Chap. VIII.

BRITAIN.

433

the base of the first pier was fixed, the depth of the excavation must be increased, till it iswithin 2 feet of the former depth.

Method of fixing the foundation according to the plan. The excavation completed, letthe 21 piles upon which the frame is to rest be driven into their proper places; thesepiles are to be 10-inch heads, and 6 feet long, supposing the gravel of equal strength withthe last; the pile heads are then to be reduced to a level, and the frame laid thereon,and trenailed down upon the pile heads. 'Then the sheeting piles are to be driven; andthese may be of oak, elm, beech, or fir, and about 6 feet in length, as these piles are to berebated, and if possible driven without splitting.

To save timber, it was recommended to groove the piles on both sides, and to nail inthe tongue, which might be of hard wood if fir piles were used; these tongues were to be1£ inches in thickness, and 1^ inches broad ; and to be let in $ inch where it fastens, and tostand out 1 inch. The tops of the sheeting piles, being reduced to the same level as thestring pieces to which they were to be spiked as they were driven; the outside was to bereduced to a regular breadth, and the notched stones taken in a line. "When this was per-formed, the rest of the bearing piles were to be driven, beginning with the outside rows,and cut to a level with the tops of the string pieces; the piles were to be 6 feet, moreor less as found requisite. ,

The setting was to be completed by first underpinning the string pieces and tie-beamsas firmly and equally as possible, by driving stones under them, and then the other spacesto be set and well driven down as before; but before ramming down, the joints were to befilled, by sweeping in dry lime mixed with sand and small gravel, so that the whole whendriven down should be thoroughly compacted together.

After the pier was raised above low water, the matter was to be taken out 4 feet in widthall around, down to the level of the top of the notch course, and then filled with goodlagging as before, standing somewhat higher than the natural bed of the river, and the restof the space covered with rubble to the sides of the dam.

The bearing piles were to be driven until the ram made them descend 1 inch for every20 blows, and the sheeting piling, until it required 40 blows to drive the same quantity ;and the latter were to be driven to a regular depth throughout.

Coldstream Bridge over the Tweed, designed in 1763 by John Smeaton , has five arches.That in the centre is in span GO feet 8 inches, the two adjoining 60 feet 5 inches, and eachof those on the land sides 48 feet. They are all parts of the same circle; the smallestarches being one-third, and the others rising higher in proportion to their span. The heightof the piers above low water, including the impost, which is 2 feet, is 12 feet precisely, sothat the shaft of the piers before the impost was put on is 10 feet.

Fig. 434. COLDSTREAM BRIDGE.

The engineer in one of his reports upon this bridge remarks, that be found the gravel ofthe Tweed lie very open, and not wrecked up with sand and matter in the usual manner,and though generally objecting to piling foundations, which are tedious and expensive,and getting down to the rock, would from the nature of the gravel make the drainageexceedingly so, he in this case recommended piling the foundation 3 feet below the bed olthe river; in general he observes that in similar cases it was usual to lay down a gratingand build upon it without any piles at all; but that here the floods might take away thegravel and leave the grating bare, which would eventually undermine the pier; and thatto defend the foundations in the usual way, by building starlings round the piers, ter-minating above low •water, would also be objectionable, as the water-way would be toomuch contracted.

Hexham Bridge , as designed by John Smeaton , had nine elliptical arches of differentspans, and extended 518 feet between the abutments, and 568 feet comprising them; thewidth measured on the soffite 20 feet, and the clear between the parapets 18 feet.

The span of the centre arch was 51 feet, the piers 12 feet 6 inches; the adjoining arches50 feet 7 inches; the piers 12 feet; the next arches 49 feet 11 inches, the piers 11 feet6 inches; the next arches 48 feet, the pier 11 feet 6 inches, and the two outer arches, span37 feet. This bridge, which was destroyed by a flood in 1782, was a considerable loss to thecontractors. Five of the piers were built in a caissoon, the internal width of which was16 feet, the length on the flat side 22 feet; the bottom was formed of 3-inch plank laidcrosswise, and the sides of 3-inch plank placed upright, supported by three ribs of timber,two on the inside and one on the out, all secured at the angles by iron bolts in a propermanner.

F f