Unconsolidated deposits and bedrock will be carefully surveyed prior to constructing Citytunneln. By taking account of existing rock conditions, it has been possible to determine the position of the tunnel for acceptable stability. It has also been important to clarify how water-bearing the different layers are so that the extent of lowering the water table can be determined. Different types of examination also show whether there is any risk of subsidence in adjacent buildings or damage to plant life.
Unconsolidated deposits along the extent of the tunnel are often between 5 and 20 metres deep. The thickest deposits are at the south-east section at Lockarp. In the northern part of Malmö, at Malmö C for example, there are filler deposits as a consequence of sea areas and old canals having been filled. Filling comprises mostly sand and moraine. Below the infill in the northern parts there are loose layers of clay and peat which can cause subsidence.
Types of rock
The rock in the Malmö area can be divided into three layers. At the top there is Copenhagen limestone under which there is bryozoan limestone rock with chalk at the bottom. The chalk is at a much greater depth than the actual tunnel. The tunnels are bored at the level where there is bryozoan limestone rock.
The Copenhagen limestone thickness in the Malmö area can be up to 8 metres thick. The Copenhagen limestone contains significant quantities of flint which is a very hard stone. The layer is often cracked which means that the rock is highly pervious to water.
The bryozoan limestone is found down to between 50 and 60 metres under ground level, and is often less pervious than the Copenhagen limestone. However, its upper layer is most often cracked. North of Triangeln, the bryozoan limestone changes character and becomes slightly more pervious. Copenhagen limestone rock is lacking in some places such as at Holma.
Ground water condition
In the northern parts of Malmö the water table in the limestone rock is close to the water level in Öresund, in other words close to + 0 metres. The water table follows the topography and is therefore higher in high areas such as in the south-east part of Malmö. The water table in the
unconsolidated deposits is normally higher than in the limestone rock. In the areas that are affected by Citytunneln, the water table varies from 1 to 7 metres below ground level.
Surface water condition
In the central parts of Malmö, surface drainage goes mostly via the day water network that leads out into Malmö’s canals. Along the southern parts of the tunnel, drainage is led westwards towards Öresund via agricultural drainage, ditches and a large day water pipe. The eastern parts of Malmö are drained via the Riseberg creek out towards the Segeå river.