Drilled Shaft Deep Foundation Design Calculator
Estimate axial geotechnical capacity of a drilled shaft using classical soil and rock-socket methods. Supports undrained clay, effective-stress clay, long-term drained clay, sand, shale, weak rock, limestone, sandstone, groundwater effects, end bearing, skin friction, moment screening, and advanced visualization.
Foundation Geometry and Loads
Resistance Caps and Method Parameters
For undrained clay, water table may not change capacity because the method depends on Su. Effective-stress clay, long-term drained clay, sand, and tip bearing methods are groundwater-sensitive.
Soil / Rock Layer Profile
Enter layers from ground surface downward. For clay, choose the calculation method. For shale and rock, enter UCS, RQD, weathering factor, rock adhesion factor, and rock bearing factor as applicable.
| Layer Name | Material Type | Clay Method | Thickness (ft) | γ (pcf) | Su (psf) | c′ (psf) | φ′ / φ (deg) | α | β | K | δ (deg) | ca (psf) | qu Rock (psi) | αr | Cb | RQD (%) | Weathering Factor | Remove |
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Formulas and References
1. Total Ultimate Axial Capacity
\[ Q_{ult} = Q_s + Q_b \] \[ Q_s = \sum f_{s,i} \pi D \Delta z_i \] \[ Q_b = q_b A_b \] \[ A_b = \frac{\pi D^2}{4} \] \[ Q_{allow} = \frac{Q_{ult}}{FS} \]2. Effective Stress and Water Table
\[ \sigma’_v = \sigma_v – u \] \[ u = \gamma_w(z-z_w) \quad \text{for } z > z_w \] \[ u = 0 \quad \text{for } z \leq z_w \]Water table directly affects effective-stress clay, drained clay, sand, and effective-stress end bearing calculations.
3. Clay Undrained α-Method
\[ f_s = \alpha S_u \] \[ q_b = N_c S_u \]This is a short-term total-stress method. Capacity is primarily controlled by undrained shear strength \(S_u\).
4. Clay Effective-Stress β-Method
\[ f_s = \beta \sigma’_v \] \[ \beta = K \tan \delta \] \[ q_b = N_q \sigma’_v \]This method is sensitive to groundwater because \(\sigma’_v\) decreases below the water table.
5. Clay Long-Term Drained \(c’-\phi’\) Method
\[ f_s = c_a + K \sigma’_v \tan \delta \] \[ q_b = c’N_c + \sigma’_v N_q + 0.5\gamma’DN_\gamma \]This option is intended for long-term drained behavior where effective shear strength parameters are appropriate.
6. Sand β-Method and End Bearing
\[ f_s = \beta \sigma’_v \] \[ q_b = \sigma’_vN_q + 0.5\gamma’DN_\gamma \] \[ N_q = e^{\pi \tan \phi} \tan^2\left(45^\circ + \frac{\phi}{2}\right) \] \[ N_c = \frac{N_q – 1}{\tan \phi} \] \[ N_\gamma \approx 2(N_q+1)\tan\phi \]7. Rock / Shale Socket Side Resistance
\[ f_{s,rock} = \alpha_r q_u F_r \] \[ F_r = F_{RQD}F_{weathering} \] \[ Q_{s,rock} = f_{s,rock}\pi D L_{socket} \]\(q_u\) is intact rock UCS, \(\alpha_r\) is the rock adhesion factor, and \(F_r\) is a rock mass reduction factor.
8. Rock / Shale End Bearing
\[ q_{b,rock} = C_b q_u F_r \] \[ Q_{b,rock} = q_{b,rock} A_b \]Rock end bearing should be capped and reviewed carefully for discontinuities, slaking, soft seams, and construction disturbance.
9. Moment Eccentricity Screening
\[ e = \frac{M}{P} \] \[ \frac{e}{D} \leq \frac{1}{6} \] \[ q_{max,min} = \frac{P}{A} \pm \frac{M}{S} \] \[ S = \frac{\pi D^3}{32} \]This is only a screening check. Final lateral and moment design should use a lateral drilled-shaft method such as p-y analysis.
References
- FHWA drilled shaft design and construction guidance.
- O’Neill and Reese drilled shaft design methods.
- AASHTO LRFD Bridge Design Specifications.
- Das, Principles of Foundation Engineering.
- Coduto, Foundation Design: Principles and Practices.
- Tomlinson and Woodward, Pile Design and Construction Practice.