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Donegan Civil Engineering

The Airfields, Deeside

Large-diameter caisson and dewatering for regeneration infrastructure at the former RAF Sealand.

By: Valentine Donegan

Published: October 9, 2025

The Airfields, Deeside

Client: D. Morgan PLC / Welsh Water

Value: £500,000

Project Description/Main Operations

The Airfields is a major regeneration site north of the River Dee and west of the A494 at Garden City, on the former RAF/MoD Sealand estate. As part of the below-ground utilities enabling works, V. J. Donegan & Co. Ltd delivered a deep, large diameter access shaft and associated civils for D. Morgan PLC on behalf of Welsh Water.

Scope and method:

Donegan constructed a 10.5 m internal-diameter, 15 m deep segmental caisson in sands and gravels, providing a durable access/connection point for future drainage assets. Given the high groundwater regime and proximity to tidal waters, open-cut was discounted in favour of caisson sinking to manage stability and surface occupation.

A ring-build with cutting shoe was advanced in controlled stages. To sustain dry working conditions, 11 deep wet wells were installed and operated in a duty/assist/standby arrangement, balancing draw-down against settlement criteria. The permanent works included a pumped base slab and mass benching placed in staged pours, followed by lifting and placing an 11 m cover slab using a 300-tonne mobile crane.

Key challenges and controls:

Groundwater/tidal influence: Multi-well dewatering with monitoring to prevent rebound and limit particle migration.

Large cranage on a constrained plot: Engineered working platforms and exclusion zones, verified ground bearing pressures, and weather/wind thresholds embedded in lift planning.

Interface management: Sequencing coordinated with principal contractor enabling works to maintain safe access and segregation.

The completed shaft provides a maintainable access point for the development’s foul/stormwater network. The chosen methodology reduced excavation risk in granular strata, contained groundwater, and minimised surface footprint-supporting programme certainty and safe delivery within an active site.

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