How railway tunnels under Amsterdam revealed a medieval treasury

(CNN) The construction of Amsterdam’s north-south metro line was a major problem – a 15-year, budget-busting operation that involved painstakingly digging under the foundations of centuries-old architecture.

Nor was it an easy task for archaeologists tasked with sifting through soft mud to preserve the history disrupted by the massive engineering project. Their potentially dangerous work took place in concrete boxes that were pressurized to keep flooding away from the Dutch capital’s ubiquitous waterways.

Today, the fruits of their underground labor can be seen at Rokin Station, one of eight stops on the route and one that doubles as an impressive underground archaeological museum with nearly 10,000 artifacts on display.

Worth a visit on its own, the train station is a testament not only to the rich heritage Amsterdam is built on, but also to the engineers and archaeologists who have worked so hard to preserve it.

The fruits of their labor are displayed in two showcases positioned between the escalators, one showcase at each end of the station. On any given day, it’s not uncommon to find a commuter riding up and down the escalators just to get a better view.

A significant number of these artefacts have been found in and around Rokin, a neighborhood that lies on the city’s main river, the Amstel, which during its development from the 13th century was the heart of Amsterdam.

Waterways tend to become garbage dumps that accumulate items over the centuries. The Amstel riverbed around Rokin was no different.

“The sheer volume of material we excavated during the construction of the north-south line was extraordinary,” says Peter Kranendonk, one of two senior archaeologists leading excavations during the subway project.

“The construction gave us a unique opportunity to excavate under the city to a depth of 30 meters,” he adds. The oldest objects found were conch shells, which are over 115,000 years old.

The artifact displays at Rokin Station are divided into different themes. In the northern exhibition, the focus is on objects related to food, science and technology, arms and armor, communications, play and recreation, personal artifacts and clothing; The southern exhibition includes objects from the fields of buildings and structures, interior design and accessories, transport as well as crafts and industry. All of these artefacts offer glimpses into Amsterdam’s glorious and sometimes unknown past.

“Some objects, like the 500-year-old coins, have a direct history behind them,” says Kranendonk. “Based on the finds, we can also say something about the use of an area,” he adds.

Negative pressure

Archaeologists worked in compressed concrete chambers.

At one site in Rokin, the excavation of a collection of chopped animal bones suggested the existence of a butcher’s shop nearby in the 17th and 18th centuries. Elsewhere, an abundance of furniture fittings confirmed the presence of a 19th-century furniture factory.

“Prior to the excavation of these artifacts, the city had an archaeological archive of only about 70,000 artifacts,” says Hoite Detmar, who served as director of the north-south subway project from 2016 until its completion. “When we built the north-south line, we found ten times as many.”

Kranendonk details the rather unconventional excavation process behind these finds.

“It wasn’t a normal excavation,” he says. “Usually, before building, there is excavation. But in this case, the blueprints had already been finalized. So we had to become part of the existing process. The civil engineering team built and we excavated.”

Working in the caissons was a new experience for the archaeological team. A caisson is a large, watertight, open-bottomed concrete space that is kept out of water by air pressure and in which construction is carried out underground or under water.

“It was an interesting experience, but also a bit scary,” says Kranendonk. “The deeper you go, the more compressed the air becomes. It’s like deep sea diving.” To acclimate to the caissons, teams had to spend time in a hyperbaric chamber before entering and after exiting, otherwise they were at risk of “the flexing” as gas bubbles build up in the body, potentially leading to paralysis .

Approximately 700,000 artifacts have been recovered from under Amsterdam.

To allow people to engage with the Rokin displays at leisure, an online database of nearly 20,000 items, Below the Surface, has been created, providing information on each and every item in the showcases. “It’s a discovery process in its own way,” says Kranendonk.

A documentary about the excavation called ‘Amstel, Spiegel van de Stad’ (Amstel, mirror of the city) and a beautiful illustrated book ‘Amsterdam Stuff’ were also made.

“We knew that we would be working in the city for a very long time and would cause a lot of inconvenience to the citizens,” says Hoite Detmar. “This was one of the many ways we were able to give back to the city.”

In addition to the two archaeological exhibits, the walls of the Rokin train station next to the tracks are covered with stone mosaics by artists Daniel Dewar and Grégory Gicquel depicting 33 of the unearthed artifacts – a keyboard, a pike, a teapot, dice, a butterfly, among others .

There is even a mosaic of a crocodile depicting an excavated crocodile jaw, a fairly unusual find for this part of the world.

An engineering feat

The new subway line had to dig under old city foundations.

Hailed as one of the most challenging infrastructure projects in the Netherlands, the north-south route was inaugurated with much fanfare in July 2018. The route is six miles long – 4.5 miles of which is underground – and runs under the historic city centre, the main train station and the IJ, a water canal that separates the north of the city from the city centre.

The line connected neighborhoods such as the northern suburbs (previously unconnected by rail) to the city center, eliminating the need to take a ferry across the IJ or go through the IJ Tunnel. It also halved the 30-minute drive time required to traverse the city from north to south. Immediately after the line opened, an estimated 120,000 commuters used it daily.

However, initial plans for the north-south line did not meet with enthusiasm. Public opposition to this project was sparked by the traumatic experience of building Amsterdam’s first metro, the East Line, in the 1970s. A large part of the Nieuwmarktbuurt neighborhood was demolished to make way for the project, causing anger and riots in 1975.

The artwork on the walls of Rokin Station represents objects found during its construction.

Construction of the north-south line began in 2003, with one of the main objectives being to preserve the existing built environment.

With this in mind, a specific route was chosen and several new construction techniques were used, including the use of a bespoke tunnel boring machine, which made it possible to dig deep into Amsterdam’s soft soil without affecting the structures above.

However, the project was overshadowed by public concerns about house collapses. In June 2008, work came to a halt when four 17th-century buildings near the Vijzelgracht station sank about 25 centimeters and became uninhabitable.

“Fortunately, no one was injured,” says Detmar. An independent assessment was carried out and work resumed in the summer of 2009. The historic houses have also been restored.

The project was fraught with many technical challenges, which resulted in a doubling of the construction budget from 1.4 to 3.1 billion euros. The original launch date of 2011 has also been pushed back to 2018.

Despite these challenges, the north-south connection has worked smoothly since it was launched.

Detmar is pleased with the appreciation of the project so far, especially for the art at each of the eight stations on the new line.

Rokin is the highlight.

“When I travel to Rokin Station, I see people really studying the archaeological exhibits,” he says. “I hope more people will take the subway to see this underground museum.”

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