Vibration isolation for Norfolk House development 

By Mason UK

Norfolk House once served as the headquarters for Operation Overlord, the Allied invasion of German-occupied Western Europe at the end of the Second World War. Now, it has been converted into a new eight-story commercial office building. However, given the building’s proximity to the London Underground and HS2, vibration control was required for the newly excavated basement level. 

Originally built in the 1930s, Norfolk House is a Neo-Georgian red brick building, located in St James’s Square. Skanska was awarded the contract to redevelop the site, in a project estimated to be worth between 60 and £72 million. Work began in October 2020 and was originally scheduled for completion in September 2022.

The redevelopment of the historic site involved the creation of an eight-story commercial office building and the reconstruction of two brick and Portland Stone facades. These facades are nearly identical to the original, but with slightly adjusted floor heights that are better suited to office use. 

The project also involved additional excavation to create a deeper basement. The new basement will provide storage for bikes, a gym, and accompanying changing rooms. Owing to the site’s proximity to both the London Underground and major train lines, it was necessary to implement acoustic fittings and vibration control measures.

The solution was a concrete floating floor. A jack up floor system was proposed which involves raising the floor off the structural slab, on either spring or rubber mounts, to interrupt the transmission path for ground borne sources of vibration. To design and oversee the installation of this system, contractor J Coffey brought Mason UK on board to invest in the quality and support package, after having worked successfully together on previous projects. 

A flexible system

From gyms to bowling alleys, studios to plant rooms, concrete floating floors are used for many purposes, usually to prevent noise passing through the floor but also, as was the case for this project, to isolate against vibration. Without a floating floor, vibration from the tube would pass into the building’s structure and be heard as reradiated noise.

The Mason jack up floating floor system, pioneered in the 1960s by Mason Industries, carries several advantages including high performance and flexibility. Although there are variations in system design and installation, the process typically involves the following steps. First, rubber or spring mounts are placed strategically on a bond breaking separation layer, covering the area where the floor is to be treated. Second, reinforced steel is added, which later provides structural integrity to the concrete. Third, concrete is poured over the area, level with the top of the mounts. 

Finally, when the concrete has cured, the floor is raised, or ‘‘jacked up’’ on the rubber elements or steel springs. Once the floor has been fully jacked to the required elevation, there is an air gap between the floating floor and the structural slab underneath, thereby breaking the transmission path for vibration. Depending on the specification and performance requirements of the floor, either rubber or spring jacks are selected. For Norfolk House’s basement the FSN Rubber concrete jack-up system was suitable.

The flexibility of the system is best seen by contrasting it with the alternative, a system of mats or pads. These are typically laid out and then boards are placed on top. As everything must line up perfectly, you have more restrictions and cannot easily accommodate design changes or unforeseen challenges which are quite common in projects like these.

The jack up system offers flexibility and can be amended quite easily, a factor which became a distinct advantage during this project. In the original design, the floating floor was to cover one continuous area. However, it was later revealed that some of this area was to be occupied by the temporary canteen and site welfare facilities. 

Mason therefore had to work around this temporary obstacle, ‘‘designing on the fly’’ as one engineer involved in the project put it. Due to the flexibility of the system, the Mason team could make design changes to accommodate the site welfare and an access corridor, laying a floor that was smaller and of different dimensions to what was proposed in the original design. Once the welfare facilities were relocated, the team returned to lay the remainder of the floor. 

However, this was not the only occasion where the flexibility of the jack up system and the ability of Mason’s onsite team to adapt to design changes would prove advantageous. As well as moving the edge of the floor to accommodate the site welfare, the team also had to work around pipes and penetrations coming through the floor required to service areas where showers and drainage would be installed. Again, some penetrations were not reflected in the original drawings Mason had received. However, as the jacks could be moved around relatively easily, they could accommodate the space needed for pipes. 

‘‘This was such an interesting project, both because of the prestige of the development and because of some of the challenges we encountered,’’ recalled Tom Van Dongen, Senior Project Engineer with Mason UK. ‘‘There were many additional details that were never included in the original design, so we had to be reactive and work closely with the contractor on site.’’

A lot goes on in the redevelopment of prestigious buildings like Norfolk House. Given the proximity of the building to major train lines and the London Underground, installing a high-performance floating floor was imperative. However, this project demonstrated that things can often pan out differently from the original design, which brought to the fore the flexibility of the jack up floating floor system and the expertise of the Mason UK engineering team on site.