Rail viaduct project blends old & new infrastructure in Tallinn, Estonia

When Selektor Projekt OÜ won the tender for the Paldiski Road Viaduct Project as part of Estonian Railway’s multi-year expansion project, the team of engineers was ready to take on the challenge.

There were three parts to it: designing a railway viaduct, a widened embankment, and a retaining wall to support a new rail line through the center of the capital city of Tallinn.

Erki Reinsalu of Selektor Projekt said: “We had already developed a vision during preparation for the tender, and now we had the chance to really design something outstanding in the very center of the city.”

The challenge - pairing engineering with aesthetics

While each aspect of the project presented an independent engineering feat, the challenges didn’t stop with combining the three elements.

The municipality of Tallinn and the National Heritage Board required the new infrastructure project to meet the codes of today, while matching the aesthetics of the existing viaduct that had been constructed in 1926.

On top of the engineering and aesthetic demands, tight timelines and an even tighter construction footprint would be required. This due to continuous railway traffic on the adjacent rail lines and viaduct throughout the project.

“Bridge engineering, as in any other structural engineering niche, is gradually becoming more than the composition of structurally sound systems,” said Reinsalu. “It is an art of bringing together multidisciplinary parties.”

That multidisciplinary initiative began with a preliminary design concept from Reinsalu and Tarmo Jõe, the other key member of the design team at Selektor Projekt.

The design used a small block, mechanically stabilised earth (MSE) retaining wall to create the space for the embankment.

However, they soon discovered that their proposed solution couldn’t meet the demands of the project.

“[That option] turned out unfeasible to implement due to very heavy soil and traffic loads and restricted earthworks,” explained Reinsalu.

“At one point we were even worried that existing railway embankment widening could not be done with such limited anchorage.”

The solution - Redi-Rock wall using the Positive Connection system

At first, the team considered a last resort plan of a cast-in-place gravity wall, but it would have exceeded the initial time and budget calculation twofold.

At last, they were presented with a solution that would check not only their design constraints but also the aesthetic concerns.

Local Redi-Rock manufacturer, Pärnu Graniit OÜ, offered a solution to use massive precast modular blocks.

By using the Redi-Rock Positive Connection (PC) system, they could meet the confined space requirements while supporting the necessary loads – and at a competitive price.

Where many MSE wall systems rely on friction or mechanical fasteners for the geogrid connection, PC blocks are cast with a center core slot where a 300-mm-wide strip of geogrid wraps through the block.

By significantly reducing the chance of a connection failure and providing a weight-independent connection, the connection strength is consistent from top to bottom of the wall, allowing designers to factor almost the entire tensile strength of the geogrid into their design.

In this instance, with just 4.5 metres of geogrid tailing behind the wall that ranged from 1.5 - 6 metres in height, the wall could support the significant load from the crest slope of the embankment.

It was also able to support the load from the railway infrastructure and traffic, where systems with less connection strength at the top of the wall couldn’t do within the confined space.

Redi-Rock blocks are manufactured with wet cast concrete, which allows the face of the blocks to carry rich, detailed textures.

The Redi-Rock Kingstone texture provided the natural stone appeal that the city planners and architects, as well as the National Heritage Board, hoped to carry over from the original structure’s limestone block walls.

The design - maximizing versatility of the Redi-Rock system

The finished retaining walls on each end of the bridge present a cohesive appearance, and the versatility of the Redi-Rock system allowed the design team to modify the engineering behind the walls to suit the varied demands of the project.

Their final design, which was third party reviewed by geotechnical consulting firm Geoman Ltd., used:

• A combination of gravity and reinforced wall sections. While the PC blocks and geogrid strips provided the structural support for the reinforced portions of wall along the embankments, a gravity wall was used in front of the structural support for the bridge.
• A blend of various batter angles for wall face. Where the new retaining wall joined the old viaduct, Selektor used zero degree batter to match the lines of the historic structure. The retaining wall then blended seamlessly into the standard 5 degree batter for the rest of the wall.
• A tight radius curve. Because the massive precast modular blocks taper from front to back, they allow for convex and concave curves. The ability to turn a tight radius with the walls allowed the pedestrian pathways adjacent to the new infrastructure to remain, making space for the new rail traffic without interfering with the existing pedestrian and vehicle traffic.
• A smooth top of wall finish. The design called for a freestanding hollow-core block for the top course of blocks to facilitate cutting them in the field to match the grade. This allowed for caps to be placed along a smooth surface, rather than having step-downs between blocks. This helped match the look of the historic structure.

Ultimately, the 600 linear metres of retaining wall allowed the embankment to be widened by 4.5 - 6 metres, creating space for the new railway track and viaduct.

The bridge itself was skewed 53 degrees and needed complicated post-tensioning over two spans of 14 metres supported mid-pier with a V-shaped body to minimise the ground area between the traffic lanes.

The installation - overcoming weather and meeting timelines

Construction of the retaining walls began in January 2021 with two parties splitting the work.

Pärnu Graniit not only manufactured the blocks, but also delivered and placed them with a crew of three and a hydraulic crane operator.

LEONHARD WEISS Viater OÜ, a LEONHARD WEISS OÜ subsidiary, managed the excavation and geogrid placement with a crew of two and an excavator.

As the work begun, weather became an additional factor to consider for the installation.

“At some point the temperature dropped below -20°C. It was challenging, but it was still possible to construct the wall,” explained Aleksei Dohin, project manager for LEONHARD WEISS OÜ.

Teet Rehtla, sales manager at Parnü Graniit, explained that the ability to install Redi-Rock in varying weather conditions is just one advantage over a cast-in-place solution, which requires optimal weather conditions for concrete curing.

“With each machine placed block, we’re installing 0.5 square metre. The speed of Redi-Rock installation makes it a great choice for situations like this where the timeline matters in keeping the entire project on schedule,” said Rehtla.

As construction progressed, what had already been a heavily watched project throughout the city garnered even more attention. As the wall took shape, it received a lot of positive feedback from passerbys who saw the wall.

The outcome - a beautiful balance of old and new

The wall was finished by May 2021, and positive feedback about the entire project continues to roll in.

The project achieved the ultimate balance – enriching the urban environment with a structurally sound solution that will last for generations to come, while also providing details that please passersby when viewed up close.

“I can honestly say Paldiski Street railway bridge stands out with its majestic curve-shaped walls that complement the modern shaped bridge deck and V-shape mid-pier – which on their own would stand out next to the existing old railway bridge,” said Reinslau.

“[They’re all] beautifully balanced and brought together with the old by rough surfaced stone blocks.”

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