Some Observations on Undrained Side Resistance of Drilled Shafts

A data base of 106 field load tests (65 in uplift and 41 in compression) is used to evaluate the undrained side resistance of drilled shaft foundations. The total stress or alpha (α) method is examined first, and a new α correlation is presented specifically for drilled shafts. The data show no significant difference in α for either uplift or compression loading. On the average, this new correlation should result in a slightly conservative prediction. Secondly, the effective stress or beta (β) method is examined for undrained loading. Back-calculated β values very dramatically with depth and strongly suggest the importance of evaluating stress history carefully and thoroughly, especially for shorter drilled shafts.