. Railroad construction. Theory and practice . re to test itsbearing-power. In a new country judgment and experience mil be requiredto decide intelligently whether to employ a simple drop-hammermachine, operated by horse-power and easily transported butuneconomical in operation, or a more complicated machineworking cheaply and effectively after being transported atgreater expense. 131. Pile-driving formulae. If R = ihe resistance of a pile,and s the set of the pile during the last blow, w the weight ofthe pile-hammer, and h the fall during the last blow, then we may state the approximate relat
. Railroad construction. Theory and practice . re to test itsbearing-power. In a new country judgment and experience mil be requiredto decide intelligently whether to employ a simple drop-hammermachine, operated by horse-power and easily transported butuneconomical in operation, or a more complicated machineworking cheaply and effectively after being transported atgreater expense. 131. Pile-driving formulae. If R = ihe resistance of a pile,and s the set of the pile during the last blow, w the weight ofthe pile-hammer, and h the fall during the last blow, then we may state the approximate relation that Rd==wh, or R =—. This is the basic principle of all rational formulae, but the maxi-mum weight which a pile will sustain after it has been drivensome time is by no means equal to the resistance of the pileduring the last blow. There are also many other modifyingelements which have been A^ariously allowed for in the manyproposed formula). The formula range from the extreme ofempirical simplicity to very complicated attempts to allow. §131. TRESTLES. 155 properly for all modifying causes. As the simplest rule, speci-fications sometimes require that the piles shall be driven untilthe pile will not sink more than 5 inches under five consecutiveblows of a 2000-lb. hammer falling 25 feet. The ^^Engineering News formula * gives the safe load as r, in which iv = s + 1 weight of hammer, h=iall in feet, s=set of pile in inches underthe last blow. This formula is derived from the above basicformula by calling the safe load ^ of the final resistance, andby adding (arbitrarily) 1 to the final set (s) as a compensationfor the extra resistance caused by the settling of earth aroundthe pile between each blow. This formula is used only forordinary hammer-driving. When the piles are driven by a steam pile-driver the formula becomes safe load = ^—r. «For ^ s + thegunpowder pile-driver, since the explosion of the cartridge drives the pile in with the same force with which it thr
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