The word metro is a delicate one in Amsterdam. Construction of the east-west line in the 1970s triggered huge opposition as buildings and neighborhoods were demolished. The post-1960s protest movement turned its attention to environmental and green issues. There were riots.
So it was not until the early 1990s that anyone dared breath a word about a North- South Line. It was even more contentious given that it would pass underneath the historic centre of the city – a world heritage site and a major tourist attraction.
The new project was to avoid any past difficulties by using modern tunneling methods, with some major innovations. It would pass underneath the city, leaving old buildings intact and unaffected.
A host of techniques would be employed, almost a catalogue of modern tunnelling and ground engineering, from advanced real time TBM control, to refinements in compensation grouting and new methods of tunnel lining. An extraordinary and unique application of immersed tube tunnelling was combined with advanced jet grouting, and vertical microtunnelling to pass beneath the historic central station. Deep excavation with diaphragm walls could make the stations. There are even caissons.
Planners and designers believed modern techniques would make the work possible. But they all posed huge difficulties. The Netherlands is famously mostly below sea-level and most of its geology formed by glacial, marine and estuarine deposits, in other words soft, water-filled ground, setting major challenges for tunnels and ground works. For good reason bored tunnels were unknown in the country until the 1990s.
Within cities it was even more of an unknown and particularly in Amsterdam where the famous tall, thin, canal-side buildings are founded essentially in a peat bog. They are carried by tens of thousands of timber piles on to a sand layer beneath, all preserved by the water around them but vulnerable to atmospheric decay.
"The city geology is one of multiple layers," explains Frank Kaalberg, tunnel design project leader for the project’s design consultant, Tunnel Engineering Consultant. The company is a joint venture between Dutch Witteveen+Bos and British Royal Haskoning.
Three layers of sand alternate with medium soft clays beneath the peaty top round, he explains.
The first is the traditional founding for historic buildings, the second at roughly 40m depth is used for modern buildings and the third 60m down for a few very high rise modern towers. There is also an intermittently occurring sand layer between the second and third layers. In places, the main layers also disappear.
The sand is firm but porous and delivers substantial water pressures underground to be coped with by any excavation or tunnelling. Hence the armoury of modern techniques and sometimes traditional techniques which have been required.
The challenges have been met, but with significant difficulties along the way; much of the work has been more demanding and time-consuming than thought originally. An infamous incident in 2008 saw leakage through a diaphragm wall, which damaged old weavers houses. Station work was suspended and the prospect of cancelling the whole project mooted.
But this nightmare low point in 2008 passed and the project turned around. One by one different underground techniques are proving their worth and the scheme is on an albeit heavily revised course. Even the notoriously prickly Amsterdam population has turned positive. Some of the damaged buildings have been repaired.
Cost of the scheme as currently estimated is over EUR 3bn (USD 3.8bn) against an initial estimate of EUR 1.4bn (USD 1.8bn) and the opening date has been put back by five years to 2017. But that now looks certain.
"There was a very closed culture for the project earlier on," says Gerard Scheffrahn, currently contract manager for the deep stations. He is a management consultant who came in after 2008 to try and help turn around a project which had become almost paralysed. "After the difficulties of the incidents, things were even more closed in, as everyone was highly defensive."
A big effort was made to change the atmosphere of the project to one of much greater transparency, both internally, and in the relations with the public. This began with an acceptance that the project should continue and that rather than assign blame for the difficulties, with potentially extended legal actions and potential breakdown, a compromise should be reached.
"There are many possible causes of the incident" says Scheffrahn," adding that he cannot comment on whether eventually there will be legal actions and claims.
"But for the moment that is to one side and a pain-gain contract arrangement was instigated to keep the project going," he says. Along with that came a complete restructuring of the management culture, including a beefing up of the city’s management oversight team. New risk assessment methods were instigated and more detailed incident protocols and sign-off procedures.
"We began by bringing in a variety of European expertise for round-table sessions to examine the incidents and the general methods of the project," says Scheffrahn. "After these meetings, which brought in insurance representative as well, things started to move along better."
Amsterdam seems on course to achieve its North-South Line.
