1526
THEORY AND PRACTICE OF ENGINEERING.
Book II.
water the pressure from the interior sometimes forces the water through the banks, unlessthis precaution is taken.
Walls constructed of earth to hold water in a reservoir or filter are set out in the samemanner, but as the pressure to which they are subjected is only on one side, the inclinationof their slopes is varied according to circumstances. Puddling a wall in the centre is always
Fig, 2803.
advisable, and before the work is commenced, it is usual to lay in the main or pipe,through which the contents of the reservoir are to be admitted or discharged ; if thismain be required of considerable size, a brick tunnel may be substituted, which permitsof attention to the hydraulic works, and gives facility for their repair.
Sea Wall at Brighton is constructed with sand and shingle taken from the beach, andcemented together by an hydraulic lime made in the vicinity; salt water was usedat first, and answered the purpose well. The wall is at the top 2 feet 6 inches inthickness ; the back is built perfectly perpendicular, and the front has a talus equal to one-third of the height; its base is on the chalk ; its height varies from 40 to 70 feet. Thelime was obtained at Bycombe, where chalk marl lies under the lower chalk formation,and is of the same quality as the Dorking and Hailing limes; it is found generallyin the north escarpment of the South Downs, and in the southern of the North Downs.After being properly burnt it was not ground but slaked at once, and at the same timemixed with three parts sand; it was then put into a pug-mill, and combined withthree parts of shingle, wheeled away and thrown down upon the wall, into a case made ofboards lodged together; the rear lodges were strutted against the face of the chalk, andthe front was held to them by iron ties, which were afterwards drawn by taking out thepins, and again used as the work advanced. The whole was executed at a price equal to3s. 4 d. per cube yard.
The Humber Dock Wall is in perpendicular height 32 feet; at top it recedes 6 feet 8inches from the perpendicular line; the thickness of the wall is 6 feet; the projection and
Fig. 2805.
DOCK WALL.
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width of the counterforts, which are 15 feet apart, is 3 feet 9 inches. Oak fenders 12inches square are placed along the walls to protect them; these project 8 inches before theface; there are also two horizontal rows of fir fenders, 7 inches square, let into the uprightsby short tenons with angle pieces, to prevent vessels catching underneath or riding upon themas the tide rises or falls.
The Humber Dock Wall has the foundations all piled, with a row of 6-inch groovedsheeting piles in front; those upon which the wall is founded are 9 inches square, andthe counterforts 8 inches in diameter, all driven with a ringing engine, and a ram of9 cwt. worked by fifteen men: longitudinal sleepers of half timber were bolted down upon