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Stabilisation of Hurst Castle using geopolymer injection solution


Site washout

Hurst Castle, located in Southern England, is a historic fortress with origins dating back to the reign of Henry VIII. Initially constructed as a coastal defence fortification, it underwent significant expansions during the Napoleonic Wars, transforming into a gun emplacement castle. This historical site faced a critical challenge due to soil erosion caused by sea tides, which led to catastrophic structural failures, including parts of the castle collapsing into the sea.


The primary concern at Hurst Castle was the erosion that threatened the stability of the entire structure. Despite its historical modifications, the castle’s foundations were compromised by the relentless action of the sea, undermining the integrity of its foundations. One significant section of the castle had a catastrophic failure and there was the imminent risk of adjacent sections falling into the sea. This necessitated an urgent and effective solution to preserve this significant piece of history.


Shingle mass

Shingle mass

Initial assessment and trial injection

The project commenced with a trial injection on the beach adjacent to Hurst Castle. A specialist geopolymer was injected into the beach shingle to assess the feasibility of creating a solid layer beneath the castle’s foundations. This initial test yielded promising results, producing a substantial piece of solidified geopolymer and shingle indicative of the potential to significantly enhance the ground’s density.

Penetration testing and laboratory analysis

Subsequent penetration testing demonstrated a marked improvement in the soil’s density, transitioning from virtually no resistance to achieving over 40 blows for every 100 millimetres. Laboratory analysis of the polymer-shingle mixture revealed an average compressive strength of nearly three megapascals, surpassing the project’s requirements for durability and strength.

Solution design

Implementation of the Treatment Plan

The successful trial and testing phases led to the development of a comprehensive treatment plan aimed at stabilising the existing foundations. The strategy involved injecting the geopolymer into a shingle beneath the foundations across ten metres on either side of the failed area. Loose shingle was strategically placed underneath the foundation, and injection tubes were installed to facilitate the geopolymer’s injection directly beneath the foundation, creating a stable and solid base.

Execution and Results

Utilising a mobile injection unit and operating between tidal cycles to avoid high tide conditions, the team successfully injected approximately 2.3 to 4 tons of polymer at 120 points. This intervention effectively stabilised the structure, preventing further movement and safeguarding Hurst Castle against additional erosion threats.

Geobear on site

Geobear on site


The innovative approach to addressing the erosion challenge at Hurst Castle not only preserved a historical monument but also demonstrated the effectiveness of Geobear’s innovation in geopolymer design to stabilise structures at risk of water-based erosion. This case study highlights the importance of adaptive solutions in the conservation of heritage sites.

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