Screwpile and Caisson Lighthouses
The Problem of Soft Ground
Rock lighthouses solved one problem: how to build on an exposed solid surface. But many of the most dangerous hazards to navigation are not exposed rock at all. They are shifting sandbars, submerged shoals, mudflats at the mouths of rivers, and the shallow reaches of wide bays where the water is too shallow for a lightship but the ground too soft for a conventional masonry foundation. On these sites, the engineer's challenge was not to resist the impact of waves against a solid tower but to find some way of providing a stable base in material that would not support a heavy structure.
Two solutions dominated nineteenth-century practice: the screwpile and the caisson. They are structurally different — one a framework on legs screwed into the seabed, the other a hollow cylinder sunk and filled — but both addressed the same fundamental requirement: a foundation in soft or unstable ground without the option of piling to bedrock. Together they produced a distinctive family of lighthouse forms, concentrated in particular on the American East Coast, the Chesapeake Bay, and the tidal reaches of the British Firth of Forth.
The Screwpile and Alexander Mitchell
The screwpile foundation was invented and patented by Alexander Mitchell, a blind Irish engineer working in Belfast in the 1830s. Mitchell had been employed in harbour and dock construction and was looking for a way to found navigation marks in soft tidal mud where conventional driven piles would not achieve adequate grip. His solution was a cast-iron pile with a broad helical flange at its lower end — a screw, in effect, formed from a 1.2- to 1.5-metre diameter cast-iron disc with a pitched edge that would wind the pile into the ground when it was rotated, developing bearing capacity from the area of the flange rather than from the resistance of the ground below the tip.
Mitchell first used the screwpile for a beacon in the Thames Estuary in 1838 and then for a lighthouse at Maplin Sands in 1838–41, building what is generally regarded as the first screwpile lighthouse: a seven-pile structure with a central vertical pile and a ring of six raking outer piles, topped by a small wooden lighthouse cottage and lantern room. The design was ingenious in its structural logic: the raking outer piles gave the structure lateral stiffness under current and wave loading that a purely vertical arrangement of piles could not have provided, while the relatively light superstructure and the open framework below kept the wind and wave loads to manageable levels.
The screwpile principle spread rapidly to North America, where it found its ideal application in the soft shoals of Chesapeake Bay and the shallow approaches of the middle Atlantic coast. Thomas and John Turner of Baltimore built dozens of screwpile lighthouses in Maryland and Virginia waters from the late 1850s onward, developing standard designs that could be erected by small crews using the materials and equipment available to port contractors. The typical Chesapeake screwpile lighthouse was a hexagonal wooden cottage on a platform supported by seven piles, with a central lantern giving a focal plane between 10 and 15 metres above mean high water.
Chesapeake Bay: The Great Screwpile Region
By 1870 there were more screwpile lighthouses in Chesapeake Bay than anywhere else in the world. The bay's physical geography made them almost the only option: the water is generally less than ten metres deep across most of its 300-kilometre length, the bottom is soft sediment over much of its area, and the positions that needed marking — the ends of extensive shoals, the mouths of river tributaries, the edges of the deep ship channel — were typically far from any hard ground.
The Seven Foot Knoll lighthouse, built at the mouth of the Patapsco River in 1855, is the oldest surviving screwpile lighthouse on the Chesapeake and one of the oldest in the United States. It is now preserved at the Baltimore Maritime Museum after being moved from its station in 1988. The Thomas Point Shoal lighthouse, built in 1875 at the tip of a shoal extending from the Western Shore of Maryland, is the only surviving screwpile lighthouse still in its original position in the Chesapeake. It is also the only unaltered screwpile lighthouse in the United States listed as a National Historic Landmark and is maintained by the United States Lighthouse Society through a partnership with the Coast Guard.
Thomas Point Shoal has a distinctive cottage design on a hexagonal platform, with a gallery around the base of the lantern room and a cluster of attached outbuildings that house the fog signal machinery. Its light character — Fl R 5s, a range of approximately 16 nautical miles — marks the southern entrance to the channel from the bay to the Annapolis anchorage. The lighthouse is accessible on guided tours during summer months and draws large numbers of visitors by water taxi from Annapolis.
Ice and the End of the Screwpile
The vulnerability of screwpile lighthouses was not structural failure in storms but destruction by ice. In the shallow, brackish waters of Chesapeake Bay and Long Island Sound, winter ice floes could exert lateral forces on the relatively slender piles that the structures could not resist. Several Chesapeake screwpile lighthouses were demolished or badly damaged by ice: the Brandywine Shoal lighthouse was struck by ice in 1881, and the following years saw a wave of replacements that substituted the caisson foundation for the screwpile at the most ice-exposed stations.
The Caisson Lighthouse
A caisson foundation is a hollow cylinder of cast iron or steel, open at the bottom, that is set on the seabed and then sunk by excavating material from inside it while adding weight at the top. When the caisson reaches the required depth, it is filled with concrete or rubble, providing a solid plug that transfers loads from the structure above to the ground below the seabed surface. The caisson provides a much greater foundation area than a screwpile of equivalent diameter, resists lateral ice loading far more effectively, and eliminates the open framework below the platform that the screwpile required.
The first caisson lighthouses in American waters were built in the 1880s at stations where ice had proved destructive to screwpile structures. The design that emerged from this period — a circular cast-iron caisson, typically 8 to 12 metres in diameter, supporting a cylindrical iron tower — is sometimes called the "spark-plug" form because of its resemblance to an automobile spark plug when seen from a distance. The caisson provides the base; the cylindrical tower provides the light room and keeper accommodation in a continuous column from water level to lantern room.
Fourteen Mile Point on Lake Superior, completed in 1894, and Orient Point on Long Island Sound, completed in 1899, are among the early examples of the American caisson lighthouse. The design was used extensively in the Chesapeake by the Lighthouse Board's Third District engineers, and variants appeared in the Delaware River, the Connecticut coast and the Cape Cod approach routes.
Caissons in British Waters
In British waters, the caisson was used most extensively in the Firth of Forth, where the combination of significant tidal range, soft sediment and severe winter conditions ruled out screwpiles and made conventional masonry foundations impractical. The Oxcars lighthouse on the Oxcars Rock in the Firth of Forth, completed in 1886 by David and Charles Stevenson, uses a caisson foundation supporting a cast-iron cylindrical tower. The Inch Keith lighthouse at the east end of Inchkeith island uses a similar arrangement. Both are still active aids to navigation.
The most elaborate British application of the caisson principle was the construction of the Royal Sovereign lighthouse off Beachy Head in Sussex, completed by Trinity House in 1971. Royal Sovereign is a post-war offshore platform rather than a nineteenth-century caisson, but it uses the same principle: a prefabricated concrete base unit sunk onto the seabed, providing a stable platform for the tower above. The tower is a squat, angular structure of reinforced concrete supporting a helicopter deck and lantern room, quite unlike the elegantly tapering towers of the Victorian period, but it stands on ground that no nineteenth-century engineer could have built on.
Open the map to find the positions of surviving screwpile and caisson lighthouses in American and British waters, where these extraordinary structures mark the shallow approaches to some of the world's most intensively used port approaches.