Evaluation of tip capacity analysis model for drilled shafts in gravelly soils

This paper examines an analysis model for predicting the tip capacity of drilled shaft foundations under gravelly soils. Forty one static compression load test data are utilized for this purpose. Comparison of predicted and measured results demonstrates that the prediction model greatly overestimates the tip capacity of drilled shafts. Further assessment on the model reveals a greater variation in three coefficients, including the effective overburden pressure ( q ), the overburden bearing capacity factor ( q N ), and the bearing capacity modifier for soil rigidity ( qr ζ ). These factors are modified from the back-analysis of the drilled shaft load test results. Varying effective shaft depths are considered for the back-calculation to evaluate their effects on capacity behavior. Based on the analyses, the recommended effective shaft depth for the evaluation of effective overburden pressure is limited to 15B (B = shaft diameter). The q N and qr ζ are enhanced while maintaining their basic relationship with the soil effective friction angle ( ), φ in which the q N increases and qr ζ decreases as φ increases. Specific design recommendations for the tip bearing capacity analysis of drilled shafts in gravelly soils are given for engineering practice.