The sink hole appeared after the collapse of the 100 year old redundant tunnel which was 15 metres below the road surface. The tunnel was used by Titan Works to gain access to both sides of the work site without using Hogg Lane. The sink hole was approx 3 metres wide. 

GS Foam Concrete established a safe method to infill the sink hole and tunnel which was to infill the sink hole in one metre stages until it was full, up to 100mm from the road surface. We were also hoping that the foam concrete would flow into the tunnel and infill the tunnel void at the same time, but with the collapse being so substantial we could not guarantee this would happen. We asked our client to build a temporary formwork wall at one end and our site team would install a 76mm delivery pipe and vent so we could infill the remaining void in the tunnel if the foam concrete hadn't managed to penetrate the collapsed material. The foam concrete was pumped into the sink hole and unfortunately the material couldn't penetrate the collapsed material so we had to use the sacrificial pipework in the formwork for the remaining void. 

The mix design used on this project was P460 @1500kgs wet density to achieve a compressive strength of 4n/mm2 @ 28 days. 

The sacrificial hoses were installed prior to the shutter/formwork being erected. The delivery hose/vent was sealed into the shutter with enough hose left showing so we could clamp our delivery hose off the concrete pump and place the foamed material through the pipeline. The vent pipe was fixed to the tunnel soffit around 8m from the shutter face. 

Due to the nature of the foam concrete a maximum pour height of one metre is the most we can place in one visit, otherwise the foamed material starts to crush under its own weight adding volume to the original order. This is due to the wetter material crushing down the dryer material. Also we did not want to place more than one metre in height as this would put excess pressure on the tunnel and potentially cause further contamination close to the void. The sink hole was completed in 5 visits and the remaining void in the tunnel took 2 visits.