Ch. 9] FOOTING FOUNDATIONS 175 



and dead loads; and allowable settlement. This is, however, an im- 

 portant phase of the economic study of the substructure and one that 

 requires careful consideration. 



Every foundation should be designed for a factor of safety com- 

 parable to that used in the design of superstructures. This factor 

 should be applied against the possibility of the foundation failing as a 

 result of applied loads exceeding the shear strength of the supporting 

 sediment, sliding upon the contact between the footing block and the 

 supporting earth material, sliding upon bedding planes of the sediment, 

 or excessive settlement due to compressive materials. This factor of 

 safety should not be less than 2%, and it rarely needs to exceed 3. It 

 is not uncommon in present practice for factors of substructure safety 

 to be as high as 20. Such a design policy rarely results in a founda- 

 tion failure in the normal sense, but it does cost untold thousands and 

 thereby represents economic waste. 



The total settlement of the structure should not exceed that which 

 may result in differential settlements in amounts capable of either 

 damaging the structure or developing a poor riding deck. Differential 

 settlement between adjacent supports will usually not exceed 50 per- 

 cent of the total settlement unless the loads are vastly different or the 

 foundations are of different types; for instance, adjacent bridge piers 

 supported by pile and footing foundations respectively. In such cases 

 the differential settlement may approach 100 percent of the total settle- 

 ment. 



Precise settlement computations are rarely justifiable for highway 

 structures except when firm support cannot be reached by piles, col- 

 umns, or caissons because of great depth, or when soft compressible 

 sediment occurs beneath the footing at a depth sufficiently shallow to 

 permit consolidation under the applied load (Terzaghi and Peck, 1948, 

 pp. 413-456; Plummer and Dore, 1940, p. 192). 



Considerable money could be saved by a more extensive use of the 

 flexible-hinge design for structures where serious differential settle- 

 ments are anticipated. Hinge design, together with facilities for 

 jacking, would, for example, make it possible on many occasions to 

 use floating abutments on fills through which piles are commonly 

 driven. 



Load tests on bearing plates, when accompanied by data that iden- 

 tify the character of the sediment within the significant depth, fur- 

 nish valuable information regarding the load-carrying ability of the 

 supporting material. Such tests, although generally considered ex- 

 pensive, can, with properly designed equipment, be conducted on a 

 routine basis at low cost. Load tests at the sites of pier locations for 



