How were the foundations of The Tower Bridge built?

Question

Me and my friend were talking about how The Tower Bridge was built and then she asked me a very confusing question, "How did they build the bit in the river that holds the Tower up when in those times they couldn't dive down and build it". Does anyone know how they built it ?

Answer 1

The same way they build most bridges of this type.

Large steel chambers are driven into the river bed and the men work inside this as the water is continally pumped out.

Answer 2

In 1878 Horace Jones, the City architect, put forward a proposal for a low-level bridge on the bascule principle - that is, a bridge on a level with the streets with two leaves or arms that could be raised to let ships pass up and down the river and lowered to let vehicles pass to and from across the waterway. Successful bridges of this type already existed, though on a much smaller scale, at Rotterdam and Copenhagen.

"Bascule" is derived from the French word for see-saw," and the bascule bridge is a kind of drawbridge which works on a pivot and has a heavy weight at one end to balance the greater length at the other. This was the type of bridge finally decided upon, and it has proved a great success.

The piers of the Tower Bridge are much more complicated structures than the piers of an ordinary bridge. In addition to supporting the towers carrying the overhead girders for the high-level footways and the suspension chains of the fixed spans, they also house the counterpoise and the machinery which operates the bascules.

The caissons used for securing the foundation of the piers consisted of strong boxes of wrought iron, without either top or bottom. To secure a good foundation it was found necessary to sink them to a depth of about 21 feet into the bed of the river. There were twelve caissons for each pier. On the north and south sides of each pier was a row of four caissons, each 28 feet square, joined at either end by a pair of triangular caissons, formed approximately to the shape of the finished pier. There was a space of 2.5 feet between all the caissons, this being considered the least dimension in which men could effectively work. The caissons enclosed a rectangular space 34 feet by 124.5 feet. The space was not excavated until the permanent work forming the outside portion of the pier had been built, in the caissons and between them, up to a height of 4 feet above high-water mark.

The method adopted in building and sinking the caissons was unusual. First came the building of the caisson upon wooden supports over the site where it was to be sunk. The caisson was 19 feet in height and it was divided horizontally into two lengths. The lower portion was known as the permanent caisson and the upper portion, which was removable when the pier was completed, was called the temporary caisson. The object of this upper portion was simply to keep out water while the pier was being built. When ready the supports were removed and the permanent caisson lowered to the river bed (this had previously been leveled by divers) by means of four powerful screws attached to four lowering rods.

After the caisson had reached the ground various lengths of temporary caisson were added to the permanent section, till the top of the temporary portion came above the level of high water. The joint between the permanent and the temporary caissons was made tight with india-rubber. Divers working inside the caisson excavated first the gravel and then the upper part of the clay forming the bed of the river. As they dug away the soil, which was hauled up by a crane and taken away in barges, the caisson gradually sank until its bottom edge penetrated some 5 feet to 10 feet into the solid London clay. London clay is a firm watertight stratum, and when the desired depth had been reached by the caisson it was safe to pump out the water, which up to this time had remained in the caisson, rising and falling with the tide through the sluices in the sides.

The water having been pumped out, navvies were able to get to the bottom of the caisson and to dig out the clay in the dry. Additional lengths of temporary caisson were added as the caisson sank, so that at last each caisson was a box of iron 57 feet high, in which the preparation of the foundations could be made. The caisson having been controlled from the first by the lowering rods and screws, its descent any farther than was desired was easily arrested by the rods when the bottom of the caisson was 20 feet below the bed of the river. The clay was then excavated 7 feet deeper than the bottom of the caisson, and outwards beyond the cutting edge for a distance of 5 feet on three of the four sides of the caisson. In this way not only was the area of the foundations of the pier enlarged but, as the sideways excavation adjoined similar excavations from the next caissons, the whole foundation also was made continuous.

All the permanent caissons, with the spaces between them were then completely filled with concrete, upon which the brickwork and masonry were begun in the temporary caisson and carried up to 4 feet above high water. The preparation of the foundations was a long and troublesome task because of the extent of the river traffic, which made it difficult to berth the necessary barges. On two occasions "blows" occurred which hindered the operations. When the cutting edge of one of the caissons had reached a depth of 16 feet beneath the river bed, water rushed into the caisson through a rent in the clay. The caisson had to be lowered still further to seal the opening when the water was pumped out.

The second blow was due to one of the stage piles between the caissons having been driven in aslant. As the caisson went down its cutting edge came in contact with the pile and thus loosened the clay in the immediate neighbourhood. Divers were sent down to ascertain the damage and the pile was re-driven. The full extent of these handicaps was underestimated and thus this section of the work occupied much longer than had been expected. Finally there emerged four feet above high-water mark two gigantic piers of concrete, granite and bricks able to withstand without settlement a load of 70,000 tons. From the river bed upwards the piers are faced with rough picked Cornish granite, in courses between 2 feet and 2.5 feet thick. The piers called for the excavation of 30,000 cubic yards of mud, silt and London clay. The material consumed in the piers was 25,220 cubic yards of cement, 22,400 cubic yards of bricks and 3,340 cubic yards of Cornish granite. The cost of the piers was £111,122. As soon as the piers had been finished the building of the towers began.

Answer 3

There is loads of information on the web about the Tower Bridge, this will sum it up for you....

Arguably the most famous bridge over the Thames – this is of course London Bridge. The very first London Bridge was built here over 1000 years ago, while the Romans were in control of London in AD43, and it was made of wood. When London was raided by the Danes in AD1013, they sailed upriver and attached ropes to the wooden struts. They then rowed downstream with all their might and managed to pull the bridge down. This is thought to be the origin of the old nursery rhyme "London Bridge is Falling Down". It was subsequently repaired in bits and pieces, but in 1209 AD in the reign of King John the first stone bridge was built. It took 30 years to complete. The piers were called "starlings" and were driven deep into the riverbed. They were very close together and caused the tidal waters to rush through underneath the bridge in rapids. A Drawbridge gate was on the southern end of the Bridge – this was the infamous gate upon which heads of traitors were spiked on long wooden poles. The bridge even had shops, houses and a Chapel. It survived the Great Fire of London in AD 1665 and lasted until 1831. It was pulled down and a new bridge erected of stone, to the designs of Sir John Rennie. This graceful structure lasted until 1967, when it was decided that the bridge must be widened to accommodate the vast increase in traffic since Victorian times. Rennie’s bridge was sold for 2 and a half million dollars to some American millionaires, who re-erected it on Lake Havasu in Arizona, where it is now a huge tourist attraction - I personally have seen it and walked across it in its new location, and it is SO narrow! The current London Bridge was opened in 1972 and is a 3-span concrete structure in a simple clean modern design, with a headroom of 29’2".

Tower Bridge (headroom 28’2") is by contrast only just over 100 years old – and yet it is probably London’s most famous landmark. It was built in 1886 to the designs of Sir Horace Jones and John Wolfe-Barry. They tried to create a structure that was in architectural harmony with the nearby Tower of London.

Work was started on the bridge in 1886, with the Prince of Wales laying the foundation stone over a time capsule containing papers and coins. The work was intended to take only three years, but parliament was asked twice for more time. Two piers containing 70,000 tons of concrete were sunk into the river bed to support the weight of the bridge, and it was on these that the towers were built. Because the central area of the river could not be obstructed, the towers were built one at a time. The bascules had to be built in the upright position, including the wood paving.

It has a unique stone clad steel frame, which supports the bascules – the lifting arms of the roadway which it carries across the Thames. Bascule is the French word for see-saw. The bascules are regularly opened to allow the passage of tall ships upriver to the Pool of London. Again, it is one of the most photographed tourist sights in London - an iconic image known all over the World.

Answer 4

Two massive piers, containing over 70,000 tons of concrete, were sunk into the river bed to support the construction. Over 11,000 tons of steel provided the framework for the towers and walkways. This was then clad in Cornish granite and Portland stone, both to protect the underlying steelwork and to give the bridge a pleasing appearance.