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Outdoor slate paving unites tradition, art and sustainability

Trovet i Trondheim med fliser av skifer på bakken. Olav Trygvasson er i midten av bildet.
When Torvet, the market square in Trondheim, Norway, was fully renovated in the summer of 2020, almost 12,000 sqm of the city's finest outdoor space were paved in slate. There were several reasons for this. Besides aesthetic requirements and a low environmental footprint, the new civic paving would have to pass the great “iron ball test”!
The square in Trondheim with slate outdoor tiles on the ground. The statue of Olav Trygvasson is in the middle of the square.

Anyone seeing the world through King Olav Tryggvason’s eyes would have spent day in and day out looking down on one of the most planned squares in Trondheim. There was never really any doubt that the newly refurbished Torvet in Trondheim would consist of slate.

– Both the architects and Trondheim Municipality wanted the surface for the ‘new’ Torvet to be slate and we gradually realised that this was a solution that worked for all parties, says Lisbeth Alnæs.

She is head of research at Sintef, which assisted the municipality in connection with the project.

Different stone for different uses

The square will be subject to a great deal of wear and therefore needed stone paving and an underlay that could withstand this. That’s where Alnæs and Sintef came in.

– We assisted in choosing the optimum material to use. The different areas of Torvet have different functions and uses, which allowed for several solutions to create a natural stone surface.

The square in Trondheim with outdoor tiles of Offerdal slate on the ground. Vår Frues Church is seen in the background.

In particular, the area that surrounds the statue of Olav Trygvasson, called the “centre area,” needed a solid underlay matched to both the load and ground conditions. Concerts are often held here, where large crowds gather. The areas on the edges of Torvet, to be used mainly as pedestrian and cafe areas, would not need such robust stone.

– One of our most important tasks was to help the project make good choices of materials and structure that were neither over- nor undersized, Alnæs explains.

To support their professional advice, Sintef set up a test field at Nyhavna in Trondheim. To ensure that the underlay could endure the stress placed on it, Alnæs and Sintef found a large metal ball.

– We made it a bit of a stunt, where we dropped the ball from different heights. This helped us ascertain that the underlay would be able to withstand accidents while rigging stages and audience seating. This new Torvet was intended to be an active urban space, so it was important that it could actually be used, she explains.

A tall crane with a man on top who will drop an iron ball down on Offerdal slate to test its strength.
An iron ball is dropped on the slate tiles from different heights.
A lady points at an iron ball that has been released from a great height down on slate tiles from Offerdal to test its strength
The slate passed SINTEF’s iron ball test – one of several methods for testing the strength of the slate.

Besides the stone and underlay having to withstand the stresses they were subject to, it was also important to have a firm underlay.

– Slate has a non-slip and even surface, which was naturally also included in the assessment. Both the municipality and the architects were aware that Torvet had to be a safe and accessible place for everyone.

Technical specifications and test results: Datasheet Offerdal slate, Datasheet Oppdal slate and Datasheet Otta slate.

Traditional material

The fact that the stone had to withstand the planned stress was naturally important, but the stone’s cultural historical origins were also important. Slate is an indigenous Norwegian material, with rich traditions both in Norway and in Mid-Norway (Trøndelag).

In total, almost 12,000 sqm of slate were laid on Torvet. Around half of the stone came from Alta and the rest from Oppdal and Offerdal in Jämtland. The various different slate types form a pattern inspired by traditions from Mid-Norway, which can be seen particularly well from the air.

– Both the architects and the municipality knew that they wanted the square to have a slate surface. The material helps to tell the story of what Trøndelag is and has been, while also meeting all the requirements for a surfacing a modern urban space, concludes Alnæs.

The new main square, designed by Agraff Architecture, is beautiful, magnificent and flexible. The floor of Trondheim’s grand living room unites art and sustainability and was awarded the Landscape Architecture Prize after its opening in 2020.

With quality requirements in the form of robust and durable materials that would stand the test of time, various different slate types in different colour spectrums were chosen to adorn the surface of the 12,000 sqm living room. The slate was to be laid in a pattern designed by artist Edit Lundebrekke, inspired by a Norwegian knitting pattern, and a traditional patterns from the Trønder region (Mid-Norway). By mixing Offerdal and Oppdal slate, the pattern clearly emerges, revealing the true grandeur of Torvet.

The combination of quality and sustainability will bring joy into the next century and beyond.
Portrett Lisbeth Alnæs Forskningsleder SINTEF

– Both the architects and the municipality knew that they wanted the square to have a slate surface. The material helps to tell the story of what the region Trøndelag in Mid-Norway is and has been, while also meeting all the requirements for a surfacing a modern urban space.

Lisbeth Alnæs
Research Manager, SINTEF

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