250 BRIDGE ABUTMENTS AND PIERS. CHAP. VI. 



Mr. E. L. Corthell gives the summary of the pressures on deep foundations in Table III. 



TABLE III. 

 ACTUAL PRESSURES ON DEEP FOUNDATIONS.* 



The data in Table III shows that great care must be used in determining on the allowable 

 pressure for any particular foundation, and that safe values for the bearing power of soils should 

 only be used as an aid to the judgment of the engineer. 



WATERWAY FOR BRIDGES. The clear waterway for bridges should be ample; great 

 care should be used to prevent floating logs and debris from clogging up the opening. The neces- 

 sary waterway depends upon the character and sizeof the runoff area, the slope and size of the stream 

 and upon other local conditions. The "Dun Drainage Table," Table IV, will be of assistance in 

 assisting the judgment of the engineer in determining on the proper waterway for any bridge. 



Many formulas have been proposed for determining the waterway of culverts and bridges. 

 The formula best known to the author is that proposed by Professor A. N. Talbot. It is 



A = cVjtfi 



where A = area of the required opening in sq. ft. ; 

 M = area of drainage basin in acres; 



c = a coefficient varying with the slope of the ground, slope of the drainage area, character 

 of the soil and character of vegetation. 



Professor Talbot gives the following values of c : c = f to i for steep and rocky ground; 

 c = \ for rolling agricultural country, subject to floods at times of melting snow, and with the 

 length of valley 3 to 4 times its width; c = f to for districts not affected by accumulated snow 

 and where the length of the valley is several times its width. 



PREPARING THE FOUNDATIONS. The preparation of the site of the abutment or 

 pier will depend upon the conditions and character of the material. 



Rock. Where the water can be excluded, the rock should be cleared of all overlying material 

 and disintegrated rock. The surface is then leveled up either by cutting off the projections or 

 by depositing a layer of concrete. 



Hard Ground. The material should be excavated well below the frost and scour line. Where 

 the foundations cannot be carried low enough to prevent undermining, piles should be driven at 

 about 25 to 3 ft. centers over the foundation. 



* " Allowable Pressures on Deep Foundations " by E. L. Corthell, John Wiley & Sons. 



