What Dislodges Sea Walls.

New Zealand Graphic, Volume XLIX, Issue 4, 22 January 1913, Page 8

What Dislodges Sea Walls.

The dislodging by wave action of large stones or masonry blocks from the face of break-waters or sea walls has been investigated by the engineering department of University College, Dundee. Such blocks weighing several tons have been forced out, even under heavy overlying courses, and it has been explained that the effect might be due to hydrostatic pressure, exerted upward and outward above the line of wave impact, or to a compression of air entrapped in openings struck by the water. In the experiment made, it was assumed that the striking wave has a maximum velocity of 80 feet per second. 'lt was found that this would give a maximum internal hydrostatic pressure—like that of a hydrostatic press—of about 2.9 tons per square foot; and that it might cause a pressure from the enclosed air of about 6 tons per square foot. The experiments proved, however, that a vastly greater force might be present—that of waterhammer action, sueh as is sometimes noticed when the flow of water through a long pipe is suddenly stopped at tha open end. This would occur under special conditions only, but it was shown that the impact upon an open joint of a wave of the stated velocity might set up a water-hammer action of 40 tons per square foot, or even more. It was suggested that walls exposed to waves should have drains or openings on the inner side to prevent excessive water, air, or water-hammer compression.