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

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Bengt H. Fellenius revisits Aleksandar Vesic’s influential 1970 tests on a strain-gauged, closed-toe pipe pile driven into sand at the Ogeechee River site. Vesic interpreted the measured shaft and toe resistances as increasing with depth over roughly the upper 10 pile diameters, then becoming nearly constant below about 20 diameters. This observation became the widely applied “critical depth” concept.<br /><br />Fellenius argues that the apparent pattern resulted from interpreting the measurements without accounting for residual force—force locked into the pile and surrounding soil after driving. Residual force alters the force distribution recorded by strain gauges; recognizing it would remove the apparent critical-depth effect in these test results. The paper illustrates how neglecting it can make lower pile sections seem to contribute little shaft resistance.<br /><br />The review describes the site’s sand profile, pile and gauge locations, and five static load tests conducted at successive embedment depths. It also notes limitations in the original records, including uncertain strain-gauge accuracy and interpolated missing readings. The tests used a manually controlled constant-rate-of-penetration procedure, which Fellenius considers poorly suited to instrumented full-scale tests because maintaining a truly constant rate is difficult and force readings during continuous penetration are less reliable as static references.<br /><br />Fellenius traces earlier discussions of residual force and explains that different studies used different definitions of pile capacity. He concludes that the critical-depth concept arose from a flawed interpretation, not a general rule for pile resistance, while emphasizing that this correction does not diminish Vesic’s broader contributions to foundation engineering.

Keywords

Bengt H. Fellenius

Aleksandar Vesic

critical depth

residual force

strain-gauged pile

pipe pile

shaft resistance

toe resistance

sand profile

pile load tests