Following structural inspections of the bridge on Newnham Hill, which spans the A45 at Daventry, it was found that the approach had settled and was becoming a safety issue for road users. KierWSP, who maintain that section of highway on behalf of Northamptonshire Highways, required a solution that would address the issue caused by badgers and poor compaction during construction.
Geobear was contacted to inspect the site and propose a solution using their specialist geopolymer injection process. The desired outcome would see the sub-base and soils compacted using a structural geopolymer, ahead of resurfacing.
The client had limited options for this work. The bridge spanned the busy A45 an important commuter route and excavation works to remediate the issue would have required partial closure. Pressure grouting was an option open to the client, but the long programme times did not meet the criteria.
We proposed to use a structural geopolymer injected at vertical depths across the affected area. Additionally, a void had opened up under the pier caps due to settlement, so horizontal injections would be required.
A Geobear Engineer designed the proposal which was accepted by KierWSP and we worked as a subcontractor for PGR Construction to deliver the works.
The first step in the process was to remove the badgers from the area beneath the bridge approach. Once this activity was completed we were able to commence our works without endangering any wildlife.
Our solution consisted of injections in a grid pattern at one metre centres across the approach. The injection points would be drilled down to 6.5 metres and upto five injection tubes placed at each point to a differing depth at one metre vertical placements. The spacing is engineered to ensure the geopolymer material is equally distributed throughout the site and provide the required load capacity and to ensure no further settlement would occur.
In order to ensure load capacity was achieved we conducted pre and post dynamic cone penetrometer tests. The tests verified that the ground strength would meet highway standards required for the road once resurfaced.
The pier cap void was filled from the embankment edges to ensure it was fully filled.This involved working on a steep slope so an appropriate anchoring system was used to ensure safety of the workforce.
We completed the work over three shifts and used two different types of geopolymer, one for the ground improvement to ensure bearing capacity and one for the void fill which ensured no further material loss would occur in future.
The client resurfaced the road following our works.