A section of the Forth Rail Bridge, between South and North Queensferry as it straddles the river Forth in Scotland. It is best viewed large.
The bridge is Scotland largest preserved monument.
Currently there is a massive program to repaint the bridge, so it has been covered in scaffolding for the past few years. It is nearing completion and they say that the new paintjob will last for 20 years.
The plastic sheeting is necessary as they need a sealed environment as they blast the old paint and rust of the steel and extract it immediately before it can fall into the river below.
This section of the bridge stands on 4 Caissons (anchors for the bridge). Caissons usually take the form of large cylinders, often circular or oval in shape, which are deep (or tall) enough to sink to the sea bed. Caissons may be made of several materials, usually steel or in some cases concrete. In the case of the Forth Bridge, each cantilever section is mounted on top of four wrought iron caissons, cylindrical in section and weighing 400 tons each. Looking at the bridge today, the top of, that is the section above the water, the caissons appear as four feet at the foot of each span.
These huge cylinders were fabricated by Arrol Brothers of Glasgow who sent them over to South Queensferry to be constructed and then floated out to their prepared sites. Once positioned in the correct place, the caissons were filled with concrete until they sunk to the sea bed. The caissons had a false bottom, below which was a seven foot steel cutting edge, this dug into the sea bed providing a temporary anchor until they could be permanently affixed to the rock below. This having been done, all the sea water had to be pumped out from the area between the base of the caisson and the sea bed.
This was very dangerous work, involving the use of compressed air to ensure that the water could not re-enter the caisson. Once the water had been evacuated workmen then had to dig away at the foundations until the caisson was level and at the correct depth. This having been achieved, the area was then finally pumped full of concrete.
By and large this work was completed remarkably smoothly, one of the caissons tilted sideways as it was floated into position. This actually happened on 1st January 1885, it is thought that the previous evening’s Hogmanay celebrations may have had some bearing on this miss-hap. It took ten months to re-float the flooded caisson.
Bearing in mind the dangerous nature of the work, surprisingly only two men were killed on this stage of the bridge building, when one of the caissons split and flooded rapidly.
Best viewed Larger.