Drilled shafts
FHWA-IF-99-025 capacity for bored piles: α / β / rock-socket side resistance, base bearing, and belled-base uplift.
A drilled shaft — also called a bored pile or caisson — carries axial load through side resistance along its shaft and end bearing at its base. PileCalc sizes that capacity with the FHWA method of O'Neill & Reese (FHWA-IF-99-025, 1999), layer by layer down the profile, and reports both downward and uplift capacity. This page explains the method and every input on the drilled-shaft tool.
How drilled shafts work
Total downward capacity is the sum of two parts: the friction mobilized along the shaft and the bearing developed under the base.
The crucial difference from a driven pile is construction. A driven pile displaces and densifies the surrounding soil; a drilled shaft is excavated, so it does not densify the ground — if anything, drilling relaxes it. That is why the FHWA correlations for bored shafts are distinct from the driven-pile formulas, and why a drilled shaft of the same size often shows lower unit side and base resistance than its driven counterpart. Compare the two side by side against driven-pile axial capacity.
Each layer contributes side resistance according to its material — an α-method in clay, the FHWA β-method in sand, and a socket correlation in rock — and the layer at the base sets the end bearing. The sections below give the formula PileCalc uses for each.
Shaft inputs
The shaft is modeled as a uniform cylinder, optionally widened to a bell at the base. You provide its geometry and the unit weight of its concrete.
The embedded length of the drilled shaft, from the ground surface to the base.
Why it matters. Length sets the side surface area — more length means more side resistance — and how deep the base reaches into a competent bearing stratum. Typical shafts run on the order of 10–30 m. Default 15 m.
The diameter of the drilled shaft.
Why it matters. Diameter sets both the side surface area (∝ B) and the base bearing area (∝ B²), so base capacity grows faster with diameter than side does. Common shafts are 0.6–2 m. Default 0.9 m.
The diameter of the under-reamed (belled) base. Set it equal to the shaft for a straight shaft.
Why it matters. A bell enlarges the bearing area and, in clay, mobilizes extra uplift breakout resistance (see Uplift & belled bases). Default 1.8 m — a true bell on a 0.9 m shaft.
The unit weight of the shaft concrete.
Why it matters. The self-weight of the shaft resists uplift, so it adds directly into the tension capacity. Reinforced concrete is about 24 kN/m³ (the default).
Soil types & resistance methods
Each layer is assigned one of three behavioural classes. The class selects which FHWA resistance method PileCalc applies — the parameter you enter for the layer changes to match.
| Soil type | Side resistance | End bearing | Key parameter |
|---|---|---|---|
| Cohesive (clay) | α-method, fs = α·Su | qp = Nc*·Su, Nc* ≈ 9 | Su |
| Cohesionless (sand) | β-method, fs = β·σ′v | SPT-N correlation | N |
| Rock | socket, fs ≈ 0.65·√(qu·Pa) | bearing on intact rock | qu |
Clay (α-method). Unit side resistance is a fraction of the undrained strength, fs = α·Su, where the adhesion factor α comes from the FHWA chart and is about 0.55 for typical shafts. The method excludes thin zones near the top and base of the shaft from side friction, where ground movement and base interaction make the contact unreliable. End bearing is qp = Nc*·Su with a bearing factor Nc* ≈ 9.
Sand (β-method). Unit side resistance is proportional to the effective vertical stress, fs = β·σ′v, with β from the FHWA depth correlation. End bearing comes from an SPT-N correlation, so the blow count drives both the side and the base in sand.
Rock socket. Unit side resistance in a rock socket scales with the square root of the rock strength, normalized by atmospheric pressure Pa:
Soil parameters
Every layer carries a top and bottom depth — the layers must tile the profile without gaps, and the deepest layer should reach the shaft base — plus a unit weight and the one strength parameter for its type.
The total unit weight of the layer.
Why it matters. It builds the effective vertical-stress profile σ′v down the shaft, which drives the sand β-method side resistance and the base stress. Typical soils are ~18–20 kN/m³. Default 19 kN/m³.
The undrained shear strength of the clay.
Why it matters. In cohesive layers it sets both the side resistance fs = α·Su and the end bearing qp = Nc*·Su. Guide values — soft 12–25, medium 25–50, stiff 50–100, hard 100–200 kPa.
Source: O'Neill & Reese (1999)
The standard penetration test blow count for the sand layer.
Why it matters. The FHWA β-method for side resistance and the end-bearing correlation in sand both key off N. Loose ~4–10, medium ~10–30, dense ~30–50.
The unconfined compressive strength of the rock.
Why it matters. It sets the rock-socket side resistance fs ≈ 0.65·√(qu·Pa) and the base bearing on intact rock. Weak rock is roughly 0.5–5 MPa. Default 5000 kPa.
Source: FHWA rock socket
Groundwater & factors
The water table sets the buoyancy in the effective-stress profile, and a pair of factors of safety turn ultimate capacity into allowable load.
The depth to the groundwater table.
Why it matters. Below the table the soil is buoyant, lowering effective stress σ′v — and with it the sand β-method side resistance and the base stress. Getting the datum right is easy to slip on; see the water-table note. Default 3 m.
The factor of safety applied to the base (end-bearing) resistance.
Why it matters. Drilled-shaft end bearing needs large base movement to mobilize fully, so it is factored down hard. A healthy ≈ 2.5 is typical (the default).
The factor of safety applied to the side (friction) resistance.
Why it matters. Side friction mobilizes at small movement, so it is more reliable than base bearing — but PileCalc keeps a separate factor so you can weight the two parts independently. Default 2.5.
Side and base are factored separately: allowable downward capacity is Q_side / FS_side + Q_base / FS_tip, not the total divided by a single number.
Uplift & belled bases
A drilled shaft resists tension (uplift) through three contributions: the side friction along the shaft, the breakout resistance of the bell, and the dead weight of the shaft itself.
The bell only helps in clay
The side term in uplift is the same friction as in compression. The weight term is just the shaft volume times its unit weight — which is why a heavier shaft and a deeper embedment both help in tension. Uplift is factored by FS uplift (≈ 2.5) to give the allowable tension load.
Reading the results
The tool reports four capacity figures, a side-vs-base split, and the uplift breakdown.
Capacity figures
- Ultimate down — the total plunging capacity, base + side.
- Allowable down — the same, with side and tip each divided by their factor of safety. This is the compression load you can apply.
- Ultimate uplift — the tension capacity, side + bell + shaft weight. Governs for tie-downs and overturning.
- Allowable uplift — uplift divided by FS. The tension load you can apply.
Side vs. base split
A bar shows how the downward capacity divides between side friction and base bearing. A side-dominated shaft is a friction pile; a base-dominated one is an end-bearing shaft, more sensitive to the quality of the bearing stratum and slower to mobilize.
Uplift breakdown
A second set of bars breaks the uplift into its three parts — side, bell, and weight — so you can see what is carrying the tension. If the bell bar is empty, the shaft is bearing in sand and the under-ream is doing nothing for uplift.