A Treatise On The Principles And Practice Of Dock Engineering

64 DOCK ENGINEERING. effect is no doubt due to the consolidation round the pile of the earth which had been maintained in a state of disintegration and vibration during a sequence of rapid blows. Limit of Driving.—The limit of adequate driving and the maximum supporting power of piles are equally moot points among engineers. To a certain extent they are interdependent. The practice at Liverpool has been to regard a total depression, not exceeding | inch in 10 blows of a 20-cwt. ram falling 10 feet, as evidence of sufficient driving, or, in other words, an expenditure of mechanical energy at the rate of 896,000 ft.-lbs. per inch. At New York river wall the piles were specified not to penetrate more than ^ foot with the last blow of a 3,000-lb. monkey falling through 8 feet, involving energy to the extent of 20,000 ft.-lbs. per inch. According to Rankine, some of the best authorities consider the test of a sufficiently driven pile to be a depression of not more than i inch by 30 blows of an 800-lb. ram falling 5 feet, or mechanical energy represented by 600,000 ft.-lbs. per inch. These standards are evidence of the great diversity of opinion there is on the subject. Supporting Power.—Various theoretical and empirical formulæ have been suggested for determining the relationship between the blow required to drive a pile to a given depth and the greatest load it will sustain without sinking further. Rankine * puts forward the following equation, supposing the pile to be supported by uniformly distributed friction against its sides :— //4 EswH 4E-s2 D2\ 2EsD ^“VK L + L2/ L~ ' ' ^ in which w = Weight of ram in Ibs. E = Modulus of elasticity. s = Sectional area of pile in square inches. H = Fall of ram in feet. L = Length of pile in feet. p = Maximum load in Ibs. D = Depression of the pile in feet by the last blow. A factor of safety of not less than 3 should be used; preferably one of 5 to 10. A very well-known, but merely approximate, rule devised by Major Saunders of the U.S. Engineers is .rølT f being the safe load in Ibs. and the other notation as before. The formula recommended by Trautwine is 51-5 w ^H ?,=’12D+1’ with a factor of safety of from 2 to 12 according to circumstances. * Manual of Civil Engineering, p. 604. ■ (2) ■ (3)