Using “marshmallows” to counter structure-borne noise
Along Zurich’s Europaallee, high-value properties are being constructed right next to the railway track area of Zurich main station. Noise per se is not the problem, but structure-borne noise – an excellent reason for the vibration protectors to brighten things up.
7.30 a.m., Europaallee, Zurich. Suited and booted commuters stride out from the railway station underpass towards the office towers, their laptops in one hand and a take-away coffee in the other. They move expertly through the development zone, ignoring the pop-up stores to one side of their path and the hoardings on the other. There are only a few places along the hoardings where a curious eye might peer through to the deep excavations behind. One such spot is our meeting point with Thomas Rupp and Adriano Manuel: construction site F, as it is known. A 54-metre-tall building complex for housing, services and retail trade is planned for construction here until 2019. The site is directly adjacent to the railway track area of Zurich main station. This is also the reason why we are here today.
Walking on marshmallows
At first glance, there is nothing remarkable about this excavation. An earthen ramp descends to the foundation slab. The usual construction site bustle prevails. And then suddenly, we notice them: black, purple, deep yellow, deep scarlet and azure mats are stacked here and already cover a significant portion of the foundation slab. “These are elastomer mats,” explains civil engineer Adriano Manuel. “They protect the building from vibrations caused by the trains and, more importantly, from structure-borne noise.” Structure-borne noise is noise generated by vibrating elements in a building. The elasticated mats mean the building is decoupled on all sides from the ground and the vibrations transferred to the building are vastly reduced. Cautiously, we step onto the yellow surface in front of us. We are amazed: our feet sink right in. Walking makes us feel almost dizzy, as the material is so soft. The orange mats are quite different. You can walk on them and the sensation is very pleasant, like walking across a springy forest floor. This clarifies what the colours mean: each colour indicates a different elasticity, and thus capacity.
A colourful image of the loads exerted by the buildings
“The load of a finished building determines the type of mat and therefore the colour,” explains vibration specialist Thomas Rupp, referring to the colourful mosaic. The mats are therefore arranged according to a strictly predetermined layout plan created by the two experts. This is because the loads are distributed very unevenly. They are highest in the area of the three towers, which will measure 38, 47 and 54 metres respectively upon completion. The towers have combined pile raft foundations. A single pile will be subjected to loads of up to 13 MN – equivalent to the weight of around 10 of the most powerful SBB locomotives. The piles are therefore separated from the building they are supporting using vibration mounts made from extremely stiff plastic mats. The black circular discs stand out clearly from the colours surrounding them. The layout of the mats resembles the playing field of a trendy new ball game. The lowest loads will occur in the areas where the two basement floors will not have anything built over them. The mats here are lemon yellow.
Every little detail is crucial
“The colours are extremely important for quality assurance,” emphasises Adriano, who is carrying out the construction monitoring. “If the wrong mat is laid in just one place, there can be serious consequences further on down the line.” The building could sit unevenly, or the protection from vibrations might not be guaranteed to the required extent. This means the expert is often on site to monitor the implementation of the project. Even the concrete for the foundation slab had to be carefully selected. “Ordinary lean concrete is too rough for the mats.” It is equally important to completely seal the mat layer before the base plate is cast onto it. Adriano and Thomas meticulously check the adhesion of the plastic film. “If concrete penetrates between two mats, the vibrations will be able to find a way into the building, and then all the work will be for nothing.”
High demands
Legal minimum requirements regarding vibrations and structure-borne noise are laid out in the BEKS*, as it is known. The owner of the high-end property on the Europaallee is aiming for a maximum noise level of 33 decibels for a passing train – roughly the same level as a modern ventilation system. “The elastic building suspension will allow us to achieve this value,” insists Thomas. Nine a.m. has rolled around – it’s break time, and the construction site empties. Everything suddenly falls quiet. Only now do the sounds of the city filter back in – the babble of a radio in a café, laughter, the shrill of the rails. “The first vibration measurements on the base plate will be interesting,” says Adriano. “Then we’ll have concrete evidence of how good our predictions were.”
* Swiss Agency for the Environment, Forests and Landscape (SAEFL, now FOEN) (1999): Directive for the Assessment of Vibrations and Structure-Borne Noise in Rail Traffic Installations (BEKS).