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Historic castle saved from coastal collapse with emergency stabilisation

Challenge

A historic fortress (Hurst Castle) suffered a catastrophic collapse as tidal erosion washed out the ground from beneath its foundations, placing the remaining structure at imminent risk.

Solution

Geobear injected a specialist geopolymer into the loose beach shingle, working between tides, to create a solid, stable foundation and help prevent further collapse.

120

Injection points

>40 blows

Per 100mm, post-treatment ground density, up from zero

~3 MPa

Compressive strength of solidified shingle

2.3 - 4 metric tons

Of specialist geopolymer injected

Collapsed rubble of portion of Hurst Castle which has fallen into the shallow sea pools

Background

Hurst Castle, a historic fortress in Southern England dating back to the reign of Henry VIII, faced a critical emergency.

Relentless coastal erosion from sea tides had led to a catastrophic structural failure, with a significant section of the castle wall collapsing into the sea.

The remaining adjacent sections were at imminent risk.

Challenge

The primary challenge was the constant tidal action, which was actively washing away the shingle from beneath the foundations, leaving them completely undermined.

An urgent and adaptive solution was required to save the rest of the historic structure.

Traditional methods were not viable in this sensitive, high-risk, and dynamic coastal environment.

Exterior view of Hurst Castle facade with sea in front, and visible gap in foundation.

Solution

Geobear provided a bespoke, engineered solution that began with a crucial trial.

Trial first: A test injection was conducted on the adjacent beach to prove a specialist geopolymer could successfully bind the loose, wet shingle into a solid, load-bearing mass.

Verification: The trial was a success. Post-injection penetration testing showed the ground density improved from "virtually no resistance" to over 40 blows per 100mm.

Laboratory analysis of the solidified polymer-shingle mix confirmed an average compressive strength of nearly 3 MPa, exceeding the project's requirements.

Execution: With the concept proven, the team worked between tidal cycles to implement the full plan. Loose shingle was placed underneath the foundation, and 2.3 to 4 tons of polymer was injected at 120 points to create a new, stable, and erosion-resistant base.

Technical drawing of Geobear's treatment plan at Hurst Castle

Results

The emergency intervention successfully stabilised the structure, preventing further movement and safeguarding the historic castle from additional erosion and collapse.

The innovative approach demonstrated a proven method for stabilising critical structures at risk from severe water-based erosion.

Facing an erosion or washout emergency?

Our specialist geopolymers are designed to stabilise ground in the most challenging conditions, including saturated and loose soils.

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