HO
DOCK ENGINEERING.
came up to 33 feet above the sill, but during equinoctial gales the waves
frequently surged to the top of the dam and broke over the roadway.
Iron Dams usually take the form of caissons, but they are by no means
common. The most striking instance of their adoption is perhaps in con-
nection with the construction of the Thames Embankment. The caissons
were of wrought-iron in half oval segments, with upright flanges at each
end, so that when the halves were bolted together they formed a complété
oval, 12 feet 6 inches long by 7 feet wide in the centre and 4 feet 6 inches
deep. Ihe plates were | and | inch thick. Angle irons were bolted round
Iig. 69. —Dam at Ardrossan.
the top of the rings, enabling them to be firmly secured to each other in the
vertical position. A watertight joint was formed by a guide pile, 10J by
6} inches section, fitting into a groove between adjoining caissons. The
dam was further stayed by a few surrounding piles which maintained the
caissons rigid and vertical in their descent. The gross cost of this dam was
£30 per lineal foot as compared with £20, the gross cost of a timber cofferdam
in a similar position. Some of the iron caissons were incorporated in the
permanent work at an allowance of £8 per lineal foot. With this qualifi-