Caerphilly Castle Silt Trap Clearance and Bank Rehabilitation
Welsh heritage service Cadw, in association with the Welsh Government and WSP consultants, appointed EDS to carry out a silt trap clearance within the moat of Caerphilly Castle and to stabilise part of the moat embankment.
Caerphilly Castle is a medieval fortification and a scheduled monument, so all works had to comply with the conditions of Scheduled Monument Consent. The construction site was located on the south-west corner of the moat, where the Nant Gledyr flows in via a silt trap. The site required diversion of public footpaths and was positioned directly opposite a primary school on a busy road with traffic-calming measures, close to the town centre. These challenges were planned into the project design in consultation with those affected, to minimise disruption to traffic, school schedules and visitors.
EDS established a safety perimeter around the site to segregate the working area and diverted pedestrian footpaths. Track mats underlain with geotextile were used to protect the ground, while a hessian silt containment and fish protection boom safeguarded marine life. NRW relocated fish from the working area before works began. The project was scheduled during the winter to avoid the nesting season of the wildfowl that inhabit the moat.
The works had two objectives:
To excavate accumulated material from the silt trap down to its base profile so that it could resume effective functioning. Additional dredging of accumulated silt downstream of the trap would also improve water quality and floodwater retention capacity.
To stabilise a section of the western moat bank, preventing further erosion and reprofiling an overhanging section that posed a safety risk.
Silt Trap Excavation
Sandy silt was excavated and placed in a bunded draining area within the trap before being removed off site by tipper lorries for soil recycling. Traffic movements were planned to avoid school drop-off and pick-up times, with marshals safely managing vehicle access. Once the silt trap was excavated to design levels, silt downstream within the moat was removed.
Due to Scheduled Monument Consent, all excavation work required archaeological supervision. Aside from a buckled bicycle and several old bottles, no artefacts of historic interest were uncovered (although they would have made for an interesting newspaper story!).
Bank Stabilisation Works
The moat embankment had become badly eroded, leaving steep, overhanging sections that posed risks to anglers and dog walkers.
To stabilise the embankment, a trench was excavated under archaeological supervision. Gabion baskets were placed and filled in situ, with coir rolls pre-seeded with wetland plants installed within them. These will grow to form a vegetated edge, protecting the waterline. The void was backfilled and profiled to the height of the gabions, creating a gentle incline with a planted edge to reduce both the risk to the public and future erosion.
Towards the end of the project, the client requested additional surface stabilisation prior to hydroseeding. An Erosaweb system was installed, filled with stone chippings, topped with soil and raked by hand. To prevent washout and contamination of the moat, hessian matting was placed over the hydroseeded areas. This protects against birds, wildlife and adverse weather, ensuring the embankment surface establishes successfully.
Despite challenges from weather and the busy location, the team successfully delivered the project ahead of programme. EDS is proud to have supported the preservation of such a beloved historic site, helping to protect it for generations to come.
