Geobear was contacted by WSP to provide a design solution for a voiding and ground improvement
This project was emergency works within the footprint of the A14 Cambridge to Huntingdon Major Improvement scheme at the Junction 14 Girton Interchange.
The affected section of the junction comprised a two-lane slip road section of motorway with cloverleaf junction that interfaces with the A428 and A14. This section of the northbound M11 carries around 40,000 vehicles a day and forms part of the strategic highway network which also serves Stansted Airport and provides access to strategic ferry ports.
Issues with the embankments to the bridge abutments on the M11 overbridge at Jct 14 had been identified by Highways England as part of routine maintenance inspections. Further investigation through visual and ground penetrating surveys and dynamic probing identified the presence of voids beneath the carriageway close to the abutments which, if left, could have resulted in significant settlement or collapse of the carriageway.
An urgent solution was needed that would meet both technical and programme requirements without compromising safety. The solution would require a material that had a compressive strength of 4MPa, implemented with minimal disruption to road users and the wider improvement scheme.
Geobear were consulted by WSP and subsequently designed an innovative and time efficient solution that could be implemented from the carriageway surface.
Emergency Ground Improvement & Void Filling Geopolymer injection scheme on 170m2 of highway to a depth of 2.5m below road surface. The works were required due to the potential for significant voiding & incompetent soils underlying the sub-base on the North & South bridge abutments (the section of highway was essentially considered to be at significant risk of collapse).
Geobear were required to install a material achieving a compressive strength of 4MPa, ensuring that the Geopolymer re-established full contact between the sub-base and underlying soils. Another important aspect to the works was controlling heave of the road surface such that it remained within tolerance of +/- 5mm whilst also ensuring that the nearby concrete wingwalls & overall bridge construction was not adversely impacted.
Due to the section of highway being heavily trafficked, Geobear were unable to carry out the desired amount of site investigation – this required flexibility on-site as voiding and underlying soil conditions were not known, the work also required the ability to adapt to circumstances such as unexpected subsurface structures.
Geobear’s methodology incorporated a set amount of ‘trigger time’ per injection (the time spent continuously injecting material) – this ensured that the effects of the expansion of the Geopolymer remained fully controlled.
Works were verified by visual assessment of the void fill (via CCTV monitoring), movement at the surface of the road verified by laser level monitoring, and the comparison between pre and post treatment Falling Weight Deflectometer testing.
The project was successfully delivered two days ahead of schedule (including additional treatment to areas outside of the initial scope). The focus throughout development and delivery was to “minimise the duration and disruption of the works without compromising safety”. This was achieved by employing four crews, two working on the north and two in south bridge abutment simultaneously.
Early completion was achieved by Geobear, Highways England, WSP and Interserve teams working collaboratively, focusing on minimising the impact on the customer and maximising safety. This result followed a thorough review of design options to select the right solution. Careful planning including opportunities to carry out works in advance; reviewing methodologies and design to maximise productivity and monitoring and managing construction activities.
Despite the design changes and challenging schedule we completed the works within budget. It is the first time a geopolymer solution has been used to this extent on such a major scheme and highlights the benefit of the real time adaptability and flexibility of this design solution under such challenging timescales. Two project engineers were on-site throughout the course of the works and adapted the design methodology subject to new unforeseen variables. For example, as variations in location & extent of expected voiding occurred on site, the decision was made to amend injection spacings & material quantities in order to ensure full coverage was achieved.
Using the Geobear methodology resulted in temporary overnight closures for a three week period. The alternative solution of a full excavation would have meant the carriageway would have been subject to full closure closed for at least an eight week period.
Such was the success of the project it was reported by BBC television and publicised by the Chartered Institute of Highways Engineers.
Customer testimonial – Karl Brooks the Highways England Project Sponsor
‘Geobear’s solution to the essential embankment repair at M11 Girton interchange has been a highlight of this year’s 2018/19 programme of works. The geopolymer injection has provided an innovative response to voiding at the scheme location, allowing Highways England and its supply chain to effectively manage what is a complex stretch motorway that reduces to dual carriageway, has tight cloverleaf interfaces with the A428 and A14 and, at the time, sat within the footprint of the A14 Cambridge to Huntingdon Major Project scheme.
The consideration given to the monitoring of existing assets, particularly in respect of pavement heave was impressive and overall the programme of delivery has seen large improvement versus comparative methods of work. Levels of housekeeping and responsiveness to Highways England’s high standards of safety have been pleasing to see on site and collaboration in working alongside WSP and Interserve (as our design and construction provider respectively) has meant we have met and exceeded in cases, the requirements of the network and our stakeholders’.
Watch the BBC News report about the project: