A Treatise On The Principles And Practice Of Dock Engineering

274 DOCK ENGINEERING. three years, was carried away bodily by a storm in January, 1877, and deposited in two pieces within the line of tbe breakwater. But even this is not the limit of wave power. During the storm of October, 1898, which is said to have been as severe as any that have been witnessed in Peterhead Bay, the waves were 30 feet in height, and a section of the breakwater there, down as far as 10 feet 7| inches below low water and weighing 3,300 tons, was bodily slewed to the extent of 2 inches, without the brickwork being dislocated. This enormous mass slid upon the surface of the course immediately below it, the blocks in which were, strange to say, quite unmoved. In the waves which were responsible for this feat, the water was thrown up to a height of about 115 to 120 feet, and the surface upon which they acted measured 33 feet by 44 feet, or 1,122 square feet. In order to form an idea of the force required to slew such a mass, the Engineer, Mr. William Shield, ascer- tained the coefficient of friction of blocks similar to those forming the breakwater, by causing them to slide upon a concrete floor. The floor was well wetted, and the average of several triais with blocks up to 68 tons weight, gave a coefficient of 07. In moving the mass, the waves must therefore have exerted a force of 2,310 tons over the whole area exposed to them, or slightly over 2 tons per square foot. Although about one-third of the mass was below the level of low water, the troughs of the waves would be considerably below its lowest point, and taking all the circumstances into consideration, little, if any, allowance need be made for flotation. If such allowance, however, be considered necessary, it is prob- able that some deduction should also be made from the area exposed to the wave-stroke, so that the above force per square foot would not be mucli affected.* After this incident it is, perhaps, not surprising to find that a 20-ton block at Ymuiden breakwater, in Holland, was lifted to a height of 12 feet vertically up the face of the pier and landed on the top of it.f The Design of Jetties, Wharfs, and Piers.—The principles of the stability of quays have already been set forth, and they are equally applicable to those wharfs of solid construction which act as retaining walls. The depth of a wharf or river wall, however, will generally require to be greater than that of a dock wall, on account of the vertical disturbance of vessels by waves. Open timber wharfs in front of pitched slopes, allow the waves to pass through and expend themselves upon the bank, so that the wharf structure does not encounter the full force of the waves, but this arrangement is only feasible in situations where the exposure is not great. In considering the stability of structures subjected to external forces of great magnitude, it will be found that there are two distinct sources of resistance, upon either of which a design may be based—viz., the resistance * Shield on “The Effect of Waves and Breakwaters,” Min. Proc. Inst. C.E., vol. cxxxviii. + Ibid.