Building underneath a valuable heritage site

Building underneath a valuable heritage site always demands extra care, but special skills and planning are needed when the soil is weak and the historic structures overhead are fragile.

Those were the conditions our engineers faced when creating a two-level underground parking garage in the Rotermann Quarter, a former industrial area in the heart of Tallinn, Estonia that has been given new life as a modern shopping, entertainment, business and residential zone.

The newly-revamped factory area turned architectural showcase has become one of the city's major attractions. There was no room for error, therefore, to incorporate a parking garage where three new shop, office and apartment structures were being built directly overhead, one of them within the old facade of a salvaged building.

Working on weak soil

The task was far from straightforward, explains Veikko Vapper, Foundation Engineering Unit Manager for Lemminkäinen in the Baltic States. "The big challenge is that this is old seabed, with soft, weak soil layers," he says.

Added to this complication was an unusually high groundwater table associated with being about 500 metres from Tallinn Bay. "The table is 1.5 to 2 metres from the ground, so we had to minimise the inflow of water in order to prevent settlements in the vicinity of the buildings and underground networks," he notes.

"Work like this has never been done in Estonia," says Deivy Paavo from Dollimar Invest, noting that, in addition to preserving the historic walls, safety was a primary concern.

Good vibes, no vibrations

As the garage's base slab is nearly seven metres below groundwater level, the enormous pressure had to be overcome. Part of the solution employed involved installing the sheet piles that surround the excavation as a site permanent structure – a first for a similarly-designed underground parking garage project in Estonia. Pumps and drainage systems also played a role.

It was essential that vibrations be kept to a minimum to prevent soil displacement and damage to the buildings above and nearby. We used a special low-vibration press to install the sheet piles, and placed sensors and hundreds of geodetical measurement points to nearby buildings to monitor the situation.

A 53-metre brick chimney standing directly atop what would become the garage area was something the team was worried about most. A combination of load-bearing support piles forming a retaining wall and tension strands was used to take the loads and keep the chimney stable.

Vital for vision

We worked with Paavo's team as well as expert consultants from Tallinn Technical University to come up with solutions. "The working relationship with Lemminkäinen has been very good," Paavo says.

The underground parking lot is a vital support function for Rotermann's shops and offices and will play an important part in attracting tenants to what will be the area's second residential building.

The result will guarantee improved public access to this popular attraction as well as preservation of the area's unique character, ensuring that the area is every bit the vibrant work, living and culture space that planners envisioned.

Text: Steve Roman
Photo: Sergei Zjuganov