Dams may be built of caissons of strong timber, filled with loose stones and covered with planking; others are filled with earth instead.
A recent writer states that weirs of solid masonry, like other hydraulic works, should be founded on the natural ground on a bed of concrete, or on piles, according to circumstances. The masonry may be built in cement or hydraulic lime; the face-work is usually in dressed stone or blocks. The stones, besides being fastened together by metal cramps, are sometimes bonded by dovetailing.
A good example of a masonry weir is that built across the Dora Baltea for obtaining a supply of water for the subsidiary canal of the Canal Cavour. This work consists of a mass of concrete faced with ashlar and blocks in courses roughly dressed. The crest is 1·20 metre in width, and the total length 200 metres. This dam cost 237,682 francs, or at the rate of 1188·41 francs per lineal metre. A layer of concrete alone forms a very effectual protection to a river or canal embankment. In rivers subject to excessive floods a rock-work consisting of large irregular-shaped blocks of stone—not less than one-third of a cubic metre each—is exceedingly useful for protecting the bottom of embankments or walls from scour.
FOOTNOTES
CHAPTER XXX
[282] Paper by Mr. G. R. Jebb in the ‘Journal of the Society of Arts,’ for 1888.
[283] Paper on “The Maintenance of Canals,” in the ‘Journal of the Society of Arts’ for 1888.
[284] Paper read before the British Association, 1878,.