How are bridges built over water?

Foundations of bridges in the water

Caisson foundation, box dam, cofferdam, caisson foundation

One of the caissons for the Brooklyn Bridge is launched (New York, 1870). The wooden one
Caisson had a floor area of ​​1632 m². Its ceiling was 6.7 m thick.

"The manufacturer and builder", May 1870

yearInvolvedEvent / project
AristotleThe philosopher describes the principle
the diving bell
1691Denis PapinIn an essay, Papin mentions the possibility in one
Build bell using compressed air
1692John WilliamsFirst patent on a caisson
1739LabelyeWestminster Bridge London;
open caisson without compressed air
1841 TrigerChalonnes / Loire;
Sinking a coal shaft with compressed air
1851CubittMedway Bridge, Rochester / England;
first compressed air foundation in bridge construction
1854BrunelRoyal Albert Bridge, Saltash / England; first
Victim of "caisson disease" in bridge construction
1859Fleur Saint-Denis
Castor company
Rhine bridge between Kehl and Strasbourg;
First compressed air foundation in France / Germany
with modified procedure
1870EadsBridge over the Mississippi in St. Louis / USA;
many deaths from caisson disease
1870RoeblingWashington Roebling suffers in the compressed air establishment
a lifelong paralysis for the Brooklyn Bridge
1878Paul BertScientific research into diving disease;
Recompression chamber recommendation
1883Harkort companyCaisson foundation lighthouse "Roter Sand";
first offshore structure in the world

Most bridges were and are being built because a traffic route meets water. In mountain regions, the occasion may often be a deep ravine, and in recent times the avoidance of level crossings of different transport systems (e.g. road / rail), but most of the time it is about overcoming a river, a lake or a strait.

The easiest and most elegant way to do this is, of course, when the water surface can be bridged with a single large jump, but this is not always possible. The realizable span of the various bridge systems developed only slowly over the centuries. The technically skilled Romans achieved arch spans of a maximum of 36 meters. The largest arch bridge of the Middle Ages in terms of span was the Adda Bridge near Trezzo in Lombardy. It was built in 1377 and had a span of 72 m. However, this bridge already occupied a special position, because the usual spans of the Middle Ages were significantly smaller.

The magical 100 m limit of a single Frückenfeld was crossed for the first time in Europe in 1820 by Samuel Brown and John Rennie when building the Union Chain Bridge in Berwick / England.

The pile grid

But that does not mean that bridge construction stopped entirely at these rather modest spans. However, the builders and engineers of the different epochs then had to deal with the question of how to found a bridge pillar in the middle of open water.

Depiction of a box dam made of toothed beams by Leonardo da Vinci

© "Codex B", Institut de France

In order to build a foundation in a river bed, you primarily need a dry excavation pit. The foundation work was therefore started in the summer when the water level was low. If you had the time to wait for an extended period of drought, the foundations could sometimes be made in an almost water-free excavation. In pre-industrial times, the actual foundation was mostly carried out by means of a wooden pile grid, which was stored on driven piles driven close together. The bridge pillar was built on the pile grid, which can be imagined as a horizontal framework, and additionally secured with large stones all around. However, such constructions were not very deep and therefore susceptible to undermining. Many bridges built in this way collapsed during floods. Again and again it had to be noted that a structure is only as durable as its foundation.

An obvious thought is of course the diversion of the whole river during the construction work. Indeed, it has been done occasionally. In the case of really large rivers such as the Rhine, Moselle, Danube, etc., however, the diversion was impracticable due to the enormous effort involved. In addition, most of the bridges had to be built in cities or their immediate vicinity and there was usually not the space required for a diversion. In order to build a bridge in such a situation, two basic technical processes have been developed: the box or coffer dam Derived from the technical term "coffer dam", the process is also incorrectly referred to as "coffer dam" and the caisson. Both methods are not only suitable for building bridge piers, but of course also for lighthouses, port facilities, etc.

The Roman box dam

A box dam consists of a round or square box, open at the top and bottom, the side walls of which have been made watertight in one way or another. The box can be prefabricated on land or made at the intended location by driving in sheet piles. Then the inside of the box is scooped out or pumped out so that a dry construction pit is obtained.

A sunken pillar of Westminster Bridge in London is being refurbished.
Charles Labelye built the bridge in 1739 using reusable caissons.

The Romans already mastered this technique so well that they were able to build bridges over the Tiber and later over much wider rivers of their empire. The exact creation of such a foundation has been preserved to us by the Roman military engineer and architect Marcus Vitruvius Pollio, better known as Vitruvius. Vitruvius lived around 65-10 BC. and described in his work "Ten books on architecture", among other things, the construction of a box dam.

Work on a Roman box dam began by driving two rows of pointed tree trunks close together, circular or oval, as deep as possible into the river bed. For this purpose, two tree trunks were clamped together at a distance of approx. 0.5 m and rammed together. At the end you had a double-walled ring with clay or clay filled in between. After this was firmly pulped, the water could be scooped out from inside the box dam. Since the Romans already had water wheels and pumping stations, which were driven by the flowing water like a mill, they were able to drain the excavation pit without much effort.

In the largely dry excavation they were able to work on the foundation base. All mud, sand and gravel now had to be removed from the inside of the box, if possible down to the stable subsurface. However, if the layer to be removed reached deeper than the box dam was high, this did not succeed. In order to still obtain a stable subgrade for the pillar, piles were driven into the river bed as close to one another as possible and cut off at the same height at the top. For better load distribution, sleepers or a sleeper grid were placed on top of which the massive stone pillar could be built. Another prerequisite for a solid bridge foundation was the Roman concrete Opus Caementitium, which was also permanently strong under water.

Jacques Triger's compressed air shaft
for lowering a coal mine
in the Loire Valley (1841)

Vitruvius also describes alternative construction methods in his textbook if the required materials were not available on the spot. The Roman Empire was very large and the building instructions had to be universally applicable.

The Kastendamm in the Middle Ages and today

After the fall of the Roman Empire, the box dam and waterproof cement were initially forgotten. The lack of these crucial keys was one of the reasons why bridges could not be built over the great European rivers for the next 1000 years. A revival of bridge construction did not begin until the 12th century with the stone bridges in Würzburg, Regensburg and Dresden. However, during the foundation work for these bridges, a particularly dry weather period was awaited.

In the High Middle Ages, the Kastendamm was rediscovered or reinvented and soon improved. There are drawings by Leonardo da Vinci as well as other representations from the Middle Ages, which show us a box dam with dovetailed wooden beams. With this construction method, tedious sealing with clay could be dispensed with.

The box dam or cofferdam is still used very often today, as it is usually cheaper compared to the caisson foundation. Steel sheet piles or duct boards are most often used, which are interlocked when driving and form a tight enclosure around the construction pit. Any water that may flow in must of course be constantly pumped out of the construction pit. However, the use of sheet pile walls is subject to technical limits with regard to the depth that can be achieved.

The caisson or caisson foundation

The caisson is mostly referred to internationally as a caisson, but this leads to a certain confusion of the terms, since the French word "caisson" simply means "box". Basically, when founding a caisson, a box that is prefabricated on land and is open at the top or bottom is swum to the construction site and lowered there at the prepared location. The bridge pier is then built on this or in it. A caisson foundation can be done with or without the use of compressed air. In common parlance, however, a caisson foundation is understood to be a process that uses compressed air.

One of the first uses of a caisson is documented in 1739 for the construction of Westminster Bridge in London. This bridge leads directly to the Houses of Parliament and the "Big Ben" over the Thames. The Swiss Charles Labelye was commissioned to build the stone bridge after explaining to those responsible several times how he intended to found the 14 pillars in the river with his novel method.

The Medway Bridge in Rochester / England (1851).
It was the first bridge that was built with the help of compressed air caissons

Labelye had a large wooden box made of wood, open at the top but closed at the bottom, which was brought into the specified position on the river. Then they began to manufacture the first bridge pier inside the box. Due to the weight of the stones, the box slowly sank to the previously dredged bed of the river. The side walls of the box were of course a little higher than the river was deep. After the completion of the first pillar, the side walls were removed and reused for the next pillar. However, the bottom of the box remained under the foundation.

Before the construction work was finished, one of the pillars tilted and sank. Labelye countered the destruction of the bridge by removing the superstructure, reinforcing the pillar and restoring the superstructure using a weight-saving construction method. The bridge was finally completed in 1750 and existed until 1854, when construction began on the iron bridge that still exists today.

Caisson foundations with compressed air

In 1841 the French engineer Jacques Triger (1801-1868) was busy digging a coal mine near Chalonnes in the Loire Valley. The constantly flowing water of the Loire bothered him considerably and finally gave him an idea. In his final report to the French Academy of Sciences, he noted:

Brunel's Royal Albert Bridge in Saltash / England (1854). The central pillar was with
a compressed air caisson 25 m below the water level of the Tamar.

© Richard & Gill Long

"Since it was now not possible to scoop out the water in my shaft, which would have meant as much as wanting to scoop up the river, I came up with the idea of ​​pushing it back with compressed air and this procedure exceeded all my expectations".

The compressed air that was blown in displaced the water from the shaft and the work could be completed without any major problems. It was obvious that this method is of course also ideally suited for the construction of port facilities, lighthouses and bridge piers in the water. Triger's invention is still the origin of all kinds of compressed air foundations and made him so famous in France that his name was immortalized on the Eiffel Tower. This honor was only given to the 72 most important French engineers and scientists.

Negative and positive pressure foundations

A whole decade passed before the compressed air foundation was first used in bridge construction, while a modified process initially caused a sensation. This method, which was first used in 1847 when building a railway bridge on the Welsh island of Angelsey, alternates between positive and negative pressure. Cast iron pipes with a diameter large enough for men to work in are rammed into the river bed. The upper opening of the pipe is sealed airtight and the air is sucked out, so that a negative pressure is created. As a result, the water that surrounds the lower end of the pipe is flushed into the interior and thus also the surrounding substrate such as sand, gravel or clay is transported into the pipe. The pipe sinks by a few cm to several dm, depending on the nature of the soil. An overpressure is now generated which displaces the water out of the pipe and enables the workers to remove the material that has flooded in. This process is repeated until the required depth is reached. This negative / positive pressure method has the disadvantage that it can only be used on very homogeneous subsoil such as bog, sand, gravel, etc. As soon as the pipe encounters large stones or a transverse tree trunk, the foundation cannot easily be continued in this way.

The foundation of the Rhine bridge near Kehl (1859).
The excavated material is carried with rotating bucket chains, which are carried by a
flooded shaft run, transported from the caisson. On the right is a security gate.

General construction newspaper (1861)

In 1851 this method was also to be used for the construction of the Medway Bridge in Rochester / England. After initially making good progress, however, the foundations of a Roman bridge were surprisingly found. Since you couldn't get any further, you switched to the compressed air foundation in a caisson and the remains of the wall could be removed without any problems. This was the first use of Triger's compressed air process in bridge construction.

Isambard Kingdom Brunel successfully used the compressed air foundation for the construction of the Royal Albert Bridge (1854) in Saltash / England. The middle pillar of the bridge had to be built in the middle of the river bed of the Tamar, at a depth of about 25 meters. Brunel built a double-walled iron pipe with a diameter of 11.4 m and a length of 27.4 m. However, the great depth and the resulting overpressure caused health problems for some workers. One of them even died of air embolism as a result of the lack of pressure equalization. Others showed symptoms such as bleeding from the ears and nose, joint pain and symptoms of paralysis. The medical causes of these problems were not fully understood at the time.

Compressed air foundations in bridge construction

In the coming decades, the compressed air method using prefabricated caissons established itself in bridge construction. First, a box girder, open at the bottom, is prefabricated on land, swam to the intended location of the pillar by ships and there lowered to the bottom of the river. Sometimes these caissons were made of wood. According to the nautical tradition, they were sealed with "caulking" with tar ropes or cotton strips. In the course of time, however, the caissons were mainly made of cast iron, steel or reinforced concrete. The caissons have blade-shaped contact surfaces on the underside with which they sit in the river bed after the lowering process. By blowing in compressed air, an overpressure is generated inside the box, which displaces the water and thus creates a dry working area. The excess air is displaced with the water under the side walls of the box and ensures that air bubbles constantly rise on the surface of the water. When the overpressure is sufficient, the workers enter the work area through sluices and begin to remove mud and debris from inside the box. The overburden is carried upwards through material locks, for which mechanical aids such as bucket chains were used early on.

For the first time during the construction of the Rhine bridge near Kehl for the railway (1859), the massive base for the bridge pillar was bricked up at the same time on the top of the caisson.Due to the mass of the structure and the simultaneous excavation of the material in the box, so-called "local ground breaks" occur on the outer edges of the caisson in the area of ​​the cutting edges, causing the box to sink further downwards. This process is continued until a stable subsurface is reached.

Also the caissons of the Firth of Forth Bridge / Scotland (1870)
already had separate shafts for people and material

Wilhelm Westhofen: "The Forth Bridge" (1890)

In particularly unfavorable soil conditions, it was sometimes even necessary to blast in the caisson. For example, when the Brooklyn Bridge was built in 1870, when large inhomogeneous rock masses were encountered and only gained 15 cm of depth per week. So that the caisson sinks downwards in a horizontal position and does not tip over, height measuring devices are attached to the corner points of the upper side in order to check the position. When the caisson has reached its end position on solid ground, it is completely filled with masonry or concrete. Before that, of course, the shafts and all equipment for ventilation and removal of the material are removed from the box. In the case of inadequate ground conditions, an additional pile foundation can also be placed below the caisson for the compressed air foundation.

In Germany, however, the compressed air foundation was carried out by French engineers and a French construction company. The compressed air foundation was first used in 1859 for the construction of the Rhine bridge between Kehl and Strasbourg. For the first time, separate shafts were used for the workers and the excavated material, which significantly increased work safety. The spoil was removed from the box with a chain of buckets that ran through a separate chute filled with water.

Only in the second attempt in 1883 succeeded in founding the "Roter Sand" lighthouse in front of the mouth of the Weser on a caisson in 24 m water depth. Two years earlier, the first caisson was destroyed by a storm while trying to lower it. The lighthouse is considered to be the first offshore structure in the world.

Health risks with compressed air foundations

Historical descriptions of the working conditions in a caisson suggest that it was an extremely dangerous and strenuous job. Initially, the work on kerosene lighting had to be done with only shovels and hoes. And of course with much longer working hours than usual today. The deeper the caisson sinks, the greater the water depth and therefore the water pressure. This means that you have to increase the overpressure inside the caisson accordingly so that the work area remains dry. The application limit for this process is set at an overpressure of 3 bar, which corresponds to a water depth of 30 m. If the pressure is even greater, human performance decreases disproportionately and there is inevitably serious damage to health.

In the case of overpressure work, it is imperative that certain times for lowering the pressure after leaving the caisson (discharge) must be observed. The human organism needs a certain time to cope with the pressure difference. If these rules are disregarded or if there is a sudden drop in pressure in the caisson, considerable damage to health and even death must be expected. The need for adequate decompression also plays an important role for divers. The consequences of insufficient pressure equalization are therefore mostly called "diving illness" today.

Working in a Caisson (ca.1880)

During the construction of the Brooklyn Bridge (1870), the overpressure caused several deaths and injuries. And that although there was already a lot of experience about this phenomenon in Europe at this point in time. Even Washington Roebling, one of the "fathers" of the Brooklyn Bridge, was not spared. Roebling often descended into the caisson to monitor the progress of the construction work. After a fire in the caisson, he stayed in the compressed air for far too long and became seriously ill from the effects of decompression sickness. He survived but remained paraplegic for the rest of his life.

James Buchanan Eads was at the same time building the first bridge over the Mississippi in St. Louis. When the first bridge pillar was founded, there were 12 deaths and a considerable number of injuries. Ead's doctor cared for the affected workers directly in the caisson and eventually showed symptoms of the disease himself. He finally realized that only slow decompression could protect against the disease and ordered appropriate safety measures. When the foundation of the last abutment was completed, there was only one further death to complain, although the foundation depth of over 40 m was far greater than that of the first pillar and exceeded the depth permitted for compressed air work today.

After these experiences in Europe and America, the dangers of founding caisson were well known and safety regulations were enacted that led to a significant reduction in the risk. However, the number of caisson foundations increased steadily in the period that followed and the process was also used in the construction of subways, port facilities and in tunnel construction. The work in the caisson remained dangerous for the workers and tragic accidents repeatedly occurred due to carelessness or technical defects with spontaneous loss of pressure. During the construction of the first Elbe tunnel in Hamburg (1906-1910), for example, there were three dead workers and several injured as a result of caisson disease.

The causes of decompression sickness were scientifically proven in 1878 by the French physiologist Paul Bert. As a result of his studies, he recommended equalizing the pressure difference slowly and in several stages. Bert also found that the symptoms of the disease could be relieved very quickly by increasing the pressure again. His findings led to the development of a recompression chamber in 1893, which was first used in the construction of the Hudson Tunnel in New York.

Underwater foundations today

A caisson for the Mackinac Straits Bridge is being lowered. The outer ring is with
Stone material filled so that it sinks down more easily. The entire caisson
was made from individual cylinders, which were intended on
Location on top of each other.

© Mackinac Straits Bridge Authority

The development of new and better work equipment and more powerful hoists meant that workers are rarely sent into a caisson today. Only a few compressed air construction sites are still being carried out in Germany today, a large part of them in Hamburg. Nowadays, foundations in open water are mostly carried out using box embankments. If this is not possible due to the required depth, prefabricated caissons are used where possible. These are open at the top and bottom so that they can be dredged from above.

When the Mackinac Straits Bridge was built (1955), the caisson was made from double-walled, stacked steel cylinders, each 13.4 m high. The space between the two steel walls was filled with stone material to lower the cylinder at the intended location. The bottom cylinder had a blade-shaped lower edge so that it could sink into the soft subsoil of Mackinac Street. The following cylinders were stacked on top of it one after another until the required height above the water level was reached. The water was then pumped out and the loose soil material dredged from the interior. Finally, the cylinders were welded together from the inside and then the entire cavity was filled with concrete.

The pylons of the Akashi Kaikyo Bridge in Japan (1988) were also founded on caissons. The load-bearing subsurface on the sea floor was about 60 m deep. Circular caissons were prefabricated in one piece in a dry dock, on which the two 280 m high towers were later erected. The two caissons consisted of 67 m high steel cylinders with a diameter of 80 m.They were transported to the intended location by barges, lowered to the seabed and then completely filled with waterproof concrete.

  • "Report of the meeting of the scientific society ISIS in Dresden", 1868
  • Jacques Triger, "About an Air Compression Apparatus for Sinking Mine Shafts Underwater", 1845
  • Rudolf Floss: "Foundation of old bridges"
  • Wilhelm Westhofen: "The Forth Bridge", 1890
  • Sven Ewert: "Bridges - The Development of Spans and Systems"
  • David J. Brown: "Bridges - bold constructions over rivers, valleys, seas"
  • Dirk Bühler: "Bridge Construction - German Museum Munich"
  • Charlotte Jurecka: "Bridges - Historical Development, Fascination of Technology"
  • Klaus Stiglat: "Bridges on the way"
  • Fritz Leonhardt: "Bridges - Aesthetics and Design"
  • Hans Wittfoht: "Triumph of Spans"
  • "Handbook of Engineering Sciences", 1863
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