Cubic ft. Ton. Tons. 30 ft. × 10 ft. × 1 ft. = 300 × 0·056 = 16·80 The submerged portion = 30 ft. × 20 ft. × 1 ft. = 600 × 0·028 = 16·80/33·60 33·60 Consequently the pressure = ————— = 1·12 ton per square foot; 30 K also a difference of 0·56 ton per square foot, or 33 per cent. less load. The difference in weight, assuming a wave of 5 feet in height, measured downwards from high water level, to simultaneously roll against the embankment upon both slopes and completely recoil, would be equivalent to the displacement of 5 feet depth of water for a strip = (30 ft. + 30 ft. + 30 ft.) + (45 ft. + 30 ft. + 45 ft.) ——————————————————————————————————————————————————————— = 105 ft. in width. 2 The cubic contents per lineal foot are— 105 ft. × 1 ft. × 5 feet = 525 cubic ft. O. The flotation power = 525 cubic ft. × 0·028 ton = 14·70 tons. For the purposes of this calculation, this weight is taken as if it were spread over the whole area of the seat of the embankment at a depth of 15 feet from the top = 45 ft. + 30 ft. + 45 ft. = 120 ft. × 1 ft. = 120 square ft. The vertical pressure per square foot therefore = 14·70/120 = 0·123 ton = 275 lbs., which is equivalent to a hammering action upon the foundations of 275/144 = say, 2 lbs. per square inch occurring each time the 5 feet waves recoil. The weight, 1·05 ton upon each slope, of the water upon that portion of the slope which is alternately submerged and unsubmerged is not considered. This wave action may, and generally will, happen upon one side only of an embankment owing to the direction of the wind, the current, and the “fetch” of the water. In that event the lateral pressure upon the embankment will also constantly change, and there will be a varying horizontal force from the 5 feet in height wave and its percussive action upon the slope tending to produce unequal strain and movement. The object of the preceding calculations is to show _the variation of pressures_ an embankment in an estuary or a tidal river has to sustain in addition to those of an ordinary embankment upon dry land, and its especial liability to slip and subside; and also to demonstrate that the vertical and necessarily the horizontal pressures may be in a perpetual state of mutation and vary considerably, and that the vertical pressure of the water outside an enclosure embankment may reach a point when the water may be forced upward upon the land side. Usually, subsidence is greatest in the wet seasons and at the time of the lowest tides.