The new Oslo opera rests on 28 000 metres of piles, and the under-stage area, which lies 13 metres below sea level, is kept water-tight with clay. Geotechnical engineering is also invisible art.
A geotechnical chance game, maintained a construction expert in 2004, when the proposed concept for the opera's under-stage area was unveiled. Today, the basement area is functioning according to intention.
"It is exciting to device new solutions which are potential cost-savers," says Kjell Karlsrud, Technical Director at NGI. "I was convinced that the solution would work, even after the debate in the media started."
Beneath the new opera there is a thick layer of clay, 40-50 metre thick down to bedrock. Statsbygg (the Directorate of Public Construction and Property) maintained that the building of the under-stage was the most complicated and challenging part of the new opera house, because it was located in the most unfavourable soil, and below sea level.
The basement is circular, with a diameter of 40 metres, and contain the opera's under-stage area with technical equipment for the stage. Four large pillars 2.5 metres in diameter were driven down to bedrock through clay and unconsolidated material. They are the largest piles that have ever been driven anywhere in Norway.
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"Even the cellar floor is designed as a two-metre thick, curved plug, cast in concrete. The plug lies 16 metres below sea level. Below the plug, there is clay, and it acts through its natural properties as a sealing agent," explains Karlsrud.
The greater part of the excavation and casting of the concrete for the cellar took place under water, with the aid of professional divers. The casting of the plug took three days.
Like much of central Oslo, the new opera lies on clay. At the deepest point, it is 60 metres down to bedrock. The opera rests on a total of 700 piles, all of which are solidly anchored in and on bedrock. A total of 28 000 piles were used over an area of 230 x 110 metres. |
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"Even though the individual operation does not offer major engineering challenges, the building as a whole presents a challenging complexity. There is extensive engineering work behind this structure," confirms NGI Project Manager Astri Eggen.
A ship barrier was built in front of the new opera
"Although the possibility of ships going off course are small, the barrier is dimensioned to stop ships of up to 25 000 tonnes travelling at a speed of two knots. It is designed not to bring ships to an abrupt halt, but to lift them, thereby absorbing their energy, and then to stop them," says Karlsrud.
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The barrier was an extensive construction underwater. It counts several drains 15 metres down into the ground. When pressure is applied to the base of the barrier, some of the water will drain out and the supporting basal clay will become firmer. A drainage membrane holds the earth in place, and gravel is filled on top. Stone fill was then added on top of the gravel cushion up to four metres below sea level.
The last part of the barrier, designed as a step in a staircase, consists of layers of stone from elevation minus four to minus two metres, the uppermost layer being a thick concrete shell. |
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