صور الصفحة
PDF

clay pnddle, mixed with gravel and sand, or a portion 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 Scoteh 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 Becond courses.

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 maybe 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

expense of laying a foundation of concrete, from four 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."

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 pitehing was often continued to the extent

[graphic][merged small]

and set on edge, as at A B, Figs. 282, 283. When 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 pitehed 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 afterwards 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 c D, Figs. 282, 2S3.

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 tilled with it. Thus, a quicksand surrounded by a strong and close encaissemcut of piling would be perfectly safe as a foundation. When there is any danger of the sinking of a structure in consequeuce of the lateral shifting of the sand, the first part of the foundation should consist of a limber 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

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 seveuty 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 wasting of the bottom may be prevented. This plan was successfully adopted by Mr. Ronnie 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

[graphic][merged small]

be placed all round the top of the inner piles, as in Fig. 284, and to be connected by wrought-iron bolts, li 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 dum 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 heu'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.

[graphic]

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 l,S001bs. weight, with considerable full, 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.

[merged small][graphic]

Nasmyth's Tatent Steam Pile-driver, which may be easily made to perform 8O strokes per minute; aud, 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 foree 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 forees 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

[merged small][merged small][graphic][merged small][graphic][ocr errors][merged small][merged small]

a tube 2 feet 6 \ inehes in diameter sunk to a depth of 32 feet 7 inches in 6 hours; whereas, a bar of iron, 3 inches in diameter, could not be driven more than 13 feet, at which depth it required 46 blows of a monkey weighing 10 cwt., falling through 10 feet, to advance it one inch. In such sandy soils, and sometimes, also, in clay soils, it is necessary to drive common piles with the large or butt-end downwards, to prevent the elasticity from forcing them up again.

These pneumatic piles have been used in the foundations of several railway-bridges. Those for Windsor Bridge were 5 and 6 feet in diameter; and at the present time Messrs. Fox and Henderson are about to put in the foundations of a bridge over the Medway, at Rochester, with cylinders 10 feet in diameter. With such colossal dimensions they cease to be piles, and become, in fact, caissons. When the hollow cylinders have been sunk, and the earth which has risen up into them been removed, they are filled up with concrete, and thus made equivalent to solid columns.1

The accompanying section and plan of the cofferdam at Neuilly, Figs. 292 and 293, will convey a general idea of the arrangements of coffer-dams. The arrows represent the direction of the stream, which was taken advantage of for turning large waterwheels placed in the river, the motion of which, being communicated to pump-machinery within the dam, raised the waters thereof, and discharged them by means of shoots over the piles into the river. The works at the left of the figures are the land abutments, dammed in on the river side, and containing also the first pier. The complete coffer-dam to the right shows the second pier in progress. Temporary bridges of planking connect the dams with the land.

2. The piers and abutments. In the construction of a bridge it must be nearly always desirable to have the smallest possible number of points of support. "Piers in a waterway intercept the current and impede the navigation; they arc most troublesome and expensive to found and form, and are most exposed to injury when they are formed. The object of the bridge itself—a convenient road over—being properly provided for, and the permanence of the structure being sufficiently considered, it is not too much to say that the aim and end of the bridge-builder should be to reduce the piers to the smallest possible number, consistently to a due regard to economy. Of all the bridges over the Thames, at and near London, the suspension-bridge at Hammersmith interferes least with the navigation of the river, and is least exposed to injury from the action of the current upon its points or support; these having, to a cortain extent, the effect of embankment-walls, which prevent the stream from spreading itself uselessly, if not injuriously, over a wide and shallow bed, and direct the current upon the mid-channel, whereby it is kept free and clear; whereas, the lumbering masses which support Putney Bridge obstruct the navigation, force the current into narrow rapids, which tend in every

(1) Supplement to Mr. Wcnlv's work on Bridges. Edited by Geurgc R. Burncl], C.E. London. 1850.

way to the destruction of the works themselves, and make the passage up the river dangerous. The laden barge of commerce and the double-banked barge of pleasure pass with or against the stream, and alike with ease and safety, under the tasteful and scientific erection, which carries a convenient and agreeable road over, and leaves the water-way uninterrupted; while both arc exposed to inconvenience and danger where the ugly piles of Putney and Battersea support narrow and inconvenient roadways over the dammedup river. In like manner, the effect of Southwark Bridge, with its two well-formed piers of neatlyexecuted masonry, is hardly felt upon the river; whilst the multitnde of wry-looking angular piers of Vauxhall Bridge, standing across a bend of the river, arc with difficulty avoided by the heavy craft, which depend almost entirely upon the current for motion. These are timber and iron bridges, of various forms and modes of arrangement; and with the materials of which they arc composed, no sensible inconvenience, and much less obstruction, should in any case have been imposed upon the navigation of the river. That neither obstruction nor inconvenience is necessary with even a bridge of masonry is shown by the new London Bridge, which contrasts advantageously, not alone with its predecessor, but with all the other stone bridges upon the river, in these respects. The infrequcucy of the piers, and their moderate bulk, together with the expanse and elevation of the arches, preserving the head-way almost unabated over a great part of the whole width of the water-way, show in these, as in other respects, an example of the highest degree of perfection in the practice of bridge-building. . . . Had Labelye and Mylnc built with granite, their works could not have been executed with the funds at their disposal, respectively; but with granite, and the means of applying it, they would possibly, or they ought to, have occupied less of the water-way with obstructions; and, with the means at their disposal of raising the approaches, they would probably have avoided making the roadways upon their bridgessosteep m to be always inconvenient. and sometimes dangerous." {Hosking.) The danger of contracting the water-way by piers is shown in the case of Hexham Bridge, over the Tyue,

[graphic][ocr errors][ocr errors][merged small]
« السابقةمتابعة »