178 harned. HIGHWAY BRIDGE FOUNDATIONS [Ch. 9 



cohesive silts or argillaceous sediment. Allowable loads in such cases 

 should be determined by use of a safety factor of not less than 2 when 

 used in conjunction with load tests approved by the American As- 

 sociation of State Highway Officials, Standard Specifications for 

 Bridges (1949). 



Settlement determinations must be made in all cases in which soft 

 plastic sediment occurs within the significant depth below the pile 

 tips, or in which the load-carrying capacity of the pile group or cluster 

 is less than the bearing value of the cumulative capacities of the piles 

 within a cluster (Seiler and Keeney, 1944; Moore, 1947, pp. 1341- 

 1358). 



CONCLUSIONS 



There is a growing insistence among bridge engineers that precise 

 foundation data are a prerequisite to efficient substructure design. 

 This insistence is commendable and completely justifiable within the 

 limits of our present understanding of nature, and it is plainly up to 

 us as engineers and earth scientists to accept the responsibility for 

 our present state of knowledge, foster the extension of our knowledge 

 and understanding at every opportunity, and encourage the prompt ap- 

 plication of practical truths to the design and construction of current 

 projects. 



The foundation specialist must have a keen sense of the economics 

 involved in his work and must not become so carried away with his 

 study program that he loses sight of his justification in a conscientious 

 attempt to know all the answers prior to construction. A foundation 

 study is not over when the borings are completed and the testing done ; 

 it should be continued through observation and study during the 

 period of construction and ultimate use. It is only by so doing that 

 we learn of our mistakes and are able to accumulate a stockpile of 

 corrected data for our future use or the use of those who follow. 



REFERENCES 



Allin, R. V. (1935). The resistance of piles to penetration: Spon, London, 130 

 pages. 



(1941). Pile driving formulas: Proc. Amer. Soc. Civ. Engrs., pp. 853-866, 



May 1941. 



(1946). Pile foundations and pile structures: Amer. Soc. Civ. Engrs. 



Manual of Engineering Practice, No. 27. 

 American Association of State Highway Officials (1949). Standard specifications 



for bridges. 

 Anderson, Paul (1948). Substructure analysis and design: Irwin-Farnham Pub. 



Co., Chicago, 305 pages. 



