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St Tudwal’s West Island

Client: Private Client

St Tudwal’s Island West (Ynys Tudwal Mawr) is a small island located on the coast of North Wales. It is approximately 670m in length and 185m at its widest point. Centrally situated are a lighthouse and the light keeper’s cottage, while a granite masonry quay can be found at the northern end of the island on its East side.

John Grimes Partnership were instructed by a private client to develop a scheme to improve access to the island. After several concepts were appraised and discussed with various stakeholders, the preferred option was to develop a steel slipway extending from the existing granite masonry quay. The slipway provides access to the island during all tidal states, from the Highest Astronomical Tide to the Lowest Astronomical Tide.

The slipway structure, measuring 40m in length and 6m in width, is formed from a braced steel deck supported by a series of portalised frames. This structure rests on piles set into the varying-level rock platforms which descend within the intertidal zone to approximately 0.5m below the Lowest Astronomical Tide. A drilling methodology was devised by JGP that utilised an Odex Rig, supported on a bespoke platform that extended from a Jack-up Barge. The pile design takes into account future erosion of the wave-cut rock platform, incorporating tolerances and functionality for the piles to serve as columns should the surrounding rock mass be lost.

The pre-existing quay structure on the island was elevated by 1m, achieved using a reinforced concrete slab supported by mini piles and secured with rock anchors. To ensure the original quay wasn’t overburdened by the new slipway and quay extension, the steel frame was integrated into the suspended quay slab. This method of construction minimises the impact of the new structure on the current quay, guaranteeing its prolonged stability.

Thanks to this design, island access remains uninterrupted across all tidal phases, ensuring year-round accessibility under the majority of weather conditions. The works, which concluded in August 2021, have since improved the transport of personnel and materials to and from the island.