174 harned. HIGHWAY BRIDGE FOUNDATIONS [Ch. 9 



EXPLORATION PROCEDURE 



The first step in the conduct of a foundation study for a highway 

 structure should consist of a complete determination and understand- 

 ing of the general geology of the area. The foundation condition at 

 each structure site, whether complex or simple, is the direct result of 

 the geologic processes, environment, and history of the area. If an 

 understanding of these factors is not acquired first, the study becomes 

 a routine matter of boring holes in the ground and conducting monoto- 

 nous tests rather than an interesting interpretation of the complexities 

 of nature. The second step consists in ascertaining the physical char- 

 acter of the sediment for the purpose of determining specific founda- 

 tion requirements at the sites of structure support. 



The depth to which borings or soundings should extend, and the 

 spacing between borings, should be dependent entirely upon the com- 

 plexity of the site, the size of the project, and the depth to compres- 

 sible sediment occurring within the limit of surface-load influence. 

 Any attempt to standardize boring depths or distances between borings 

 is obviously dangerous and to be avoided. Each structure site re- 

 quires its own individual explorational treatment, and any attempt to 

 predetermine the study requirements or procedures usually leads to in- 

 efficient operations and faulty results. There is no substitute for ade- 

 quate training and experience when the proper study procedure for a 

 foundation problem, or the accurate determination of the time when 

 further study will cost more than it is worth, is being selected. 



FOOTING FOUNDATIONS 



In those cases in which scour or lateral erosion is not indicated and 

 in which the boring logs and sounding patterns or both do not reveal 

 the existence of soft compressible sediment within the significant depth 

 of surface-load influence, the structure may be supported by footing 

 foundations resting upon near-surface sediment (Taylor, 1948, p. 560 ; 

 Terzaghi and Peck, 1948, p. 56). All footings must be located below 

 the influence of frost and seasonal moisture. 



No precise statement can be made with regard to the depth to which 

 footing foundations may extend and render structural support in 

 economic preference to pile foundations, because this determination 

 must be made for each substructure in the light of allowable bearing 

 value of the sediment, depth of seasonal volume change due to moisture, 

 frost, plants, etc., depth of scour, span lengths; live, wind, earthquake, 



