Ch. 8] SAFE BEARING CAPACITY 157 



structures have been built. Field tests have been conducted at Possum 

 Kingdom Dam (Niederhoff, 1940) on the Brazos River in Texas, at 

 the location of Watts Bar Dam (Rountree, 1940, pp. 1538-1543) on the 

 Tennessee River, and at the Lake Lynn Dam on the Cheat River in 

 West Virginia, to mention only a few. Tests of bearing capacity, 

 shearing and sliding strengths, and reaction to repeated loading and 

 unloading have been performed. 



Blocks of concrete are poured directly upon a surface of shale pre- 

 pared as if it were to be the foundation for the project structure. Un- 

 der a vertical load supplied as in a common load test on a bearing 

 pile, the block is subjected to horizontal force by jacks. Both vertical 

 and horizontal forces may be increased, or the horizontal force alone 

 may be increased. Suitable instrumentation permits the recording of 

 deformation and the applied forces. The test is continued usually until 

 failure of the foundation is achieved either under continuously in- 

 creasing load or repeated reloadings. After the failure, horizontal 

 loading may be applied again and an indication obtained of the co- 

 efficient of friction. If the failure occurred in the rock, the coefficient 

 will refer to the friction of rock on rock and will be of value in analyz- 

 ing the rock foundation; however, if the failure occurred at the con- 

 tact of the concrete and the rock, the subsequent loading will indicate 

 the frictional value of concrete on rock. The data derived from such 

 field tests are indicative of the conditions prevailing at the location 

 of the test block and may be extrapolated to the remainder of the 

 foundation area in relation to the similarity and homogeneity of the 

 rock. 



A characteristic of shales and other argillaceous sediments that is 

 commonly overlooked in relation to foundations is the tendency to con- 

 solidate under a load less than required to produce failure. In field 

 and laboratory testing of the less elastic shales the rate and amount of 

 consolidation are measured in relation to time and to the increased 

 loading. Because many structures impose varying loads, the rebound 

 or return toward original volume is also determined with the release of 

 loading. Cyclic loading and unloading in the range below failure may 

 be applied until the shale approaches a degree of consolidation where 

 effects of successive cycles are essentially those of preceding cycles. 



In summation, it is a practical impossibility to define the ultimate 

 mass strength of a foundation, although quantitative indications may 

 be obtained of the strength of the rocks by means of field and labora- 

 tory tests (Burwell and Moneymaker, in press) . It is the writer's im- 

 pression that most foundations are considerably stronger en masse than 

 the localized tests indicate. 



