The ground on which Amsterdam is built is constantly shifting. Engineers from the Noord/Zuidlijn Project Office are therefore constantly monitoring the underground layers of soil and the buildings within a broad zone along the route of the future metro line.  
  
Monitoring for safe construction
Before, during and after the construction of the Noord/Zuidlijn, measurements will be taken 24 hours a day in a zone of some 30 metres on either side of the tunnel-boring route.

An advanced measuring system using computer-driven measuring devices relays measurement data at set times to the engineers of the Noord/Zuidlijn Project Office.

Upon interpreting the data, the engineers are able to notice even the slightest settling of the ground or of buildings early on. This will allow the construction processes to be responsibly managed.

Due to the nature of the building activities (involving construction pits), the zone around the three ‘deep’ stations (Rokin, Vijzelgracht and Ceintuurbaan) is about 70 metres wide. We refer to this zone as the area of influence. In such areas, deformations exceeding one millimetre may occur.

Aboveground monitoring
Reflectors (also known as measurement mirrors) of about 3 centimetres in diameter are attached to the fronts of houses and buildings within the area of influence. These reflectors are constantly being monitored by computer-driven measuring devices – the so-called total stations – to determine any changes in their position and thus any shifting of the buildings.

The results of all these measurements are transmitted immediately to computers and are interpreted by the engineers of the Noord/Zuidlijn Project Office. This means that we will always be aware of any shifting that takes place in the surrounding buildings.

Underground monitoring
Any movements below the surface within the area of influence are also measured. Special measuring instruments are positioned up to 40 metres below the surface, from where they transmit the measurement data to the computers.

Safe construction
As soon as the measurement data register trends of unacceptable deformations, the engineers will immediately analyse the problem. If necessary, the Noord/Zuidlijn Project Office will actively intervene in the building activities to keep any deformations within acceptable levels.

The monitoring is successful
The effectiveness of the monitoring system was proved in October 2004 when there was leakage in a circular cofferdam at the corner of Nieuwe Vijzelstraat and Weteringschans. Groundwater was leaking through the wall of the cofferdam, causing the ground to settle beneath the houses beside the cofferdam.

The monitoring system immediately registered the movements in the soil. As Amsterdam’s soil is constantly shifting, such movements are nothing out of the ordinary. But in this case, the level of the shifting became alarming.

After analysing the data that the system delivered every hour, the contractor decided to take action. To prevent matters from getting worse, the circular cofferdam was put under water. Besides stopping the flow of water that was leaking out, this also stopped the soil from settling further.