176 haened. HIGHWAY BRIDGE FOUNDATIONS [Ch. 9 



the Plaza Garage Overhead structure on U. S. 99 north of Visalia, 

 California, proved conclusively that footing foundations will safely 

 support loads of 3 tons per square foot at a depth of 7 feet. The cost 

 of the tests was only a small fraction of the saving realized as a re- 

 sult of pile elimination. The load tests in this case were conducted in 

 large-diameter holes drilled with a power-driven bucket auger on plates 

 having a surface area of 2 square feet. The load was applied by means 

 of a power-operated hydraulic pressure cell. Constant pressure was 

 maintained by use of a nitrogen-loaded gas accumulator and a mer- 

 cury pressure switch. Twelve-inch expanding anchors fixed in drill 

 holes furnished adequate load reaction. 



Load tests on bearing plates must be used with a full appreciation 

 of the limitations of the method. It should be kept in mind that the 

 test reflects only the character of the sediment to a depth of about 

 twice the diameter of the bearing plate, whereas the footing foundation 

 will exert an influence on the underlying sediment to a depth at least 

 as great as the footing width for ideal conditions, and to a depth 

 several times the footing width if the sediment becomes softer and 

 weaker at depth. 



The depth below the footing elevation to the water table is of con- 

 siderable importance, particularly in the case of footing foundations 

 supported upon cohesionless sediment. If the water table rises within 

 the significant depth, the factor of safety should be increased from 

 2.5 to 4. 



Large footings settle more than small ones if unit loads are the 

 same. For example, allowable loads should be reduced from 6 tons 

 to approximately 5 tons per square foot on dense sand where the 

 footing width is increased from 5 to 10 feet. It is not advisable to 

 attempt to support footing foundations for highway structures on soft 

 clay or plastic silt except as a last resort. Differential settlements up 

 to 6 inches are commonly unavoidable in such cases, and total settle- 

 ments amounting to nearly a foot have been measured. 



It is not advisable to apply loads to footing foundations supported 

 by plastic sediment in amounts that exceed the unconfined compressive 

 strength of the weakest material encountered. If weak plastic sedi- 

 ment occurs at some depth below a firm stratum upon which footings 

 are to rest, a calculation is required for the purpose of determining 

 the load distribution at depth and, thereby, the amount of load that 

 will be applied to the weak sediment. A safety factor of 3 should then 

 be used against overstressing the weak stratum. 



