Moat Bank Stabilisation at Caerphilly Castle

EDS successfully completed reservoir safety works at Caerphilly Castle to ensure the long-term stabilisation of the outer moat embankment.

Caerphilly Castle, one of the finest medieval castles in Western Europe, is the second largest castle in the UK. It attracts thousands of visitors each year and is also a popular site for local recreation and fishing. EDS has a long-standing relationship with the site, having delivered several projects for Cadw, the Welsh Government’s historic environment service.

The outer moat had suffered erosion along the upstream face toe caused by local wildlife (particularly geese) and at the outlet to the auxiliary overflow. EDS was engaged to stabilise the upstream embankment toe and provide long-term erosion protection through the installation of rock mattresses, rock rolls and coir rolls.

Key Challenges:

  • Steep upstream embankment face and crest path used by tourists to access the castle and its grounds
  • Navigating water-based plant within the moat and beneath the visitor centre entrance bridge
  • Fluctuating moat water levels during both summer and winter working conditions
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Solutions:

  • A water-based construction methodology was deployed, using an excavator mounted on a floating barge constructed from individual steel pontoons
  • A trench was excavated along the upstream embankment toe under archaeological supervision, enabling the placement of a soft engineering solution comprising rock rolls and coir rolls, which were then backfilled to reinstate the toe
  • Rock and coir rolls were chosen over harder solutions such as gabions or rock armour, as they allow natural sediment accretion, create a living revetment, and encourage rapid vegetation establishment
  • Stability calculations supported the pontoon arrangement, considering plant positioning and materials storage throughout installation
  • Careful planning and navigation of pontoons minimised the impact on flora and fauna, enabled safe passage beneath the visitor access bridge, and accommodated fluctuating water levels

Before stabilisation

After stabilisation

Despite poor weather conditions, which would have suspended works from the embankment, EDS’ innovative water-based approach enabled the project to be delivered successfully with minimal disruption to the historic site, the reservoir embankment, and public access routes.

EDS has a strong track record of working with reservoir owners and operators to develop and deliver engineering solutions in challenging locations, contributing to safer and more resilient environments.