Floating pontoons were used as work platforms for the construction of piers in rivers. They had to be strong enough to carry all the construction equipment including cranes, hydraulic gear, as well as the pier components and workers. The pontoons also had to provide a stable platform regardless of tides and currents of the water on which they floated.
The pontoons consisted of watertight tanks with a space between them through which the structures for pier foundations (caissons) could be sunk. A leg was attached to each corner of the pontoon and extended down to the river bed to anchor it. The height of the pontoon could be adjusted to a suitable level for work on the caisson cylinder by the use of hydraulic jacks to lengthen or shorten the legs.
However, Arrol utilised the lifting effect of the rising tide to help raise the structures into position.
Sections of the caisson cylinders were constructed on the shore and transported out to the pontoon and moved into position. The interior wall of the caisson was lined with bricks and once the desired depth had been reached it was then filled with concrete and then lowered by hydraulics onto the river bed. The cylinder was forced downwards and material from the river bed was removed from inside the bottom edge of the caisson. In constructing the Tay Bridge, the material was loosened using hydraulic water jets and then removed by grab dredges which scooped the material out. However, for the Forth Bridge the caissons were sunk under compressed air. A working chamber at the base of the caisson was filled with compressed air to prevent the high pressure at depth forcing water into the caisson. Workmen entered this chamber via an airlock into a shaft passing down through the concrete above. In the chamber men would dig the slit and clay from the river bed and the material was carried to the surface via another shaft in the concrete.
Working under compressed air was dangerous. Men would get headaches and earaches, or in the most severe cases severe joint pain, paralysis and even death. This is now known as decompression sickness, or the bends, and is caused a rapid change from high to low pressure resulting in air bubbles forming in the tissues. Based on the experience of other bridge projects, Arrol limited these workers to 6 hours per day or 4 hours at the highest pressure and 20-35 minutes to decompress in order to minimise the effects.
William Arrol invented a hydraulic spade which had a ram which could force the cutting and lifting plate into the hardest boulder clay, making it easier and faster for the men to excavate the river bed.
William Arrol used steel caissons which although more expensive than the traditional wood, enabled the work to progress at a faster rate.