Ch. 9] PILE FOUNDATIONS 177 



PILE FOUNDATIONS 



Pile support for highway structures should be used only in those 

 cases in which footing foundations cost more or are unsafe because 

 of the possibility of erosion, or where settlements are apt to be ex- 

 cessive. 



The engineer has, through experience, acquired a deep respect for 

 the complexity and variable nature of foundation conditions. As a 

 result, pile foundations occupy a hallowed niche in the engineering 

 profession and are apt to be used, not necessarily when needed, but 

 whenever possible. The limits of possibility, when confronted with 

 engineering ingenuity, have, needless to say, been pushed back quite a 

 way. 



Piles may be classified on the basis of load-transfer method into two 

 general types: 



(1) Friction piles. Those which transfer the imposed structure load 

 to the surrounding sediment largely through skin friction. 



(2) Bearing piles. Those which transfer loads to a firm or hard 

 material at depth largely through the pile point contact. 



It should be noted that friction piles support a portion of the load 

 in the manner of bearing piles and that bearing piles transfer a por- 

 tion of their load to the sediment through skin friction. Therefore, in 

 order to estimate the load-carrying capacity of a single pile, both fric- 

 tion and point-bearing data must be obtained in most cases. 



Carefully conducted wash borings, together with a few spot samples, 

 usually suffice for sediment classification purposes. Drive or push 

 soundings when accompanied by penetration tests will yield valuable 

 data regarding pile length requirements and point bearing values. Pull 

 tests spaced at proper intervals of time and conducted at significant 

 depths, and locations throughout the structure site as previously deter- 

 mined by borings, will permit reasonably accurate estimates of skin- 

 friction values. Friction values determined by the pull-test method 

 in plastic sediment may be checked by means of direct shear or un- 

 confined compression tests, and those in granular sediment by the 

 standard penetration or triaxial shear tests or both. A few test piles 

 and full-scale load tests should be insisted upon in all cases in which 

 structural support is to be derived from friction piles. 



The use of pile-driving formulas for measuring the ultimate bearing 

 capacity for individual piles should be avoided in all cases in which 

 resistance to pile penetration is due to friction between the pile and 



