Bearing capacity from field load tests

6 BEARING CAPACITY FROM FIELD LOAD TESTS 

Obviously the most reliable method of obtaining the ultimate bearing capacity at a site is to perform a load test. This would directly give the bearing capacity if the load test is on a full size footing however, this is not usually done since an enormous load would have to be applied. Another factor is that the bearing capacity obtained is for that size and if there is more than one size then additional tests would be required. For the test just described, the cost could be very high. 

The usual practice is to load test small plates of diameters from 20 to 30 inches or square of side 12x12 and perhaps 24 x 24 inches. These sizes are usually too small to extrapolate to fiill size footings which may be 1.5 to 5 meters square. Some of the factors causing the extrapolation to be questionable are  

Extrapolating load test results to full size footings is not standard. It is common for clay soils to note that BNy term is zero so that one might say that quu is independent of footing size giving 

In cohesionless soils, all three terms of the bearing capacity equation applies and noting the Ny term includes the footing width, one might say 

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