Bible Encyclopedias
Pontoon

Military Pontoon Bridges

From time immemorial floating bridges of vessels bearing a roadway of beams and planks have been employed to facilitate the passage of rivers and arms of the sea. Xerxes crossed the Hellespont on a double bridge, one line supported on three hundred and sixty, the other on three hundred and fourteen vessels, anchored head and stern with their keels in the direction of the current. Darius threw similar bridges across the Bosporus and the Danube in his war against the Scythians, and the Ten Thousand employed a bridge of boats to cross the river Tigris in their retreat from Persia. Floating bridges have been repeatedly constructed over rivers in Europe and Asia, not merely temporarily for the passage of an army, but permanently for the requirements of the country; and to this day many of the great rivers in India are crossed, on the lines of the principal roads, by floating bridges, which are for the most part supported on boats such as are employed for ordinary traffic on the river.

But light vessels which can be taken out of the water and lifted on to carriages are required for transport with an army in the field. Alexander the Great occasionally carried with his army vessels divided into portions, which were put together on reaching the banks of a river, as in crossing the Hydaspes; he is even said to have carried his army over the Oxus by means of rafts made of the hide tents of the soldiers stuffed with straw when he found that all the river boats had been burnt. Cyru$ crossed the Euphrates on stuffed skins. The practice of carrying about skins to be inflated when troops had to cross a river, which was adopted by both Greeks and Romans, still exists in the East. In the 4th century the emperor Julian crossed the Tigris and other rivers by bridges of boats made of skins' stretched over osier frames. In the wars of the 17th century pontoons are found as regular components of the trains of armies, the Germans using a leather, the Dutch a tin and the French a copper "skin" over stout timber frames.

Modern military pontoons have been made of two forms, open as an undecked boat, or closed as a decked canoe 'or cylinder. During the Peninsular War the English employed open bateaux; but the experience gained in that war induced them to introduce the closed form. General Colleton devised a buoy pontoon, cylindrical with conical ends and made of wooden staves like a cask. Then General Sir Charles Pasley introduced demi-pontoons, like decked canoes with pointed bows and square sterns, a pair, attached sternwise, forming a single "pier" of support for the roadway; they were constructed of light timber frames covered with sheet copper and were decked with wood; each demi-pontoon was divided internally into separate compartments by partitions which were made as water-tight as possible, and also supplied with the means of pumping out water; when transported overland with an army a pair of demi-pontoons and the superstructure of one bay formed the load for a single carriage weighing 27.75 cwt. when loaded. The Pasley was superseded by the Blanshard pontoon, a tin coated cylinder with hemispherical ends, for which great mobility was claimed, two pontoons and two bays superstructure being carried on one waggon, giving a weight of about 45 cwt., which was intended to be drawn by four horses. The Blanshard pontoon was long used in the British army, but was ultimately discarded; and British engineers came to the conclusion that it was desirable to return to the form of the open bateau to which the engineers of all the Continental armies had meanwhile constantly adhered. Captain Fowke, R.E., invented a folding open bateau, made of waterproof canvas attached to sliding ribs, so that for transport it could be collapsed like the bellows of an accordion and for use could be extended by a pair of stretchers. This was followed by the pontoon designed by Colonel Blood, R.E., an open bateau with decked ends and sides partly decked where the rowlock blocks were fixed. It consisted of six sets of framed ribs connected by a deep kelson, two side streaks, and three bottom streaks. The sides and bottom were of thin yellow pine with canvas secured to both surfaces by india-rubber solution, and coated outside with marine glue. The central interval between the pontoons in forming a bridge was invariably maintained at 15 ft.; for the support of the roadway five baulks were ordinarily employed, but nine for the passage of siege artillery and the heaviest loads; they fitted on to saddles resting on central saddle beams. The pontoons were not immersed to within i ft. of the tops of their "coamings" when carrying ordinary loads, as of infantry in marching order "in fours" crowded at a check, or the 16-pounder R.M.L. gun of position weighing 43 cwt.; nor were they immersed to within 6 in. when carrying extraordinary loads, such as disorganized infantry, or the 64-pounder R.M.L. gun weighing 98 cwt. In designing this pontoon the chief points attended to were-(I) improvement in power of support, (2) simplification in bridge construction, (3) reduction of weight in transport, and (4) adaptation for use singly as boats for ferrying purposes. One pontoon with the superstructure for a single bay constituted a load for one waggon, with a total weight behind horses of about 40 cwt.

The following table (from Ency. Brit. 9th ed.) shows the powers of various pontoons in use by different nations in the past. Modern improvements are comparatively few. The "working power of support" has been calculated in most instances by deducting from the "available buoyancy" one-fourth for open and one-tenth for closed vessels: In the English and French equipment the pontoons were originally made of two sizes, the smaller and lighter for the "advanced guard," the larger and heavier for the "reserve;" in both equipments the same size pontoon is now adopted for general requirements, the superstructure being strengthened when necessary for very heavy weights. The German army has an undivided galvanized iron pontoon, 24 ft. 6 in. long, handy as a boat, but of inadequate buoyancy for heavy traffic, with the result that the span has to be diminished and ipso facto the waterway obstructed. The Austrian and Italian pontoons are made in three pieces, two with bows and a middle piece without; not less than two pieces are ordinarily employed, and the third is introduced when great supporting power is required, but in all cases a constant interval is maintained between the pontoons. On the other hand, in the greater number of pontoon equipments greater supporting power is obtained not by increasing the number of supports but by diminishing the central interval between the pontoons. Within certain limits it does not matter whether the buoyancy is made up of a large number of small or a small number of large vessels, so long as the waterway is not unduly contracted and the obstruction offered to a swift current dangerously increased; but it is to be remembered that pontoon bridges have failed as frequently from being washed away as from insufficient buoyancy. In Austria efforts have been made to diminish the weight of the Birago equipment by the substitution of steel for iron. The present pontoon, in three pieces, is of steel, and 39 ft. 4 in. long, like the old pattern.

In the British army Colonel Blood's equipment was later modified by the introduction of a bipartite pontoon designed in 1889 by Lieut. Clauson, R.E. Each pontoon is carried on one waggon with a bay of superstructure, and consists of two sections, a bow-piece and a stern-piece, connected together by easily manipulated couplings of phosphor bronze. Decks and "coamings" are dispensed with, and the rowlock holes are sunk in a strong gunwale. The detachable saddle-beam, which receives the load on the centre of the thwarts, is made in sections, so as to form a continuous saddle of any length required. The baulks (or road-bearers) and chesses (or planks) remain unaltered, but chess-holders and chess-bearers are added for use in constructing light bridges for infantry in file. In this kind of bridge each pontoon section is used separately, with a roadway of chesses placed longitudinally four abreast. In the normal or medium bridge two sections, and in heavy bridge three sections are joined together. The chief advantages of the equipment are (I) the buoyancy of the piers can be proportioned to the weight of traffic and to the roughness of the water; (2) owing to the special design of the bows, boats and rafts are easy to row, while the pontoons in bridge oppose little resistance to the current, and so require less anchor power; (3) transport rafts, pierheads and flying bridges can be constructed with great ease, owing to the flush gunwales on which baulks can rest if necessary; (4) the pontoon sections are convenient to handle, easy to ship or to transport by rail, and can readily be replaced singly if damaged in bridge. A canoe pontoon and superstructure adapted for pack transport has also been adopted from designs by Colonel (Sir) Elliott Wood, C.B., R.E. The pontoon consists of four sections laced together, each section being a framework of wood covered with waterproof sheeting. Three pontoons and eight composite planks form a "unit," from which can be constructed 48 ft. of bridge for infantry in file, 84 ft. for infantry in single file, or a raft to carry 15 men or an empty wagon.

For the British army in India the standard pontoon for many years was the Pasley; it was seldom used, however, for boats could almost always be procured on the spot in sufficient numbers whereever a floating bridge had to be constructed. Later an equipment was prepared for the Indian army of demi-pontoons, similar to the Blood pontoon cut in half, and therefore more mobile; each has a bow and a square stern, and they are joined at the sterns when required to form a "pier"; they are fitted with movable covers and can therefore be used in much rougher water than pontoons of the home pattern, and their power of support and breadth of roadway are the same. The Chitral Relief Expedition of 1895, however, revealed certain defects. The shape of the bow was unsuited to rapid currents; the balance was not satisfactory, and the copper sheathing cracked. Experiments were then undertaken with the bipartite pontoon.

The india-rubber pontoon does not appear to have been generally employed even in America, where it was invented. The engineer officers with the army of the Potomac, after full experience of the india-rubber pontoon and countless other inventions of American genius, adopted the French equipment, which they found "most excellent, useful and reliable for all military purposes." The Russians, in crossing the Danube in their war with Turkey in 1878, employed the Austrian equipment. Aluminium pontoons have been tried in Germany, but have not been adopted.

For light bridging work the Berthon and other collapsible boats have been adopted in Germany and Great Britain, especially for cavalry work in advance of the army. The German folding boat is made of wood framework and canvas skin; two boats are easily carried on one "folding-boat wagon." The total length of the three sections together is 21 ft. 6 in. The British field troop R.E., attached to cavalry, carries two collapsible boats 18 ft. 6 in. long.

The methods of constructing pontoon bridges have been simplified of late years in most armies, and are usually restricted to (I) adding pontoons one by one to the head of the bridge; (2) connecting rafts of two or more pontoons into bridge by intermediate bays of superstructure; and (3) swinging across the river a bridge previously prepared alongside the shore. The formation of a bridge from rafts touching one another consumes an excessive amount of equipment, and opposes unnecessary resistance to the stream; it is therefore being discarded in most armies. "Booming out" the bridge bay by bay from the shore until the head reaches the opposite bank is unsuited for rapid currents, and is almost obsolete except for light infantry bridges.

In every army the pontoon service is in the hands of technical specialists.' But there are many other forms of military bridging, in which the specialist only supervises the work of the ordinary soldier, or indeed, takes no part whatever. Troops of all arms are expected to be familiar with certain methods of rough temporary bridging. In the British service the forms of temporary timber bridge usually employed are called trestle, lock and floating. The trestle bridge in it3 various forms consists of a series of two-legged or three-legged trestles carrying the road-bearers and chesses which form the roadway. Trestles can be improvised, but some are carried, ready for use, by mobile engineer units and they are frequently combined with pontoon bridges at the shore ends, where holding ground for the feet of the trestles is found. Lock bridges never touch water, forming single spans over a chasm. These consist of spars made into frames of which the feet rest in the banks of the river and the heads are interlocked, the whole being securely lashed. Another type of frame-bridge is the cantilever, which has been used in Indian frontier expeditions to bridge swift ' In Germany, however, as mentioned below, light bridging material has been placed in the hands of the cavalry. This tendency, in accordance with the needs of modern armies, will probably become more pronounced in the future. It began with the provision of demolition equipment for the cavalry pioneers.

steep-banked streams. Improvised suspension bridges are also used. Floating bridges are made not only of pontoons but also of boats of all sorts, casks lashed together, and rafts. They are almost always combined with one or two bays of trestle bridging at the shore ends.

The organization of bridging personnel in different armies shows as much divergence of opinion as the design of pontoon equipment. In Great Britain, since the divisional reorganization, the bridging trains have been assigned to the "army troops," which include two "bridging trains," totalling 14 officers and 454 men with 92 vehicles, most of them six-horsed. Each train carries 32 pontoons and 32 bays of superstructure, as well as 16 trestles and 8 bays of the appropriate superstructure, and can construct 200 yds. of medium bridge in all. Besides these trains the divisional engineer units (2 field companies per division) bear with them in all 4 pontoons and 4 trestles, with the necessary bays of superstructure, their total bridging capacity being about 40 yds. of medium bridge. In France each army corps has a bridging train which admits of the construction of bridges to the extent of about 120 yds. of medium and 140 yds. of light bridging and bears besides 2 "advanced guard" trains which can provide 33 yds. of medium bridging each. Besides the corps trains there are also "army" trains, five in all, which can furnish 280 yds. of medium bridging apiece. These would be allotted in accordance with the requirements of particular campaigns. In Germany the increasing importance attached to independent cavalry operations has led to the assignment of a folding-boat wagon to every cavalry regiment. The regimental equipment provides for a ferry, capable of taking 25 to 30 infantrymen, one artillery vehicle or four horses at one journey, a foot-bridge 22 to 35 yds. in length, or a light bridge of 8 to 13 yds. By assembling the material of a whole cavalry division of 6 regiments, a foot-bridge of I10 to 210 yds. or a light bridge of 57 to 70 yds. can be constructed. The corps 'bridging train of a German army corps can construct 140 yds. of medium or 170 yds. of light bridging, and each of the two divisional trains, 40 yds. of medium and 48 yds. of light bridging.