Triger Process

The procédé Triger is a technique for digging excavations or sinking foundations in wet ground, based on the use of a caisson pressurised with air. It was introduced and put into practice by French engineer Jacques Triger in the early 1840s.

Jacques Triger's Work

In 1839, working in Anjou (Maine-et-Loire) in connection with coal mining, Triger began to consider how to reach solid rock beneath about 20 metres of permanently waterlogged ground - effectively quicksand. He became convinced that he could use compressed air to help penetrate this layer using a pressurised caisson. With the financial and administrative support of Emmanuel de Las Cases, Triger sank a wrought iron cylinder, using pumps of his design to provide a smoothly-delivered supply of compressed air of sufficient pressure to force out all the water, enabling the men to excavate the sand, gravel, and stone to such a depth that when the cylinder was sunk to a water-tight stratum, the compressed air was no longer necessary. Some sources credit Triger with devising the airlock, essential to allow the men to enter and leave. However, this was clearly anticipated and described - although not constructed - by Thomas Cochrane in his 1830 patent.

Triger recorded the first cases of decompression sickness in miners. A lesser nuisance was the rapid combustion of the candles, but this was obviated by the substitution of flax for cotton wicks.

According to Pernolet, in 1876, the first tube sunk was 1.033m diameter (1839? 1841?). In 1845 he sank a well 1.80m diameter

The first well took 6 months to dig to a depth of 25 metres. Triger had decided to limit the pressure to a maximum of 3.5 bar. A fortuitous accident in well No. 2 allowed a lower pressure to be subsequently used here. It is claimed that a miner pierced the tube with his pickaxe. Air rushing out seems to have aerated the surrounding water, reducing its density, such that it could be excluded by an internal pressure of just 2 bar! (De l'air s'était engouffré par l'ouverture du coup de pioche, et avait formé avec l'eau une sorte d'émulsion. La densité du fluide s'en trouva alors très nettement amoindrie. Pour équilibrer la même hauteur d'eau, à peine 2 bars suffisaient!). The tube was made of wrought iron plate, which would presumably have been between about 6 and 12mm thick - hardly susceptble to an accidental pickaxe blow!

Triger dug five wells in Anjou. Today, the wrought iron tubes are flooded but still in situ; their upper part emerges very clearly from the Loire plains of the Corniche Angevine.

In 1852 Triger he was awarded the Prix de Mechanique for the invention of the Caisson sinking process.

Triger's process was soon known outside France, for example the 1841 report below appeared in many UK newspapers, but the process does not seem to have had much impact elsewhere until the 1850s.

There follows a selection of articles and onservations relating to the process as applied in France and later overseas.

Press Reports

1841 'Penetrating Through Sand by Pneumatic Pressure.— A French engineer has recently made an ingenious application of pneumatic pressure, which is peculiarly interesting here at the present moment. It appears, that in the department of Maine et Loire, there is an extensive deposit of coal, covered to a depth exceeding in some places sixty feet, with an alluvium, which are several strata of sand communicating with the river. The difficulty of working through this wet sand, which flows in as rapidly as it can be thrown out, has been always considered as insuperable, and the coal near the Loire has been consequently neglected. But M. Triger, engineer of Mans, has bethought of making his way through the sand, not by pumping it out, but by pressing it back. For this purpose he uses a machine like a diving bell, into which the air forced till it has the pressure of two or three atmospheres. Against such pressure the sand rises but little way in the bell, which is gradually lowered as the workmen clear their way to the solid stratum underneath. This process has proved quite successful. The following phenomena have been observed in the compressed air. The compression produces at first a pain in the ear, which ceases in a few minutes ; it accelerates combustion in a remarkable manner, so that under a pressure of three atmospheres (which is the pressure in a diving bell at depth of about sixteen fathoms), it was necessary to substitute candles with thread wicks for those with cotton wicks, which latter burned most rapidly, and with an insufferable smoke. Under the same pressure, a man cannot whistle, the sound the voice grows nasal, and the work of climbing ladders less fatiguing than in the free atmosphere. A deaf workman found that he could hear well in the compressed air. This ingenious contrivance of M. Triger is essentially the same as that on which Mr. Bush has calculated for fixing the foundations of his proposed lighthouse on the Goodwin Sands, the commencement of which work has been delayed a series of accidents. The possibility of keeping the semifluid sand out of a caisson by atmospheric pressure, has been hitherto doubted by many; but M. Triger has furnished the proof.— Athenaeum.'

Another source states that 'In 1831, Earl Dundonald, then Lord Cochrane, took out a patent for a device for sinking tubular shafts through earth and water, by means of compressed air. His air-lock was much like modern ones, and was to be placed at the top of his main shaft. His invention, though stated in very general terms as an improvement "in the means of, and in the apparatus for building and working under water," was made with a view to its use in tunnelling under the Thames, and similar enterprises." The same sources states that 'In 1841, Mr. William Bush took out a patent for a plan of sinking foundations, consisting of a caisson with air-chamber, and immediately above it an air-lock, into which excavated material was to be thrown. Above the lock came an open shaft. A German, by name Gr. Pfaun Muller, made a somewhat similar design for a bridge at Mayence, in 1850; but as his plan was not executed, it was, like the patents of Cochrane and Bush, little known till legal controversies in regard to patent-rights dragged them from obscurity. .... In 1845, Triger read a paper on the sinking of a tube about six feet in diameter to a depth of 82 feet by the same method, and suggested the use of the method for the construction of deep foundations for bridges. The application of Triger's method was made at Rochester, England, in 1851, where Mr. John Wright [? surely John D'Urban Hughes] sunk tubular foundations for the bridge over the Medway, working his men to a depth of 61 feet. In the construction of the central pier of the Royal Albert Bridge at Saltash, England, in 1854-5, Brunel surpassed all similar previous exploits. His men worked at times under a pressure of 40 pounds above the normal, in small compartments so situated that in some cases the entrance and exit were exceedingly slow and tiresome. The whole work was excessively difficult, and required much skill, energy, money, and time. The result was a pier built with one solid, continuous base, instead of a cluster of separate piles. The Saltash pier was essentially constructed by the old method of a coffer-dam. Compressed air was in use only to make the bottom of the dam water-tight.'

An informative Paper on pneumatic foundations in the bridges of the first Italian railways states that 'The process developed by Triger began to be applied in other sectors and within a few years was being used in the construction of underwater foundations in incoherent soils.', and further, that John Hughes introduced important innovations at Rochester: he enlarged the airlock, equipped it with a double compartment in order to improve the regulation of the air pressure and optimised the construction process by combining the Triger and Potts systems. In Italy, pneumatic foundation construction for large river bridges on the railway between Turin and Novara was undertaken by Fox, Henderson and Co, who had been engaged at Rochester.

1845 'INFLUENCE OF COMPRESSED AIR ON HEALTH.
M. Triger, civil engineer, having to establish a mine-shaft in the midst of the alluvia of the Loire, and not being able to exhaust it of water, since it would have been necessary to empty the river itself, conceived the idea of doing so as far as possible by means of pumps worked by a steam engine. In consequence of this arrangement, the workmen worked in air compressed to three atmospheres. The effects produced by this air were the following :—From the first strokes of the piston a more or less violent pain was felt in the ears. This pain ceased in every one as soon as the mercury reached in the manometer the height of three centimetres. The motions of deglutition immediately dissipated it, probably by driving into the Eustachian tubes a certain portion of air; the latter, by reestablishing, as regards the membrane of the tympanum, the equilibrium of pressure within and without, arrested the tension of this membrane, which the excess of pressure of external air, penetrating by the conduit onditif, had pressed On the caisee of the drum. The habits of individuals contribute to render this pain more or less violent. Drunkenness is a certain means of rendering it intolerable, even several hours after the intoxication has gone off. The workmen complain much of the cold produced by the stoppage of the internal air when the communication with the external air is restored. There results from this stoppage of air a very cold fog, and so much the thicker as the capacity of the box in which the men work is greater. Every one speaks through his nose and loses the power of whistling in three atmospheres. Two workmen, after having passed seven hours in compressed air, suffered, half an hour after leaving the shaft, violent pains; one in the left arm, the other in the knees and left shoulder. Frictions with spirit of wine dispelled these pains, and these workmen did not discontinue working the following days.'

1846 'The coal mining company of Douchy, in France, is at present making some rather interesting experiments, to cause the water, or wells, in coal pits, to rise to a level, by the assistalce only of atmospheric pressure, already employed with such success by M. Triger, in the coal basin of the Maine and Loire. Ihis operation has been carried on with success, and everything announces that it will terminate in the same manner; it will be a most fortunate innovation, if introduced, in the workings of the mines generally, and worthy of the consideration of the most practical savants. Up to the present time they have arrived at the bottom of the principal level of the water, and the miners are working in a pressure of two atmospheres and a half. Hitherto no inconvenience whatever for them has been noticed in the works so operated, and they are nearly always accompanied by their director of the works. Perhaps they have become a little thinner duing the course of those works, where they inspire or inhale an exceedingly compressed oxygen air; but on the whole there is no doubt of their success, and what is more, no accident has happened in this arduous operation. Mining Journal.'

1870 TUBULAR FOUNDATIONS.
Some short time ago we gave a description of screw piles, with especial reference to their suitability for foundations in situations where, from the nature of the ground, scarcely any other means were available. There are, however, various other principles of foundations which have their own proper sphere of utility and application, where they become in their turn the correct means to employ.
Among these, is the hollow or tubular system, which has been used in numerous works of magnitude with great success, and is perhaps better adapted for a very large scale of construction than the solid principle. Tubular foundations consist of hollow cylinders, either of wrought or cast iron, which present nothing particularly worthy of notice. It is the manner in which these cylinders are "got down" that is the interesting fact in connection with the use of them. For effecting this purpose there are two methods more recognised and more generally known than others.
The one is the vacuum or Potts' method ; the other Triger's, or that of compressed air.
A brief examination of their relative merits will be not without interest, and we will commence with the former. If we imagine a hollow tube, or better still, a hollow cylinder, hermetically sealed by a strong and closely fitting cap at the upper extremity and open at the lower, and the interior of it put in communication with an air pump, and the air inside exhausted, the following result will ensue. So soon as a tolerably perfect vacuum is formed, and the equilibrium between the external and internal pressures destroyed, the preponderance of the former becomes manifest. This is demonstrated by the tendency of the water to force its way into the cylinder, which after disintegrating and undermining, as it were, the earth in the immediate vicinity, it finally accomplishes, and a mixed mass of water and the debris of the substrata is driven into the tube. At the same time the cylinder descends through the loosened stratum by virtue of its weight, and that of the atmospherical pressure generated upon its top by the destruction of the normal state of equilibrium. Affairs progress in this manner until a sufficient quantity of debris is collected in the cylinder, when by means of suitable manholes, provided for the purpose, it is removed, and the operation commenced de novo. An examination into the rationale of this process will at once indicate that there are three principal agents concerned in its accomplishment, which may be termed water, pressure or weight, and air. These are dependent upon other conditions, which may be different, individually or conjointly, according to the special circumstances of the ease. Thus the upward or disturbing tendency of the water will be in proportion to the height of any given vertical section of it—in other words, to its depth. The effect produced by gravity or insistent weight alone will depend upon that of the cylinder itself and its appendages, while the action of the air will be in proportion to the more or less perfect manner in which the vacuum is made and maintained, and also to the superficial area of the top of the cylinder. As it is essential that the subtratum should be of a nature that will permit water to partially percolate through it, and break it up so to speak, it is clear that dense soils, including stiff clays, and very close and compact gravel, are not the proper kind upon which to employ this method of getting in foundations. It is manifest from this that too much caution and care cannot be exercised in ascertaining the exact character of the underlying strata, for it would be a great mistake to commence the process and subsequently discover before solid ground were reached, that it was inadequate to complete the work intended. The preliminary borings should always therefore be carried down to the same depth as that to which the foundations are to reach, or otherwise there is no reliable information to depend upon. Many serious errors have resulted from an undue attention to these points, errors which have only been rectified afterwards at a very considerable outlay of time, trouble, and money.
The inventor of the compressed air method was M. Triger, an able engineer, attached to the staff of the Ponts-et-Chaussees. He used the principle, firstly, in sinking a shaft for a coal mine. A portion of the strata it was sunk through consisted of a gravel so excessively permeable that it allowed the water to flow through in such quantities as to fill the shaft, and completely stop all operations. Under these circumstances M. Triger placed on the top of the cylinder or shaft an air bag, drove out the water by compressing the air in the cylinder, and then excavated the gravel by the ordinary means.
This method, it must be acknowledged, is sometimes termed Hughes' method, although it would appear from the evidence that the balance is in favour of the honour of its invention belonging to the French engineer. Hughes' claim rests upon the fact that he was the first to apply the principle to the foundations of bridges, which he did at Rochester. It was originally intended to use Potts' method, but the divers came upon the remnant of some old Roman masonry, which was of such a hard and impenetrable character as to leave no hope of that principle being successful. But, after vanquishing this difficulty by compressing the air, another arose. The soil at a certain depth became so impermeable and dense that the water could not get away at the bottom of the cylinder, which, in nautical language, was completely "waterlogged." In this emergency a syphon tube was introduced, the longer branch of which descended into the cylinder, and the shorter passed through the top. The water, by the compression of the air, was driven up the long branch and discharged through the shorter, outside the cylinder. Once the advantage of this additional contrivance was seen and recognised, it became a permanent feature of this system ever afterwards. Since air, or any fluid in a confined space, presses equally in all directions, it is evident that the effect of the compressed air will be felt upon the inside of the top of the cylinder, as well as upon the water it forces out, consequently if this upward pressure be greater than the insistent weight of the cylinder, the latter will have a tendency to rise. This must be counteracted by weighting or loading the cylinders with an extraneous amount of material, which, in fact, has a double duty to perform. One consists in resisting the upward tendency, and the other in causing the cylinder to descend as the earth beneath becomes loosened and excavated. In this method, therefore, there appears something slightly paradoxical, as one of the agents is employed in nullifying what is partially effected by another. Regarding one as plus, and the other as minus, the absolute work done towards causing the descent of the cylinder would be expressed by their algebraical sum. It is frequently a consequence of this ambiguity that the cylinders descend with considerable irregularity, sometimes sinking almost imperceptibly, and at others going down with a sudden and violent jerk. This would not signify to any great extent, were it not that a sudden descent is rarely uniform. One part of the cylinder generally becomes tilted up, and much time is expended in restoring it to the perpendicular position. It is easy to perceive that a constant succession of these tilts, first on one side, and then on the other, would seriously interfere with the operation of sinking.
Probably one of the most remarkable instances of the application of this method is that of the centre pier of the Saltash Bridge, erected by Brunel over the Tamar, near Plymouth. The rapidity of the tideway rendered the operations particularly difficult and hazardous, but the great engineer triumphed over all obstacles, and the Saltash Bridge justly ranks as one of the most remarkable feats in national engineering. '

1879 'AMERICAN LONG-SPAN BRIDGES
Foundations by compressed air, invented in France in 1841 by M. Triger, and applied ten years later to the reconstruction of the Rochester Bridge, were introduced iu America in 1855. They have been notably applied there, on a scale unknown in Europe, to the two great bridges at St. Louis and New York. Among other differences one remarks.-
1. The immense area of the caissons (reaching nearly 16 acres in the New York Bridge).
2. The substitution of massive timber work for iron in the caisson of this same New York bridge, where the thickness of the platform reached 7 metres.
3. The employment of a machine called a sand pump in the St. Louis bridge.
4. The great working depth (33.70 m. under water which was reached in this same bridge.
5. Lastly, and above all, the location of the air chambers at the bottoms of the wells.
Attacked on a grand scale from the end of 1869, the foundations of one of the piers of the St. Louis bridge were far advanced when I visited there in September, 1870. I was very much struck with the air locks which, instead of being placed above the level of the water and removed whenever it became necessary to lengthen the wells of access to the working chamber, were established as a fixture in the chamber itself. The descent was thus made in a central well in the ordinary atmosphere by means of a large cage, 3 metres in diameter. In the well a circular staircase was built, which later gave place to an elevator.
The descent was thus made to a point 6 feet below the ceiling of the caisson, then by a door on a level with the air chambers, which was two meters in diameter. When the equilibrium of pressure was once established the outside door opened itself, and nothing more was to be done but to jump to the earth from a height of about 0.8m. In air thus strongly compressed it became at least necessary to reduce the time of labour to less than an hour. What an advantage not to be obliged to subtract the time necessary to ascend and descend a height equivalent to 10 stories of a Parisian house. What relief to the labourers, generally overcome with fatigue and reeking with perspiration at the end of their task. What convenience for the transmission of orders, for the introduction of tools, in fact, for all kinds of communications. Besides the space necessary for the compressed air was much reduced, and it was no longer necessary to construct the portion of the wall situated above the chamber of heavy plate iron.'

Similar Systems

The idea of using compressed air to dig ground aquifers is credited to Thomas Cochrane, who obtained a patent in 1830 under the title: "Apparatus for facilitating excavation, digging and mining" (patent No. 6018). The proposed system remained purely theoretical, and there is no evidence that Triger was aware of Cochrane's patent.

A pneumatic process for sinking cylindrical iron piles and bridge piers was invented and patented by Laurence Holker Potts in the early 1840s. Potts's process was very different, using an air pump to evacuate the air from with the cylinder, which caused it to sink quickly in suitable ground under the action of external atmospheric pressure, with no digging involved. It met with some success, but it was found that on encountering sold obstructions, no further progress occurred until the obstruction was removed. It was intended to employ Potts's method to sink the piers of Rochester Bridge (commenced about 1849), but this was thwarted by the remains of an ancient bridge. 'It then occurred to Mr. J. Hughes, the engineer in charge of the work, to reverse the process, and to pump air into the cylinders to force the water out, so that the men could work at the bottom of the cylinders, as in a diving-bell. As the material was excavated from the space covered by the cylinders they sank by their own weight. An ‘air-lock’ provided the means of ingress and egress to the cylinders. An account of the work was read by Hughes before the Institution of Civil Engineers in 1851 (Proceedings, x. 353). It was afterwards pointed out that the same method had been previously used in France, though on a very small scale.' See Laurence Holker Potts.

In fact it was Hughes who pointed out - in his Paper, not afterwards - that he became aware of Triger's work in France after he had constructed the apparatus, but before it was put into use at Rochester. At an early stage, anxious to learn about the safety aspects of working in compressed air, he sought information about experience with diving bells, but his attention was drawn to an article about Triger's work in France in Dr. Ure's Dictionary of Arts and Manufactures and Mines. (In Hughes's own self-effacing words: 'When the apparatus which has been described was made, and was in the course of fitting at Rochester, the author's attention was called to an article in Dr. Ure's Dictionary ...'). Keen to learn more about Triger's experience, he obtained the relevant copy of 'Comptes Rendu de l'Academie des Sciences'.. The Paper was presumably "Mémoire sur un appareil à air comprimé, pour le percement des puits de mine et autres travaux, sous les eaux et dans les sables submergés", Jacques Triger, 2 November 1841. Académie des Sciences.

Following John Hughes's reporting of his work at Rochester, William Bush took legal action in 1852 against Hughes, Charles Fox and John Henderson for infringement of his patent. Bush's patent 'Improvements in the Means of and the Apparatus for building and working under water' was dated 21 September 1841, Specification dated 21 March 1842, but he had had equipment manufactured and ready for use on Goodwin Sands by September 1841. This was a few weeks before Triger's work was published in France. The arguments were reported at length (44 pages, much relating to the media through which the caisson is driven, and to legal arguments and precedent), and are of interest for a number of reasons, not least being the inclusion of Thomas Cochrane's 1830 patent. This was highlighted by the defendants' lawyers to show that Bush's patent had been anticipated by Cochrane's. It included the use of sections of flanged cast iron cylinders bolted together according to the required depth to be sunk, and included a description of the airlock, which he called the 'ante-chamber'. Surprisingly, not only was there no mention of Triger and his process, but any direct reference was specifically excluded. The reported proceedings included an edited version of Hughes's 1851 Paper, but omitted the historical backround in that Paper, which included a description of Triger's work. That section was replaced by an inserted statement: 'The paper here contained extracts from Beckman, Ure and other publications, describing prior operations carried on under water by means of diving-bells and other operations'.

See Also

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Sources of Information

• [8] Scientific American, Caissons for Pier-Building. August 7, 1869