Evaluation of Drained Axial Capacity for Drilled Shafts

Abstract

The drained side and tip resistances of drilled shafts in cohesionless soils were evaluated from field load test case histories with axial uplift and compression loading. The side resistance was evaluated using the effective stress or beta (β) method, and the tip resistance in compression was evaluated using bearing capacity theory. For the side resistance, the results show that β values can reach 6.5 at shallow depth, but they decrease with depth. The β values in uplift and compression are essentially the same and generally vary by less than 4%. Comparing the measured and predicted (traditional) values of β, the predicted β values are low and overly conservative. For the tip resistance, the average ratio of the measured to predicted values is about 0.3 at a displacement/diameter of 4%. This ratio shows some scatter at shallow depths, but the scatter decreases with increasing depth. To mobilize the tip resistance fully, a displacement/diameter of about 10% is required. Based on these data and evaluations, design recommendations are proposed.