Geobear was contracted by Skanska to design a solution for void filling between a new railway box structure and existing railway viaduct near Bermondsey.

The box structure, called the Bermondsey Dive Under, was designed and introduced to provide grade separation between the Kent and Thameslink lines, which would enable the continual operation of the two lines without signalling interruptions.Bermondsey Dive Under drawing

Once the box structure and adjoining line were constructed a narrow gap (0.6m) was left between the new structure and the adjacent viaduct. This void needed to be filled as it was an unprotected drop and could capture debris which forms a potential fire hazard. Geobear was contacted to fill the void using their lightweight geopolymer resin, a material that expands to fill the void on site.


There were no feasible alternatives for the customer to use on this project, in terms of materials a cementitious fill was not viable due to the weight and access restrictions also necessitated a solution with limited plant.

Geobear solution

In order to fill the void a glass reinforced plastic former was used as a soffit for fill material. This base required a lightweight fill material and is where Geobear geopolymer material is ideally suited.

One of the mobile Geobear units could access the site from below and our material was pumped via a hose into the void from above. We filled the void in a layering system to ensure it was completely filled, the area was finished with a preformed flashing put on top to prevent any water ingress.

The main works took two days, however, we also provided additional void filling of brick barrels built into the original viaduct construction which took a further day to complete.

Bermondsey Dive Under void filling