Facts about the subsidence of 2 July 2011
Size of the subsidence
The visible opening of the subsidence had a diameter of about 50 centimetres; its depth directly beneath the opening also amounted to about 50 centimetres. Beneath the turf, the hole had a larger diameter, which is why around one cubic metre of sand was needed to fill it on Saturday 2 July.
Location of the subsidence
The hole occurred in the lawn of a private property in Hamburg-Lurup. It is located above the tunnel route, around one metre beside the tunnel axis. In this place, the tunnel lies around 25 metres beneath the surface, in the ground water.
How was the subsidence discovered?
The nine-year-old daughter of the neighbours broke into the hole, which was still covered with turf at the time. The girl was unharmed and able to free herself.
Temporal connection with the tunnel construction
The tunnel boring machine worked beneath this place on 4 June 2011, that is about four weeks before the discovery of the hole.
Subsidences in tunnel construction
During every tunnel construction, the ground sags a bit – forming an elongated, shallow subsidence – and normally comes to rest after a few days. On 2 July 2011, 1715 metres of the approximately two-kilometre-long tunnel for the European XFEL were already completed. The measurements carried out so far along the tunnel route indicated that the surface is to be regarded as extremely stable: a lowering was measured in the range of only two to three millimetres.
Comparison with the subsidence in a horse pasture in November 2010
This hole was located at a distance of about 1700 metres towards the west. It had a depth of 1.20 metres and a volume of around 15 cubic metres. At this site, the tunnel is located 12 metres beneath the surface. This depression is thought to have been caused by an exceptional geological layering which was only found in this place along the tunnel route. Subsequently, the control mechanisms of the underground tunnelling process were carefully analysed once again and further optimised.
