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tage of a firm bottom at very moderate cost, compared | of ancient masonry existing in this country, consist with the other situation.

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In the old bridges, if the bottom proved unsound, piles, 3 or 4 feet apart, were driven down over the site of the proposed piers; and instead of cutting off the pile heads evenly, and planking them over with timber, as is now done, it was customary to fill in between them with a species of coarse concrete, which being brought to a level surface, formed the bed on which the first course of stone was placed. Another ancient method of using piles for building in water, and still used by the French, who call it encaissement, consisted in driving in main piles with sheet piling between them, secured and bound round with waling as in the modern coffer-dam, but only one row of piles was driven all round the space of the pier or abutment to be founded, and not a double row of piles to be filled with clay, as in a coffer-dam. The encaissement being formed, and the loose soil, &c. got out, a mass of concrete or dry rubble stones is thrown in until a sufficient foundation is formed reaching to the level of the water. After being allowed some time to settle, the dressed masonry is laid in courses upon the foundations thus formed. A design by Mr. Semple will illustrate this mode. In a river 6 feet deep he proposed to drive sheeting piles about 10 feet in length, to a depth of 4 feet in the ground all round the site of each pier, and to fill the space or coffer thus formed with a bed of concrete 6 feet in depth. The top of the concrete being level with the surface of summer low-water mark, the masonry was to be commenced therefrom and carried up to springing height clear of the water.

Mr. Hughes remarks that " on examining the foundations of old bridges, particularly in this country, they are all found to be extremely massive, and the piers were even carried up above water of a thickness quite incommensurate with the necessity of the case. On inspecting the masonry of their foundations, however, it is found that no very great attention has been paid to the regularity of courses, or to the perfection of beds and joints. Some of the strongest specimens

(1) Called by the French béton. It seems now to be generally admitted, that the use of a concrete mixture composed of lime and coarse gravel, was common among the Romans. The use of concrete in modern times on a large scale, was introduced by Mr. Peter Semple, architect of a bridge over the Liffey at Dublin.

of a kind of building little superior to rubble walling, with this most important qualification, that the mortar is always of an excellent description, and in most cases by no means inferior in hardness and cohesiveness to the stone itself." The mortar usually contained a great number of small stones or pebbles, some of them equal in size to a pigeon's egg, and was altogether much coarser than that used at the present day. Another point of difference between ancient and modern bridges is, that in modern times engineers form their bridges with as few arches as possible; the ancient structures "consisted of a long low series of culverts, hardly deserving the name of arches, with intervening piers often of greater thickness than the span of the arches they were built to support." The weight of such a bridge distributed over a great many points, such as the 20 or 30 piers sometimes built in the old bridges, each pier had scarcely more to support than its own weight. "The ancients in their bridges throughout the whole structure, substituted quantity for quality, that is to say, large masses of rough undressed masonry, or rubble, instead of the firm, compact and elegant piers of modern bridges, which, above the bed of the river at all events, are invariably built with the most durable stone, of wellsquared dressed ashlar fronts, and suitable filling within." In some cases, however, there is no objection to these coarse and massive subaqueous structures; and provided they do not obstruct the free course of

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Fig. 281.

ROME.

THE BRIDGE OF FABRICIUS AT ROME.

the river, they may be of great advantage, especially in projecting sea piers and similar structures.

In modern times, the invention of the coffer-dam has enabled the engineer to secure a firm foundation in the bed of the river. Suppose a bridge is to be built over a river with 5 feet depth of water at the lowest summer floods; that the breadth of the waterway is great; and that it is impracticable to lay dry the bed by turning the course of the river. In such a case, the best method is to drive a coffer-dam all round the space which is to be occupied by the piers and abutments. The depth of water being 5 feet, suppose that of the unsound bottom to be 25 feet: the piles must not be less than 45 feet long, and must be driven into the solid ground as far as they will go, which may be from 8 to 10 feet. For such a depth of water, a double dam, with three rows of piles, will be required. The coffers between the rows of piles should be 6 or 7 feet apart, and filled with a retentive

(2) A series of papers on the Foundations of Bridges. By T. Hughes, Civil Engineer,-inserted in Mr. Weale's work on the Theory, Practice, and Architecture of Bridges.

clay puddle, mixed with gravel and sand, or a portion | expense of laying a foundation of concrete, from four of pounded chalk. All the water and soft material must then be got out of the dam. A steam-engine is used for clearing the space of water, and for keeping it dry during the progress of the building. The earth is removed from the coffer-dam by means of buckets, drawn up by windlasses erected on a temporary stage or platform across the dam, and the contents are discharged into barges alongside the dam. When the soft material has been removed, and the bottom is found to consist of hard gravel or clay, the building may be commenced, after sinking into it about two feet. Broad and large-bedded stones should be laid on the foundations, to give as much base with as few joints as possible for the superstructure to rest on. The first and second courses should be built with large stones, and, for the first few feet, with offsets, so that each course from the bottom may project all round at least six inches beyond the course immediately above. If, after the dam has been excavated, the bottom should appear unsound, piles must be driven all over the space to be occupied by the building, and extending about a foot beyond it. The piles may be of almost any kind of timber, such as Scotch fir, beech, elm, birch, &c. &c. eight or nine inches square, or round timber of this diameter: they should be driven in rows, from two to three feet apart, as far as they can be forced into the solid ground, or until each pile will not sink more than a quarter of an inch with twenty blows from a solid ram of 1,500 lbs. weight. When all the piles have been driven, their heads should be cut off quite level, and about a foot in depth being excavated between them, the space up to the level of the pile-heads should be filled with broken stones, grouted with good lime and sharp sand. A platform of oak, beech, or elm plank, from four to six inches thick, should then be placed across the pile-heads, and secured to them with spikes, bolts, or with trenails of hard wood. Another similar planking is usually laid across the first, and closely jointed thereto; and upon this upper platform the building, with brick or stone, is commenced. The masonry should be laid with offsets, and large stones should be used for the first and second courses.

to six feet in depth, and extending about two feet round the space to be occupied by the building, ought, in all such cases, to be considered, and compared with the expense of piling, which will probably never be found more safe than a body of concrete, unless the bottom be so soft as to allow the concrete to sink into it, and thus entirely cease to support the building. For its durable and almost imperishable nature, concrete cannot be too much esteemed; and, besides being quite as safe, and, perhaps, more durable than piling, its cost will in most districts be found much less. Where the piers are to be built either of brick or stone, a great saving will be effected by the use of concrete underground, as the expense of this substance will in no case exceed one-third the price of any description of brick-work, and in some cases it may not cost more than a sixth."

This method of laying foundations by means of a coffer-dam and piling is very expensive. In situations where the river can be turned, a far less costly method may be adopted for giving stability to the unsound bottom. Mr. Hughes recommends that it be covered over entirely with cross-sleepers of Memel logs, and on these to lay a covering of planks closely jointed; while further security may be obtained by introducing inverted arches, above the planking, between the piers, and extending under each of the abutments. It will not be necessary to introduce a platform of timber to support the inverts, unless the ground be too soft to construct them upon it; and, in some cases, the platform without the inverts may be sufficient to carry the bridge. In many cases, concrete may be used as a substitute for piling, in order to secure a solid foundation. "The

It was Mr. Telford's practice, when the site of the foundations was unsafe, and the expense of driving piles through a very deep bed of loose material would be considerable, to lay an inverted arch between the piers and abutments. In other places, where the ground was more solid, but still doubtful, a pavement of broad stones was placed over the whole bottom, extending also under the abutments and wing-walls. These also were tied across the whole bottom by a row of the same description of broad-bedded stones, placed on edge; and this kind of pitching was often continued to the extent

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Fig. 282.
A

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Fig. 283.

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of the wing-walls, and then tied across by a row of the same stones, scabble-dressed, rough-shaped, jointed,

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and set on edge, as at A B, Figs. 282, 283.
these stones could be procured in lengths of 6 or 7
feet, from 3 to 5 feet broad, and from 6 to 9 inches
thick, Mr. Telford preferred them to timber as a
covering. Where large stones could not be readily
obtained, he directed a pitched bottom to be formed
with stones of about two feet superficial area, and not
less than a foot deep. These were pitched endwise,
close-jointed throughout their whole depth, and set
with considerable care; and they were never allowed
to be set on sand or any material that could after-wasting of the bottom may be prevented. This plan
wards be carried away by the water. This pitching
was usually secured at the end of the abutments and
wing-walls by a longitudinal sleeper, placed along the
outside, with sheeting piles driven down in front to
a depth of from six to ten feet, according to the
nature of the bottom. This method is shown at CD,
Figs. 282, 283.

When | prevent its disturbance. Such a covering may be
found necessary after the erection of a bridge, to
prevent a sandy or silty bottom from being carried
away by the current, which is often increased in
velocity by the contraction of the water-way. By
covering the bottom with clay for a distance of sixty
or seventy feet above and below the bridge, and then
overlaying the clay with stones varying in weight
from 200 lbs. downwards, the whole artificial covering
not exceeding two feet in thickness, the further

Sandy foundations present a great variety. The sand may be rounded or angular, the latter forming what is called a sharp sand; or the particles may be minute, as in fine sand, or very large, as when the sand approaches the nature of gravel. Sandy soils are also distinguished from each other by the proportions they contain of clay, or other earthy matter, which, in combination with the sand, is termed silt. Fine sharp sand is usually found in currents sufficiently rapid to carry away the particles of earthy and other matters, commonly deposited with the sand in still waters. When the currents are still more rapid, the beds may consist of gravel and sharp angular sand, with no silt. The stronger the current, the coarser the gravel, and the smaller the proportion of sand mixed with it. A quicksand is formed by the action of water on a bed of this material, whether of a silty or pure sandy nature; and the danger of its giving way, when any weight is placed upon a quicksand, arises from its tendency to escape with the water, and pass from under the pressure. If this can be prevented, the foundation is a good one; for sand or silt, in a state of rest, is remarkably solid, the smallest vacuities being filled with it. Thus, a quicksand surrounded by a strong and close encaissement of piling would be perfectly safe as a foundation. When there is any danger of the sinking of a structure in consequence of the lateral shifting of the sand, the first part of the foundation should consist of a timber platform resting upon the sand, and of sufficient area to extend two or three feet on each side beyond the base of the structure to be raised upon it. It may be laid with sleepers, ten inches wide, and six inches thick, placed at the distance of three feet apart, from centre to centre: these must be closely covered with four-inch-thick planks, closely jointed, and secured to the sleepers with trenails.

In rivers where there is very little current, and the sand bottom has a covering of two or three feet of clay, or heavy gravel, which is not liable to be disturbed by floods or other causes, a building may safely rest on a sand bottom, without the accompaniment of a platform. In other cases, an artificial covering may be placed over the sand, and thus

was successfully adopted by Mr. Rennie in the case of the Lary Bridge at Plymouth.

Some idea of the method of forming a coffer-dam may be gathered from the following details. Suppose a bridge-pier is to be erected in a tide-river, where there is 10 feet depth of water at the lowest spring tides, and that the bottom consists of 12 feet loose gravel and sand, with clay underneath. If we suppose the depth at high water to be not less than 28 feet, (making the whole depth, from the surface of high water, through the loose bottom, down to the clay, 40 feet,) the coffer-dam must be formed of four rows of piles, and the clay puddle will occupy three distinct. spaces, or puddle-walls, between the four rows of piles. The outer row of piles, to be driven down to within a foot of low-water mark, and 5 feet into the clay, must be 28 feet long; the two middle rows, also to be driven 5 feet into the bottom, and to stand 3 feet above high-water mark, must be 48 feet long; the inner row, to be driven to about 11 feet above lowwater mark, and 5 feet into the clay, will be 38 feet. The clear breadth between each two rows of piles to be 6 feet; the outer row of piles to be half-logs, of 12 inches by 6 scantling; the middle row to be

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be placed all round the top of the inner piles, as in Fig. 284, and to be connected by wrought-iron bolts, 14 inch square. Other lines of waling must be fixed round the coffer-dam, and connecting pieces must be introduced at intervals of 4 feet, from bolt to bolt, all round the dam. All the sand and gravel within the space to be occupied by the dam must be excavated, or it will permit the water to penetrate through; and this may be done by dredging before the piles are driven. In making the dam water-tight by puddling, clay alone should never be used, as it is subject to great changes, according to the alternations of heat and cold, drought and moisture. In very dry weather it will crack, and separate into irregular fragments, which will not unite again so as to form an adhesive water-tight substance. Mr. Hughes recommends 3 parts of pure clay, 2 of chalk, and 1 of fine gravel, as a good compound for filling the dam. These materials should be well mixed together, the chalk and clay chopped small, and no stone larger than a hen's egg allowed to pass amongst the gravel. It is usual in large dams to cover the top of the puddle with bricks, or with good strong gravel, grouted with lime, to the depth of a foot. All the piles used for the dam should be shod with wroughtiron shoes, not less than 10 lbs. each, and hooped with iron rings, three inches broad, and three-quarters of an inch thick, to prevent the timber from splintering or giving way under the driving. The following figures represent some varieties of iron shoes for piles. Fig. 288. Fig. 289. Fig. 290.

Fig. 286. Fig. 287.

OOO

In constructing a coffer-dam, the first operation is to drive the guide-piles, so called from being the first in each row, and thus serving as guides to the rest. They are usually placed about ten feet apart, and the barge or other vessel containing the pile engine should be moored stem and stern alongside the line of the intended dam. A ringing machine, and wooden monkey, of about 800 lbs. weight, are first used for driving; and when all the guide-piles of one row have been fixed, the walings should be fastened to them, and the intermediate piles, ten in number, between each pair of gauge-piles, may be driven down. When the piles cease to sink, after repeated blows from the monkey, a heavy iron ram, of 1,500 or 1,800 lbs. weight, with considerable fall, may be used to complete the driving. When the heads of the piles get bushy or besomy, they should be squared off, that the force of the blow may not be deadened.

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Nasmyth's Patent Steam Pile-driver, which may be easily made to perform 80 strokes per minute; and, in ordinary ground, piles of 14 inches square are driven at the rate of upwards of 10 feet per minute. The inventor says: "Instead of attempting to obtain high momentum from a small mass of iron falling from a great height, I employ the momentum resulting from a great mass falling with moderate velocity from a small height, (3 feet,) and, instead of having one blow per minute, I employ from 70 to 80 blows per minute. The result is that, while the pile is driven with ease and rapidity into the most rigid and resisting soil, the head of the pile is so little injured as to present a neater appearance after having been driven than at the commencement of the action. All this advantage arises simply from employing the mechanical force in its proper condition for the performance of the required duty. If we desire to split and shatter to splinters, let us give the blow from a small mass, travelling at the highest possible velocity; if, on the other hand, we desire to propel and push forward such a mass as a log of timber, let us do just the reverse, namely, give it a blow from a heavy mass, moving at a low velocity." The inventor justly remarks that, although we have equivalent mechanical forces in the case of

4 lbs. falling with a velocity of 4,000 feet per second, and 4,000 lbs. falling with a velocity of 4 feet per second, yet how very dissimilar are the effects of each, when

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employed to perform such duties as that of driving a is open, and being placed upon the ground, of whatpile!1

A remarkable application of the science of pneumatics has been made by Dr. Potts, in the construction of an apparatus for sinking foundations by means of atmospheric pressure, in deep water, moveable sands, mud, shingle, or bog. It consists in the use of hollow tubes, usually of cast-iron, of any size, and almost of any shape, which are sunk into their places by means of atmospheric pressure. The lower end of the tube

ever nature it may be, the air, water, or semi-fluid material in the inside is extracted by pumps, or by other means. When the more solid materials are removed, the air in the interior of the tubes is rarefied, by placing them in communication with large vessels from which the air has been previously withdrawn, by means of a pipe and stop-cock. As soon as the communication is effected, the air in the interior of the tubes rushes into the empty vessels, leaving the

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atmospheric pressure upon the pile-head without any | they are thrown out, so as to attain the greatest
counteracting resistance. If the strata to be traversed possible rarefaction of the air; and the operation is
be of a yielding semi-fluid nature, they are also acted repeated until the piles are fully driven. A succession
upon by the same cause, and flow up into the tube of tubes may be placed upon the first by means of
or hollow pile, which at the same time descends with flanges or other joints, so that they may be driven
corresponding rapidity. The materials thus intro- of any length required.
duced are removed, or, if the strata be more resisting,

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The chief advantage of this system is, that no vibration is communicated to the ground, and the elasticity of the strata traversed is never brought into action. A remarkable illustration of its advantages in this respect occurred on the Goodwin Sands, where

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