Revisiting Vesic (1970): Tests on Instrumented Piles, Ogeechee River Site

In a paper in the ASCE Journal, Vesic (1970), presented a case history on static loading tests on strain-gage instrumented pipe-piles that gained a wide national and international attention. The paper evaluated the case records and derived a concept later called “critical depth” that became generally accepted as basis for the design of piled foundations. That concept stated that, down to the “critical depth”, starting at a depth of about to 10 pile diameters, the ultimate unit shaft and toe resistance of a pile would develop in conformity with the effective overburden stress. Below this depth, the unit resistances would cease to correlate directly to the effective stress and become constant with increasing depth below 20 pile diameters. The concept was oblivious to the presence of residual force, which, had it been considered, would have removed the appearance of a critical depth.