Compression Top Load Reaching Shaft Bottom—Theory Vs. Tests

This paper investigates the percentage of top load P, on a top-loaded compression drilled shaft (or bored pile), that reaches its tip in end bearing, B, with top compression movements mostly in the typical working range of 3–15 mm, and at "effective" length/diameter (L/D) ratios of less than 25. The authors begin with a review of elastic theory results for predicting B/P. They then show that the results of B/P predictions from finite element studies using simulated sand gave much higher B/P ratios. Finally, they introduce comparative B/P ratio data as obtained from their review of a sample of 50 full scale, randomly selected, Osterberg load tests performed on a great variety of shaft sizes, loads and geological conditions. They included many rock sockets and 15 shafts where the. test showed disturbed bottom conditions. All this produced much scatter in B/P. A literature check of 21 instrumented, conventional top-load tests gave similar B/P results, but even more scattered. All 71 Osterberg and conventional tests gave an "upper bound" of B/P = 0.56 at an average L/D = 7. Our cited elastic solutions gave B/P = 0.03 to 0.07 at L/D = 7, which indicates that they can seriously underpredict the availability and potential importance of end bearing of real shafts in real geotechnical materials. Finite element studies suggest nonlinearity, effective stress, slip, dilatancy effects and layering as reasons for the underprediction.