280 TIMBER BRIDGES AND TRESTLES. CHAP. VII. 



dissipated and fail to come up in the immediate vicinity of the pile, the utility of the jet is uncer- 

 tain, except for a part of the penetration. 



(n) A steam or drop hammer should be used in connection with the water jet, and used to 

 test the final rate of penetration. 



(12) The use of the water jet is one of the most effective means of avoiding injury to piles 

 by overdriving. 



(13) There is danger from overdriving when the hammer begins to bounce. Overdriving is 

 also indicated by the bending, kicking or staggering of the pile. 



(14) The brooming of the head of a pile dissipates a part, and in some cases all, of the energy 

 due to the fall of the hammer. 



(15) The weight or the drop of the hammer should be proportioned to the weight of the 

 pile, as well as to the character of the soil to be penetrated. 



(16) The steam hammer is more effective than the drop hammer in securing the penetration 

 of a pile without injury, because of the shorter interval between blows. 



(17) Where shock to surrounding material is apt to prove detrimental to the structure, the 

 steam hammer should always be used instead of the drop hammer. This is especially true in the 

 case of sheet piling which is intended to prevent the passage of water. In some cases also the 

 jet should not be used. 



(18) In general, the resistance of piles, penetrating soft material, which depend solely upon 

 skin friction, is materially increased after a period of rest. This period may be as short as fifteen 

 minutes, and rarely exceeds twelve hours. 



(19) In tidal waters the resistance of a pile driven at low tide is increased at high tide on 

 account of the extra compression of the soil. 



(20) Where a pile penetrates muck or a soft yielding material and bears upon a hard stratum 

 at its foot, its strength should be determined as a column or beam; omitting the resistance, if any, 

 due to skin friction. 



(21) Unless the record of previous experience at the same site is available, the approximate 

 bearing power may be obtained by loading test piles. The results of loading test piles should 

 be used with caution, unless their condition is fairly comparable with that of the piles in the 

 proposed foundation. 



(22) In case the piles in a foundation are expected to act as columns the results of loading 

 test piles should not be depended upon unless they are sufficient in number to insure their action 

 in a similar manner, and they are stayed against lateral motion. 



(23) Before testing the penetration of a pile in soft material where its bearing power depends 

 principally, or wholly, upon skin friction, the pile should be allowed to rest for 24 hours after 

 driving. 



(24) Where the resistance of piles depends mainly upon skin friction it is possible to diminish 

 the combined strength, or bearing capacity, of a group of piles by driving additional piles within 

 the same area. 



(25) Where there is a hard stratum overlying softer material through which the piles are to 

 pass to a firm bearing below, the upper stratum should be removed by dredging or otherwise, 

 provided it would injure the piles to drive through the stratum. The material removed may be 

 replaced if it is needed to provide lateral resistance. 



(26) Timber piles may be advantageously pointed, in some cases, to a 4-in. or 6-in. square 

 at the end. 



(27) Piles should not be pointed when driven into soft material. 



(28) Shoes should be pfovided for piles when the driving is very hard, especially in riprap or 

 shale, and should be so constructed as to form an integral part of the pile. 



(29) The use of a cap is advantageous in distributing the impact of the hammer more uni- 

 formly over the head of the pile, as well as to hold it in position during driving. 



(30) The specification relating to the penetration of a pile should be adapted to the soil which 

 the pile is to penetrate. 



(31) It is far more important that a proper length of pile should be put in place without 

 injury than that its penetration should be a specified distance under a given blow, or series of 

 blows. 



