1520

THEORY AND PRACTICE OF ENGINEERING.

Book II.

of the dam, 8 feet in depth and 27 feet wide; the piles were of Memel timber, 12 inchessquare and 36 feet in length; these were all driven till their tops stood 4 feet 6 inchesabove Trinity high-water mark of ordinary spring tides ; to these were attached the waling-pieces ; and the outer sheet piles, formed of whole timber 13 inches square, and of similarlength to the others, sawn perfectly square, were driven close together and firmly bolted tothe waling-pieces. The inner sheet piles were of half timber, of the same length, anddriven to the same depth : between these rows of piles, the space above them was .occupiedwith horizontal pieces, bedded down to them, and made fast with bolts to furring pieces,inserted above the waling of each gauge pile. Throughout the whole length of 920 feetthe dam was secured by diagonal braces, extending back to the old river wall, against whichthey had an abutment: the outer and inner rows of piles were held together by three rowsof 2| inch wrought-iron bolts: when all the piles were driven, the whole space betweenwas cleared down to the clay, and then filled up with stiff clay, mixed with a smallquantity of gravel.

The foundations of the river wall were got out in lengths of 50 feet, and the footingslaid on a bed of concrete varying in thickness from 1 to 6 feet; this was composed of sixparts of gravel and sand to one of ground lime, made from the lower stratum of the chalkformation. Along the face of the wall was a row of elm sheet piles, 8 inches in thickness,and from 10 feet to 12 feet in length; after being driven close to each other they werespiked to an oak waling-piece, and further secured by inch wrought-iron bolts at every 4feet distance, which passed 6 feet into the wall, where they were held by cast-iron washersbedded between the courses.

The two lower courses of the wall were formed of York landings, 11 feet in width, and6 inches in thickness; on these were laid two courses, 15 inches thick, of Bramley fall-stone, and above the stone facing of the wall, Aberdeen or Cornish granite was laid incourses varying in thickness from 26 inches to 19 inches at top; the front had a curveof 100 feet radius, and the whole filled in at the back with brickwork ; the total thicknessat bottom was 7 feet 6 inches, and at top 5 feet; at every 20 feet was a counterfort, 3 feet9 inches in width, projecting 3 feet 4^ inches; the height above the footing is 25 feet 6 inches.

The face of the foundation of the main building is 28 feet 9 inches behind this river wall,and the space between the two was entirely filled up with concrete, formed of ten parts ofgravel and one of ground lime : the cost of the excavation, cofferdam, and river wall was74,373/.

Excavations are usually performed by a class of men called navigators, who also leveldown the bed to receive the foundation: to assist the labour of wheeling out the soil, it is

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Fig. 2785.

DRAG EMPLOYED AT ORLEANS BRIDGE.

often necessary to lay down temporary bridges, and to construct different heights ofscaffolding; the method of loading and unloading the boats employed for this purposehas already been described.

Dragging is generally resorted to for removing the soil which lies below the level ofthe water, which is done with an iron shovel or scoop with a long flexible handle, or withthe dredging-machine. The drag used at the bridge of Orleans was moved by a windlass,,and could be lifted at pleasure.