A section of Aycliffe Road in Borehamwood, Hertfordshire, had been experiencing frequent resurfacing needs due to problems with the soil beneath the concrete slabs. Water ingress had caused washout of fines in some areas, while weakened soils at depth had contributed to continued fracturing of the surface, affecting over 415 meters of single carriageway.
The regular maintenance required was both costly and disruptive to the community, prompting Hertfordshire County Council to seek a long-term solution. They approached Geobear, who proposed a solution to prevent future surface failure, rather than a full reconstruction of the site, which would have been more disruptive and environmentally damaging.
Hertfordshire CC approached Geobear due to its reputation for providing comprehensive solutions to stabilise slabs prior to resurfacing. Geobear was tasked with understanding the causes of the slab movement and providing a long-term solution to prevent it from recurring. They provided a one-stop-shop for the client, from site investigation, drainage repairs, utilities search and mark-up on site, to delivery of their injection scheme.
Geobear conducted a site investigation to understand the issue in full. The investigation involved boreholes to sample the soil and a GPR survey to identify any voids or buried structures. Geobear also completed the utilities search before marking out the site for the injection scheme.
The investigation revealed that defects in the drainage system had caused washout of some of the fines, destabilising the slabs. Additionally, the ground was not well compacted beneath the concrete slab, and one area had issues with weak soil that could not withstand the required axle loads. Both of these issues had led to rocking of the slab, causing movement and breaking of asphalt on the surface.
Geobear first addressed the drainage issues, which took one week to complete. This was followed by 13 consecutive days of ground improvement and stabilisation works using their geopolymer injection technology. One Geobear injection unit was on-site throughout the works, along with an additional drilling team.
The geopolymer stabilisation works involved 1400 drilling and injection points. A geopolymer resin was injected as a two-component liquid into each point, expanding and solidifying upon reaction in the ground, filling micro voids and strengthening the ground. Two different treatments were used in two areas, subsurface stabilisation just beneath the slab, and ground improvement to a greater depth.
Laser monitoring was used to register movement in the stabilisation area, and when 0.5mm of movement was recorded, it signalled that the area beneath the slab had received sufficient material and was stable. Dynamic cone penetrometer testing showed a 60-140% improvement in the top layers of the ground where ground strength was required, and at depth, the results indicated a 50% increase in ground strength.
Overall, Geobear’s solution provided a long-term and sustainable solution for the client, avoiding the need for disruptive and environmentally damaging reconstruction of the site.