FOUNDATIONS AND RETAINING WALLS 199 



recently wood was the only material used for piles, and they were 

 either driven by hand with sledges, or by means of a block, usually 

 of metal, which was raised between two upright guides and allowed 

 to fall on the head of the pile. The latter form of pile driver is still 

 in frequent use for driving wooden piles, and is called the drop- 

 hammer pile driver. 



In 1839 Nasmyth invented the steam pile driver, which consists 

 essentially of a steam cylinder supported vertically above the head 

 of the pile by- two uprights fastened to a cap which rests on the 

 pile. The hammer in this case is a weight attached to the piston 

 rod, and delivers a blow on the head of the pile at each stroke 

 of the piston. The uprights which support the cylinder also serve 

 as guides for the hammer, which varies in weight from 550 Ib. to 

 4800 Ib. This form of pile driver owes its efficiency to the rapidity 

 with which the blows can be given, the number being from sixty 

 to eighty per minute, thus preventing the soil from recovering its 

 equilibrium between strokes, and greatly decreasing its resistance to 

 penetration. 



In modern engineering practice cast-iron and concrete piles are rap- 

 idly coming into use, and as neither of these materials is capable 

 of standing repeated blows, piles of this kind are usually driven by 

 means of an hydraulic jet. The jet is attached to the point of the 

 pile, thus constantly excavating the soil in front of the pile as it 

 descends, and enabling it to sink into place with little or no assist- 

 ance other than its own weight. 



The rational formulas in ordinary use for determining the bearing 

 power of piles are based upon the assumption that the pile is driven 

 1>\ a drop-hammer pile driver, and express its bearing power in terms 

 of the amount of penetration at the last blow. Since the bearing 

 power of a pile is due hi part to the friction of the earth on the sides 

 of the pile, as well as to the resistance of the subsoil to penetration, 

 and also since part of the energy of the hammer is absorbed by the 

 friction of the guides, in compressing the head of the pile, in compress- 

 in L: the hammer, in overcoming the inertia of the pile, etc., a rigorous 

 formula is too complicated to be of much practical value, although 

 there are a number of elaborate discussions of the bearing power of 

 piles which take all of these elements into consideration, notably the 



