279. Three methods of founding under water may be noticed,
By driving piles.
By coffer-dam.
By caisson.
In very shallow water, where no danger arises from contracting the water-way, we may throw in loose stones until the surface is reached; and commence thereon the lower courses of the masonry. This is termed “Enrockment.”
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,