462
HISTORY OF ENGINEERING.
Book I.
Gloucester Stone Bridge , built by Mr. Telford, has but one arch, of 150 feet span and35 feet rise. The idea of this bridge is taken from that over the Seine at Ncuilly built byPerronet. The voussoirs or external arch-stones have the same chord as the inner arch,but its segment only rises 13 feet. By this means the arch has the form of a funnel, whichsuits the contracted passage of the waters, and lessens the flat surface opposed to thecurrent when the waters rise above the springing of the ellipse, that being at 4 feet abovethe level of low water.
This work was commenced in July, 1826, upon a soil, the stratification of which wasfound by boring through to be from the surface of the ground, 11 feet of loam, 12 feet ofblue silt, 5 feet of peat moss, 5 feet of brown clay, 3 feet of strong coarse indurated gravel,and 8 feet of finer gravel or coarse sand, in all 44 feet.
The foundations were laid upon the indurated gravel, at about 15 feet below the bed ofthe river. A space of 40 feet square was excavated to the depth of 33 feet below thesurface of the meadow, and a very strong cofferdam was made to protect it, as the floodsoccasionally rise 6 feet or more above the banks of the river. The cofferdam was formedof piles of Memel timber, 32 feet in length, with a space of 5 feet between the outer andinner circumference of piles ; this space was filled with clay worked into water-tight puddle.
After the gravel was made level, a course of large and flat bedded rubble stone was laidover the whole space, and upon this was carefully bedded the timber platform. Thisconsisted of thirteen pieces of Memel timber made straight and level; those laid at rightangles with the stream were 37 feet in length, and those placed up and down the stream,40 feet in length. These pieces of timber were crossed, and laid at equal distances, thesquare spaces between them being filled with rubble masonry well grouted. Upon thesepieces of timber, or sleepers, was a covering of 4-inch beech plank, planed, closely jointed,and spiked down, thus forming a level platform, 40 feet by 37 feet, and upon this themasonry was laid.
The stone employed for backing above low water mark was brought from the quarriesat Highley and Alveley, 6 miles from Bewdley; its thickness varies from 14 to 24 inches,and its weight from one to three tons. They were squared on all sides, and well bedded ;every course was brought to a level and grouted before any of the next were set. All theexternal masonry was from the quarries of Colford and Quitchurch in the Forest ofDean .
The abutment on the west side of the river was of the same dimensions, the onlydifference being, that the gravel was found at 27 feet below the surface of the meadow.No piling or platform was used to the wing-walls, and they are in consequence defectiveon the eastern side.
The centre was supported by six parallel rows of piles, fixed in the current of the river,each row being connected by cross braces and caps; each supported a rib which formedthe actual centering. The whole was steadied by diagonal braces; and between the capsof the piles, and the ribs which rested on them, the wedges were placed by which thecentering was slacked or lowered after the masonry was keyed.
The depth of the arch-stones at the springing is 5 feet 6 inches, and at the key 4 feet6 inches ; this sunk 10 inches after striking the centre. The thickness of the abutments atthe springing of the ellipse is 27 feet 2 inches, besides the wing-walls, which are 7 feeteach, the spandrill walls are 3 feet 6 inches thick, exclusive of the pilasters.
The four longitudinal walls of the interior, which support the platform of the roadway,are each 2 feet thick ; the width of the carriage-way is 27 feet, with a footpath of 4 feet oneach side.
Centering —A platform was prepared, perfectly level, rather larger than the intendedcentre, on which it was struck out the full size, the centres of the different radii beingfixed. Dantzic timber was employed in scantling 15 inches square.
Tlie piles were of Memel, with wrought-iron shoes, and caps at the top to the proper