Lighthouses of Australia Inc

South Channel Pile Light in Port Phillip Bay, Victoria is an example of a straight pile, sheltered lighthouse Photo: Rick Lloyd, Parks Victoria

Pharology 101 - Lighthouse construction methods

By Denise Shultz, LoA President

Lighthouses come in many different shapes and sizes which depend on their function, location, geography and special demands. Politics, cost, and technology available at the time also influences the lighthouse design.

Tipara Reef Lighthouse in South Australia was an example of an exposed screw pile lighthouse Photo: Bob Duthie

The early lighthouses were nothing more than towers made of wood or stone, which was just piled up, or at best, bound together with mortar. As human knowledge grew, lighthouse construction methods and technology had to be developed to satisfy ever increasing demands of maritime safety. As a result, suitable lighthouses were built even at places inconsiderable before, like wave swept rocks or moving shoals.

Top & side view of screw on end of each leg, which are screwed into the sea bed. Images: Night Beacon

Pile foundation was one of the designs which revolutionised the method of building lighthouses in shallow water. 

The first straight pile lighthouse was built in 1773 at Small Rocks off the west coast of Wales. Nine strong oak posts (one in the middle and eight around) were driven into holes in the rock. The posts carried a platform with living quarters and the lantern. This method was only suitable where the base was relatively hard and stable and it was not necessary to drive the posts very deep.

Improvement came with the invention of the screw pile lighthouse by Irish engineer Alexander Mitchell. The posts, which were usually made of wrought iron, were fitted at the ends with cast iron corkscrew - like flanges (below) that help to drive the posts deeper into the base. The screw pile technique is most often used where the ground is soft and where the instability of the bottom can be compensated for by the depth to which the piles can be driven. It is not surprising then that lighthouses of this type can be found in bays and sheltered waters. Typically, they are made of wood and are not usually very tall, because their light’s range does not have to be too long.

Race Rock Lighthouse in Long Island Sound, USA stands on a man-made island Photo: East End Lighthouses

The screw pile technique was also used in more exposed places, like reefs. In this case the light range had to be much longer, the tower had to be taller and carry more powerful and therefore heavier optics. Large iron foot plates need to be fitted to each pile above the screw flange to spread and diffuse the pressure of the heavy structure above. In these cases, the screw pile design was chosen because it was thought that in stormy conditions, the thin but strong piles would pose less resistance to the battering waves. The testimony to the resilience of such structures is the Cape Jaffa platform, still at its place and doing well, though minus its lighthouse. 

Spectacle Reef Lighthouse on Lake Huron, Michigan, USA was built using a cofferdam Photo: US Coast Guard

Other interesting methods of building lighthouses in shallow waters were used overseas. When it was necessary to keep the construction site dry, a cofferdam was built to keep the water at bay. The barrier walls were made on shore and transported to the site, where they were assembled into a closed area in the water. After the walls were sealed, water was pumped out from the inside and the construction could start as if on dry land. The cofferdam method was also used when the lighthouse had to be built on a reef that was exposed only during low tide, for example James Douglass’ Eddystone tower.

A similar method was used on the Great Lakes in America. Crib foundation technique makes use of wooden cribs constructed ashore, which are then towed to the site and sunk by being filled with stone. Once the crib had settled in the right position, it was capped with concrete or other masonry. At this stage, it was possible to adjust the level by adding weight to one side or another as required. The lighthouse could then be built on the prepared foundation.

When, for various reasons, the dry site methods could not be used, underwater foundations were made. Heavy boulders were brought to the site and laid side by side to the bottom and on top of each other, until the sea bottom was suitably raised. Protracted and expensive projects of this type were used in places with strong tidal currents which were nevertheless sheltered from the wind and waves.

Resources: 
Lighthouses - US Coastguard
Evolution and Typology  - Dr. Robert Browning
Classic Encyclopaedia - Lighthouses

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