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Kentish Town Slab Track Treatment

Geobear produced an innovative design solution to stabilise a section of rail slab track between Kentish Town station and St Pancras station, London.

We were presented with a significant geotechnical challenge where the slab track had experienced rapid deterioration which manifested in concrete slab cracking in numerous locations. The main drive for the deterioration was the soft underlying ground conditions combined with poor drainage; materials under the slab were washing out and appearing on the cess and 6ft sides of the slab resulting in voiding and separation at the base of the slab.

Rail site

Location of slab with injection tubes in place

A design solution was engineered which would utilise one of Geobear’s expansive geopolymer materials, injected beneath the slab, to strengthen the sub-grade. The pattern, depth and control of the geopolymer material was critical to stabilise the area with minimal affects to the existing asset.

A critical requirement for the client was a ground-strengthening solution without having to remove anything. This was the first occasion where a geopolymer injection design had been used as a permanent solution on a live rail slab. With Geobear’s design, we could strengthen the ground without removing the slab track, keeping everything in situ – utilising the residual life of the rest of the asset.

Geobear geopolymer injections were used to stabilise the 10m section of the slab on the Down and Up lines, respectively. The method involved drilling and injecting expansive geopolymers under the concrete base slab. The injection points were in pairs near the rail seating area at 0.75m intervals and at depths of 1.1m and 2.1m below. As the geopolymer is injected under the slab, it will start to fill any voids, push away the water and strengthen the subgrade.

Collaborating with Amey further works on-site included the installation of a transition slab and renewal of track base plates to restore track geometry. We developed the injection plan with Amey to accommodate the new transition slab within our design work. On-site, the ground conditions were as expected, and with real-time control over geopolymer injections, we maintained alignment design with tolerances.

Injection tubes

Injection locations at two depths

The delivery of the Geobear works was completed in four shifts during one possession over the August bank holiday. The fast delivery time of the Geobear method of works equated to a vast reduction in line speed restrictions when compared to a replacement of the section. A traditional approach would have required weeks to complete with maximum disruption, this would result in extremely high possession costs, in the regions of hundreds of thousands.

Using conventional methods would also have meant a litany of complex site logistics, a high amount of spoil generation that would need removing, a significantly higher carbon footprint and no utilisation of existing asset residual life.




On this scheme, Geobear produced 75% less carbon emissions than a traditional solution (verified by Carbon Footprint Ltd). Additionally, no spoil was generated that required removal from a difficult-to-access site.

The result of Geobear woks saw track deflection reduced by up to 44% – a challenging requirement from the client in order for them to run a regulation line speed.

Finalist in GE Awards

The asset’s life was extended by up to 15 years and the programme was delivered within a small window, reducing the period line-speed reductions were in place and the delays associated with conventional replacement.

Following the completion of the injections, we collaborated closely with Amey and new base plates were installed along with concrete stitches, which resulted in substantial improvement and reinstatement of track quality (i.e removal of existing tracks faults).

Network Rail collected measurements independently and quantified the level of improvement in track deflection.

The project has been nominated as a finalist in the 2022 Ground Engineering Awards.


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