Drilled Shaft Axial Capacity Calculator
Deep Foundation Engineering: Estimate ultimate and allowable axial compressive load capacity ($R_{ult}$, $R_{all}$) for drilled shafts / bored piles per FHWA-NHI-10-016.
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Axial Compressive Load Capacity
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Drilled Shaft Axial Resistance Formulations (FHWA-NHI-10-016)
Drilled shafts (also called bored piles or caissons) transfer heavy column loads to deep bearing strata through a combination of shaft circumferential skin resistance ($R_s$) and base end bearing ($R_p$).
1. Total Ultimate Axial Capacity
The total compressive resistance is the sum of side and base components:
R_ult = R_s + R_p = ∑ (π · B · Δz · f_s) + A_b · q_p R_all = R_ult / FS
2. Cohesive Soils (O'Neill & Reese Method)
Unit side resistance and ultimate base resistance in clay:
f_s = α · S_u (α = 0.55 for S_u/p_a ≤ 1.5) q_p = N_c · S_u,base (N_c = 9.0 for deep circular base)
Frequently Asked Questions
What is the FHWA alpha (α) method for drilled shafts in clay?
The FHWA $\alpha$-method (O'Neill and Reese 1999) calculates unit side friction $f_s = \alpha \cdot S_u$, where $S_u$ is undrained shear strength. The adhesion factor $\alpha = 0.55$ for $S_u / p_a \le 1.5$ and decreases linearly for stiffer overconsolidated clays. Top and bottom casing zones (top 1.5 m and bottom 1 diameter) are excluded from side resistance due to soil drying and stress relief during drilling.
Why is settlement mobilization different for side resistance vs end bearing?
Drilled shaft side shear resistance mobilizes rapidly at small axial displacements (typically 5 to 10 mm, or 0.5% to 1% of shaft diameter). In contrast, base end bearing requires significant settlement (typically 4% to 10% of base diameter) to reach ultimate mobilization. Consequently, allowable capacity often depends on serviceability settlement limits.
How does the beta (β) method evaluate drilled shafts in sand?
In cohesionless soils, unit side friction is proportional to vertical effective stress: $f_s = \beta \cdot \sigma'_v$. The coefficient $\beta = (1 - \sin\phi') \cdot (\sigma'_v / p_a)^{\dots}$ or empirically $\beta = 1.5 - 0.245 \sqrt{z}$ (bounded between 0.25 and 1.2), reflecting stress relaxation along the shaft wall during open excavation.