Measures to ensure safe construction
A launching shaft has been made in the water of the Damrak in front of Central Station. That is where the tunnel-boring machines will be assembled. From there, the boring machines will travel towards Scheldeplein. In several places along the way, the machines will be passing relatively close to the foundation piles of some buildings and bridges.

Mitigating measures
To ensure that not even the slightest settling of the soil will damage the foundations while the tunnel-boring machines are passing by, various precautions have been taken. As a group, these precautions are referred to as mitigating measures. One of those measures is soil reinforcement. Others include the restoration of foundations and the use of a computer-driven measuring system, the so-called monitoring system. In use for several years already, this system keeps constant tabs on the buildings along the route of the tunnel-boring machines.

Soil reinforcement
Most of the Noord/Zuidlijn route runs deep beneath roads and streets and sometimes below houses or other buildings. In a few places, the tunnel-boring machines will be passing close to the foundations of those buildings. Extra measures are being taken in such places (e.g. the Munttoren on Muntplein and De Bijenkorf department store on Dam Square).

There is no way to guarantee that the tunnel-boring machines will not cause the soil to shift slightly deep below the surface. Such movement can be cancelled out by pumping up the ground, as it were, in a number of places under the foundations. This involves injecting small amounts of grout – a mixture of water and cement. With long, thin boring lances, this grout is introduced into the ground from a deep vertical shaft. This technique has already been used successfully in the construction of the Jubilee Line Extension in London and of the high-speed rail line beneath Antwerp’s Central Station, among other projects.

Three phases of construction
The construction activities for reinforcing the soil can be divided into three phases:

• Carrying out preparatory building activities such as diverting cables and pipelines and moving street furniture.
• Setting up the building site, making the shaft and fastening the boring lances.
• Injecting the ground with grout while the tunnel-boring machines are passing by. To ensure the greatest accuracy during the grout injections, some buildings are being furnished with extra measuring devices that will register any movements in the ground.