner with 28 nominations, the outstanding Guggenheim Museum in Bilbao, Spain by Frank Gehry. With two nominations as first choice, the LHC at CERN ranked fourth in the list of most significant works of architecture created so far in the 21st Century.

It is not clear if it is the vast man-made caverns of the underground nuclear physics laboratory or the incredibly intricate and fabulously looking machines built to go inside them that inspired two famous architects to select the LHC installation but it is certainly worthy of the accolade.

The enormous detector hall at the Atlas cavern complex measures 35m wide x 56m long x 42m high and 1,380m2 in cross section and has directly on to its vault the junction of two large diameter access shafts. Perpendicular to it is the 23m wide, 65m long and 18m high, 338m2 section computer hall that had to be excavated and finished and handed over for installation of computer equipment before excavation of the detector hall began. The caverns at the CMS detector complex on the opposite side of the 27km particle accelerator tunnel ring are equally as impressive.

One of the massive LHC machines

What makes them more spectacular is the fact that these massive caverns are excavated in geological conditions that are far from favourable for their size and complexity. They had to be built in to the existing particle accelerator tunnel ring, had to be the size they are, and had to be built in close juxtaposition to limit the lengths of data transmission cables from the machine to the computer control rooms - despite the geological conditions. These were just some of the many tall orders imposed by the scientists on the civil engineers. That all was achieved without fault or failure is credit to the skill and expertise of the design and construction civil engineers involved.

The recognition of the LHC installation at CERN brings to mind other underground structures of distinction. In compiling a list of iconic underground structures, the criteria are that they be man-made structures and structures for civil underground use rather than those created by the mining industry.

With those criteria in mind, two are nominated to follow CERN the top of the list. First is the underground mountain hall ice hockey arena at Gjøvik in Norway and second, the underground car parking structure for the Sydney Opera House in Australia.

Gjøvik Mountain Sports Hall

Space inside the mountain

Built to host the ice-hockey competitions of the 1994 Winter Olympics in Norway, the Gjøvik Olympic Cavern Hall is still the world's largest underground cavern for public use. Designed to seat 5,500, the cavern is 91m long x 25m high and 61m wide.

The hall is located 120m inside Hovdetoppen, with the centre of the town, at the end of Gjøvik high street. More than 141,000m³ of rock was excavated to create the cavern. It was built in about two years and cost at the time NKr 134.7 million or US$20 million. The complex also includes an underground swimming pool that had been built earlier.

Professor Eivind Grøv, Chief Scientist of SINTEF and President of the Norwegian Tunnelling Society (NFF) explains that sufficient high horizontal stress that makes such a large underground cavern feasible. "The in-situ stress situation may vary from stress released rock bodies through a pure gravitational stress situation, to stresses resulting from long tectonic history of the rock mass. In situ stress measurements for the mountain hall presented a result of sh=3-5MPa at a depth of 25-50m, which is far more than the theoretical gravity approach."

Grand opening with an audience of 5,500

While the rock hall was more expensive to build that a surface structure, it has long term operations cost savings. As the assistant manager of project said: "There are no windows to wash or fix, no outside walls to paint, no roof to repair and it costs about half as much to heat as a regular building."

Gjøvik is only one of many, many underground sports and recreational halls built by Norway and designed to double as civil defence shelters since the 1970s. The peacetime activities now outweigh the civil defence needs and more underground facilities are being built to take advantage of the operational cost savings. In a survey by Engineer Jan Rygh of those who use and work in underground leisure facilities, lack of daylight was sited as a universal disadvantage. Little can be done to rectify that reality, but proposed improvements for new underground facilities include more efficient dehumidifying systems, better ventilation and better acoustics.

Sydney Opera House car park

Design of the double-helix car park

The underground car park for Sydney Opera House is unique in shape and size. It is the first helical underground parking station and claims to be possibly the widest shallow-cover rock cavern in the world.

With a capacity for 1,100 cars, it is designed as a huge doughnut-shaped cavern, with a span of up to 19m, an outer radius of 75m, and a 12 story high, free-standing double-helix internal concrete ramp structure that operates on a one-way only traffic flow. Cars travel down the ramp to park diagonally into the parking space and continue going down to exit on the double helix up ramp. Cross passages through the centre core of the doughnut provide a cross cut to the exit ramp rather than having to travel the full 12 stories to the bottom to link with the reverse helix ramp.

Plan of the structure under the Royal Botanical Gardens

A competition was held to find the best design for the 'missing' Opera House car park that had to be built underground, under the adjacent Royal Botanical Gardens on Bennelong Point. The competition was won, not by a civil engineering firm, but by a car park operating company. They had the design for the helix as the most efficient method of entry and exit while providing the greatest number of car parking spaces. Civil tunnelling engineers designed the spectacular excavation in Sydney's favourable Hawkesbury Sandstone. A description of the cavern on the ATS (Australasian Tunnelling Society) website explains that the vault is the key feature of the cavern. It has a span of between 17.5m and 19m and is beneath 7m and 8m of variably weathered sandstone cover that has a strength of 15MPa to about 40 MPa. It is not supported with a formed concrete arch but rather has internal reinforcement comprising about 2,000 tensioned Macalloy bar anchors up to 7.5m long and untensioned galvanised dowels up to 4.5m long.

Full depth excavation

There are important design features of the roof.
• It is almost flat, as this is found from both analytical studies and experience to be appropriate in horizontally bedded strata with a relatively high horizontal stress field.
• The capacities and distribution of the reinforcing elements were designed so as to tie together the horizontal beds of sandstone (ranging in thickness from 1m to 3m) to act as a single pseudo-elastic no-tension linear arch.
• The roof surface is covered with a 150mm skin of reinforced shotcrete and fibrecrete, which acts as a membrane between the reinforcing elements.

There were other special challenges for the project.
• Portions of the underground excavations passed beneath outbuildings of the NSW Government House.
• The cavern is within 60m of Sydney Harbour and extends 28m below sea level.
• All work had to be done without disrupting the surface other than for the two 9m wide accesses which had to pass over the Sydney Harbour Tunnel and under, in places, rock cover as low as 2.5m.

The crown section of the main cavern was excavated via successive widening of an initial outer 6m wide heading. With support installed, the heading was widened from 6m to 10m to 15m to 18m. Once the crown was fully excavated and supported, a D10 bulldozer fitted with an impact ripper was used for bulk excavation of the full 30m or so depth of the cavern. Slots cut into the cavern walls provided for ventilation risers, stairwells and lift shafts.

Excavation of the cavern and associated tunnels, involving some 130,000m³ of sandstone, started in late 1990 and was completed in April 1992. The twelve story concrete helix was completed in September 1992 and the parking station opened officially on March 17, 1993, six months ahead of schedule and at a cost of about Aust$40 million.

It is said that the 30m depth of the excavation uncovers many millions of years of geological time from the top to the bottom and that while, the concrete helix ramp structure inside the cavern is freestanding, the walls of the excavated rock cannot be seen. It was thought that patrons to the Opera House would not be comfortable seeing the rock face of the walls as excavated and walls were provided on the inside and outside of the ramp structure. It is possible though to go behind the internal drip-shed roof of the cavern and see the original vault excavation, a rare treat for those invited to take that special tour.

References

Spectacular excavations for physics research - TunnelTalk, August 2001
CERN
Norwegian Tunnelling Society
Australasian Tunnelling Society

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