274 FOUNDATIONS 



formulas for the bearing power of piles determine the safe bearing 

 power from the weight of the hammer, the length of free fall of the 

 hammer, and the penetration of the pile. The penetration of the pile 

 for any blow of the hammer depends on the condition of the head of 

 the pile, upon whether the pile is driving straight, and upon the 

 rigidity of the pile. The penetration of a slim, limber pile with a 

 broomed head is very misleading, and any formula will give values too 

 large. 



The Engineering News formula for the safe bearing power of 

 piles is most used and is certainly the most reliable. It is 



P 2 Wh 



'T+T 



where P = safe load on pile in tons; 



W = weight of hammer in tons ; 



h = distance of free fall of the hammer in feet ; 



j = penetration of the pile for the last blow in inches. 



If the pile is driven with a steam hammer the factor unity in the 

 denominator is changed to one-tenth. This formula is supposed to give 

 a factor of safety of about 6, and has been shown by actual use to give 

 values that are safe. 



Where piles are to be driven through gravel or very hard ground 

 the lower ends are often protected with cast iron or steel points. The 

 value of these points is questionable and most engineers now prefer to 

 drive piles without their use, simply making a very blunt point on the 

 pile. In driving piles, care must be used where small penetrations are 

 obtained not to smash or shiver the pile. Piles driven to a good refusal 

 with a penetration of, say, I inch for the last blow, with a fall of 20 ft. 

 and a 2OOO-lb. hammer will safely support almost any load that can be 

 put on them. 



Piles are usually driven at about 3-ft. centers over the bottom of 

 the foundation. After the piles are driven they are sawed off below the 

 water level and ( i ) concrete is deposited around the heads of the piles, or 

 (2) a grillage or platform is built on top of the piles to support the walls 



