PILE DRIVING.
This operation has for its object the consolidation of naturally weak bottoms; for piles driven close together tend to prevent that compression that might take place under a heavy structure. Piles may resist either by friction against the soils through which they are driven, or by bearing upon a firm substratum at too great a depth to be reached by uncovering. Piles driven in clay have sometimes acted as a conductor to water, which, insinuating itself along the side of the wood, produced settling which would not otherwise have taken place.
Experience has shown that four feet apart from centre to centre, when there is a good substratum, is near enough to bear the heaviest loads.
The fact that a pile refuses to enter further, does not show that it has reached a bed strong enough to bear the required load; for though it may bear upon a solid bottom, or resist penetration by side friction, when the load has been for some time upon the pile, it may be found too weak to stand. Piles have in some cases refused to enter the ground from the blow of a 1,500 lbs. ram, falling twenty feet, when first driven, and have afterwards gone down three feet from a ram of 1,000 lbs.
The following formula, showing the resistance which a pile should offer, is given by Weisbach in Mechanics of Engineering, Vol. I. p. [285]. First, when the ram remains upon the pile after the blow,
P s = G′2 × H
G + G′.
And, second, when the ram does not remain upon the pile,
P s =(G′
G + G′)2 × GH
Example.—A pile weighing five hundred lbs. is driven two feet, by forty blows of a 1,000 lbs. ram falling six feet. Required the weight which may be safely supported by the pile without further penetration.
The notation in the formula above is thus,
G = the weight of the pile.
G′ = the weight of the ram.
H = the fall of the ram.
s = penetration per blow.
P = the weight in lbs.
The penetration per blow will be 2
40 or .05 feet; and the formula for the second case
(1000
1000 + 500)2 × 500 × 6
.05 = 48,000 lbs.
Of which one tenth or one twelfth only is the maximum load which should be placed upon the pile permanently. The surest test of the power of a pile is to load it temporarily, when the time and place admit.
Perronet considered fifty tons, or 112,000 lbs. as not too great for a twelve inch pile; and allowed twenty-five tons for a pile of nine inches in diameter.
That the point of the pile may not be shattered by contact with the hard earth, an iron shoe is sometimes fitted to the lower end; and that the head may not split, an iron ring is driven on to the top.
The force of the blow given by a ram depends upon the weight of the ram or monkey, and upon the velocity at which it strikes the pile; the velocity depends upon the height from which it falls. The velocities of bodies falling freely being as the times, and the spaces fallen through as the squares of the times, we have the following rules; and from them the table succeeding.
Given the velocity of a body to find the space through which it must fall,
(Velocity in feet per second
8)2 = space in feet.
Thus a weight to acquire a velocity of two hundred feet per second, must fall through a height of
(200
8)2 = 625 feet.
Given the space fallen through, to find the velocity.
√height in feet × 64.3 = velocity in feet per second.
Thus the velocity of a body falling twenty feet will be
√20 × 64.3 = 36 feet per second.
Momentum is the product of weight by velocity; therefore, to find the force of the blow given by a ram of given weight, falling a given height, we find, first, the velocity by rule two. Also, given the weight of ram, the necessary velocity to produce any required effect being found, it is easy to find the height, and the reverse.
Examples.—Suppose we have a ram weighing 2,000 lbs. and wish to strike a blow of 25,000 lbs.; the velocity must be
25000
2000 = 12½ feet per second;
and to acquire that velocity, the height fallen must be (rule one)
(12½
8)2 = 2.43 feet.
Again, if we have a pile-engine which admits of a fall of fifteen feet, and we wish to strike a blow of 18,000 lbs., we first find the velocity (rule two) thus:—
√15 × 64.3 = 31 feet per second nearly,
whence the weight
18000
31 = 581 lbs.
The form of the common pile-engine is too well known to need description.
Mr. Nasmyth’s system of pile-driving consists in forcing the pile into the ground by a great number of blows following each other in rapid succession. Piles were driven by his engine at the United States Dry Dock, at Brooklyn, (N. Y.,) as follows: A pile was sunk fifty-seven feet by a hammer of 4,500 lbs.; it was driven forty-two feet in seven minutes by three hundred and seventy-three blows.