Data.—Sand embankment: Width of formation, 30 ft.; inclination of the slopes, 3 to 1; height of embankment from the surface of the ground, 30 ft.; depth of water, 20 ft.; height of wave, 5 ft.
As an illustration of the deleterious effects of the recoil of waves may be mentioned that a high vertical wall with a parapet has been found to endanger the toe, but when the parapet has been removed in order to allow the head of the waves to leap over the top of the work, the structure remained stable. To prevent the recoil of the sea and the scooping away of the base and the foreshore in much exposed situations, and consequent slips and subsidences, breakwaters which simply act as wave screens, and not as wind screens, are sometimes kept a little below high-water mark so that the heads of the waves may have free action, although their onward motion is prevented. On the contrary, light open work, such as a lattice screen, although it may somewhat lessen wave action, does not prevent it passing through.
The principal causes of the failure of vertical walls when placed upon an easily eroded foundation are by the scouring action of the recoiling waves, therefore their magnitude should be reduced to the lowest limit: by the impounded air driving out particles of the structure: by waves travelling upon the top, therefore their forward motion should be deflected and rendered vertical: by the hammering action of the mass caused by its being alternately quickly submerged and unsubmerged; the practical effect being that the foundation is intermittingly released of a portion of the load and then fully strained, therefore the height of the waves should be reduced as much as possible. Respecting the action of falling water, experiments were recently made in India, which proved that “the greatest intensity of pressure does not exceed that due to a column of water of a height equal to the fall;” the greatest intensity of pressure being always fractionally under the hydrostatic head.
As an illustration of the constant change of the load upon the foundations of an embankment in tidal waters, the following calculations have been made.
The weight of a cubic foot of sea water is taken as 0·028 ton.
The weight of a cubic foot of sand is here taken as 0·056 ton.
A. The weight of a lineal foot of the embankment when unsubmerged equals 201·60 tons, computed as follows:—
| Cubic ft. | ||||
|---|---|---|---|---|
| The central portion | 30 ft. × 30 ft. × 1 ft. = | 900 | ||
| The two inclined portions | 90 ft. × 30 ft. × 1 ft. = | 2,700 | ||
| Ton. | Tons. | |||
| Cubic contents | 3,600 | × 0·056 | = 201·60. |
B. At high water the weight of the embankment is reduced by the weight of the water displaced, which equals 84 tons, calculated as under.
The submerged contents of the embankment are—