Some 25 metres beneath the surface of its streets, workers are toiling in fine sandy soil to construct a 3.8km underground railway tunnel that will connect the northern and southern parts of the city. Once completed, the Noord/Zuidlijn will measure 9.7km.

Amsterdam's soil consists of sand with over-consolidated clay layers, which are, in turn, overlaid with a soft Holocene layer of alternating peat and clay. The historic buildings are founded on wooden piles, approximately 15 metres long, laid over this relatively thin layer of sand. About 20 to 30 metres below the surface, the tunnel is being built in another sandy layer. Because Amsterdam is settling at a rate of approximately 2mm per year due to natural soil compression, it is of vital importance to carry out settlement monitoring projects. These constitute one of Europe's biggest monitoring contracts.

Mega-construction projects such as the Noord/Zuidlijn inevitably cause some shift in surrounding structures.

To monitor the effects of drilling and construction on houses, 74 automatic total stations (Leica TCA 1800 survey instruments) have been mounted down a 30-metre-wide corridor running along the axis of the tunnel; the corridor broadens to 70 metres closer to the sites of projected stations. More than 7,500 miniature prisms were fixed to measuring points on over two thousand buildings, bridges and quays, more than two thousand levelling surface settlement points and a significant number of remotely monitored sub-surface instruments.

The precision total stations perform continuous measurement cycles and send the data to a central computer system that calculates and compares new data with original figures. In September 2008 the calculations revealed major subsidence of up to 25cm at some control points. Surveyors were called in during the middle of the night, angry residents had to leave their homes and police were brought in to restore order.

The reason for the subsidence was found to be a leak in the retaining underground wall, allowing in groundwater which was, causing the buildings to sink.

Acknowledgements
The contribution of Projectbureau Noord/Zuidlijn is much appreciated.