Before construction could start the site of the dock and quays, covering 200 acres (81 ha), had to be clear of water.

Three dams were built from the island to the mainland.
The center dam divided the dock area in half, another was further west and a third dam extended east across what would be the entrance.
The two outer dams completely closed off the site from the sea.
The center dam was built without much difficulty by simply tipping material to form an embankment, although some of the earth sank into the mud, so more had to be added.

The western dam caused much more trouble, since it rested on mud that varied in depth to upwards of 40 feet (12 m).
The ends of the dam were formed by tipping earth from wagons run out from the mainland and the island.
In the centre, the earth sank into the deep mud and slid away with it.
A viaduct of timber piles was built across the gap, to carry loaded trucks from which the earth was thrown out.
As the ends approached each other, the tide current was too fast.
The contractor twice tried to close the gap with earth at low-water neap tide, but each time the water broke through to make a gap 80 feet (24 m) wide through which the tide poured at 5 miles per hour (8.0 km/h).

The problem was solved in July 1885 by dropping shutters between horizontal timbers attached to the viaduct piles when the tide had receded, then backing up the shutters with as much stone and earth as could be delivered from pre-loaded trucks.
This worked.
A cast iron pipe 40 inches (1,000 mm) in diameter had been laid through the dam to form a sluice, with a flap on the outside that was closed at high tide and opened as the tide receded.
By this means the west part of the works were drained to the level of the pipe, and the remaining water was pumped out at an average rate of 150,000 US gallons (570,000 l; 120,000 imp gal) per hour by a Cornish beam engine brought down from the Severn Tunnel works.

The causeway along the dam permanently linked Barry Island to the mainland.

The eastern dam was made of piers of masonry with marl foundations, backed up with earth, leaving four 15 feet (4.6 m) openings through which the tide flowed.

It included a temporary stone dam where the entrance to the docks would be built.

In March 1886 the openings in the eastern dam were quickly closed with planks, backed with concrete.
Later the planks were removed and the concrete faced with brickwork in cement mortar.
Three 12 inches (300 mm) pipes with valves ran through the lowest part of the concrete wall, allowing the water to drain to this level while excavation proceeded.

The remaining water was pumped out.

Gunpowder was used to loosen the marl, which was then removed by steam shovels.
Various other steam-powered devices were used to remove mud, clay and rock.
All the hard material was used for embankments and quay roads around the docks.
The mud was placed behind these, and in trenches to seal the works from water, using special side-tipping wagons.
Before construction could start the site of the dock and quays, covering 200 acres (81 ha), had to be clear of water. 
Three dams were built from the island to the mainland. The center dam divided the dock area in half, another was further west and a third dam extended east across what would be the entrance. The two outer dams completely closed off the site from the sea. The center dam was built without much difficulty by simply tipping material to form an embankment, although some of the earth sank into the mud, so more had to be added.
The western dam caused much more trouble, since it rested on mud that varied in depth to upwards of 40 feet (12 m). The ends of the dam were formed by tipping earth from wagons run out from the mainland and the island. In the centre, the earth sank into the deep mud and slid away with it. A viaduct of timber piles was built across the gap, to carry loaded trucks from which the earth was thrown out. As the ends approached each other, the tide current was too fast. The contractor twice tried to close the gap with earth at low-water neap tide, but each time the water broke through to make a gap 80 feet (24 m) wide through which the tide poured at 5 miles per hour (8.0 km/h).
The problem was solved in July 1885 by dropping shutters between horizontal timbers attached to the viaduct piles when the tide had receded, then backing up the shutters with as much stone and earth as could be delivered from pre-loaded trucks. This worked. A cast iron pipe 40 inches (1,000 mm) in diameter had been laid through the dam to form a sluice, with a flap on the outside that was closed at high tide and opened as the tide receded. By this means the west part of the works were drained to the level of the pipe, and the remaining water was pumped out at an average rate of 150,000 US gallons (570,000 l; 120,000 imp gal) per hour by a Cornish beam engine brought down from the Severn Tunnel works.
The causeway along the dam permanently linked Barry Island to the mainland.
The eastern dam was made of piers of masonry with marl foundations, backed up with earth, leaving four 15 feet (4.6 m) openings through which the tide flowed. 
It included a temporary stone dam where the entrance to the docks would be built.
In March 1886 the openings in the eastern dam were quickly closed with planks, backed with concrete. Later the planks were removed and the concrete faced with brickwork in cement mortar. Three 12 inches (300 mm) pipes with valves ran through the lowest part of the concrete wall, allowing the water to drain to this level while excavation proceeded. 
The remaining water was pumped out.
Gunpowder was used to loosen the marl, which was then removed by steam shovels. Various other steam-powered devices were used to remove mud, clay and rock. All the hard material was used for embankments and quay roads around the docks. The mud was placed behind these, and in trenches to seal the works from water, using special side-tipping wagons.
Barry Docks