Introduction | Ground
Conditions | Construction Stages | Slideshow
| Exercise
Ground conditions
The ground is soft and wet.
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Most of the construction is in the alluvium layer. This is soft
silty clay deposited by the river since the most recent ice age (about
10 000 years ago). The melting ice caused the lake level to rise
temporarily by several metres, which in turn caused the valleys to silt
up.
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Below the alluvium is a layer of sand and gravel overlying
sandstone from the Triassic period.
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The groundwater level is about 2 m below the surface, level with the water
in the lake.
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The underside of the basement slab is 8.8 m below original ground level
(i.e. 6.8 m below groundwater level).
The groundwater is under high pressure in the sand and gravel layer
and the sandstone through which it can flow freely. Water flows
less freely through the fine-grained alluvium layer. As excavation
proceeds and the weight of alluvium inside the walls decreases,
there is a danger that the water pressure in the gravel will lift the Alluvium
and flood the excavation. This phenomenon is known as piping.
Sheet piles can be driven easily in these soft deposits making this
form of construction cheaper and quicker than the concrete alternatives
such as diaphragm walls or contiguous bored pile walls. Due to the use
of rather flexible sheet piles instead of much stiffer concrete diaphragm
walls, bracing is needed to prevent the inward deflection of the walls.
Preventing inward deflection of the walls is important because it may result
in ground settlements behind the walls.