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Army Sea Forts, Mersey Estuary, site of
Liverpool Bay, Merseyside, UK
associated engineer
Guy Anson Maunsell
Sir Alexander Gibb & Partners
date  October 1941 - July 1943
UK era  Modern  |  category  Defence Structure  |  reference  SJ277981
Following the success of his pioneering Navy Sea Forts installed in the Thames Estuary on the approaches to London, Guy Maunsell designed forts to defend the Mersey Estuary. Though Liverpool was an enemy target in World War II (1939-45), the three forts completed in the Mersey never saw action and were all demolished afterwards.
After the Navy forts were finished, the Admiralty commissioned Maunsell to built sea forts for the Army to use. The Mersey Estuary has many sandbanks and he decided that its shifting sea bed would be too soft to support the kind of flooded-pontoon-founded forts he had built for the Navy in the Thames Estuary. Instead, the Army forts were founded on a self-burying grid of concrete members integral to the rest fo the structure. The idea was to not impede the natural movements of sand and shingle.
Furthermore, each Army fort was not one structure but seven individual towers linked by tubular steel walkways. They were set out in relation to each other in a formation known to be successful for land-based fortifications. Each tower consisted of four slim raking reinforced concrete legs founded on one of the self-burying grids. The legs supported a two storey octagonal steel accommodation pod with military equipment on the roof.
Originally 38 towers were intended for the Mersey but only 21 were built (three forts). Plans for 13 towers in the Humber Estuary were abandoned. A further 49 towers were planned for the Thames Estuary but only 21 were built three complete forts, of which two are extant.
Each fort had four 94mm heavy anti-aircraft gun towers, one 40mm Bofors light anti-aircraft gun towers, one searchlight tower and one central control tower equipped with radar. The Bofors towers were deployed first, so that they could defend the vessels and crews installing the rest.
The Mersey forts were fabricated, fitted out and equipped at Bromborough Dock. Each tower took about eight weeks to build. Individual towers were then towed into position suspended between two specially modified barges. Each took eight hours to install. Although the Mersey forts were constructed first and were in deeper water, their above-waterline design survives in the Thames Army forts, which used the same template.
The 305 tonne base grid for each tower comprised four hollow members arranged in a square with members extending out in each direction (like a straightened hash symbol: #). Each member was 25.9m long, 1.8m high and 2m wide at the bottom and 1.7m wide at the top, joined with solid concrete.
Four hollow cylindrical concrete legs reinforced with 32mm diameter steel bars supported each tower. Their external diameters were 900mm and wall thicknesses 300mm, and were precast in three sections. A 4.3m square x 1.2m deep concrete cap with a 1.8m diameter hole through it was cast in situ to join the legs to the steel pod above. This was connected to two 13.1m long steel joists embedded in the cap.
The octagonal accommodation pods were made from 6.3mm steel plate. Their parallel walls were 11m apart. The walls of living spaces were insulated with hardboard and all floors had a 19mm thick layer of asphalt. Armoured parapets surrounded the armour-plated top deck (pod roof) and the magazine chambers.
The three forts were positioned between 7th October 1942 and 25th July 1943. The October 1941 tender price for the 21 towers was 336,000.
The Mersey Estuary sea forts were never engaged in enemy action. After the war they were dismantled as they obstructed shipping lanes.
Contractor: Cleveland Bridge & Engineering Co Ltd
Windows: Critall
Research: ECPK
"Maunsell: The Firm and its Founder" by Nigel Watson and Frank Turner,
AECOM Technology Corporation, 2005

Army Sea Forts, Mersey Estuary, site of