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.
Released applicability
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. Omit it for a straight shaft; a supplied bell must be larger than the shaft and no more than three shaft diameters.
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.
A belled request also requires the vertical bell height. When cohesive soil is present at the base, FHWA-IF-99-025 Eq. 11.16 excludes the bell height plus one base (bell) diameter from compression side resistance; omitting that geometry would overstate the active socket.
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* = 6.5 → 9 | Su |
| Cohesionless (sand) | β-method, fs = β·σ′v | SPT-N correlation | N |
| Rock | socket, fs = αE·0.65·√(Pa·min(qu, f′c)) | 2.5·qu when socketed ≥ 1.5 base diameters in massive rock, else qu | qu |
Clay (α-method). Unit side resistance is a fraction of the undrained strength, fs = α·Su. The FHWA adhesion factor is α = 0.55 for Su/Pa ≤ 1.5, decreasing linearly to 0.45 at Su/Pa = 2.5. That is the released upper limit: stronger material is rejected rather than silently treated as clay. 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 Nc* interpolated from 6.5 at Su = 24 kPa through 8.0 at 48 kPa to 9.0 at 96 kPa and above, using Su averaged over two base diameters below the base; for a base shallower than 3 diameters, qp is reduced by the O'Neill & Reese shallow-base factor 2/3 + z/(9D), which grows from 2/3 at the surface to 1.0 at z = 3D.
Sand (β-method). Unit side resistance is proportional to the effective vertical stress, fs = β·σ′v, with β from the FHWA depth correlation, scaled by N/15 for loose sand with N < 15, and then bounded to 0.25 ≤ β ≤ 1.2 (FHWA-IF-99-025 Eqs. 11.18–11.19); unit side resistance is capped at 200 kPa (p. 281). End bearing is qp = 57.5·N kPa (0.6·N tsf, Eq. 11.4) with N averaged over two base diameters below the base, so the blow count drives both the side and the base in sand. The released correlation requires 0 < N ≤ 50; larger values are rejected instead of capped.
Rock socket. Unit side resistance in a rock socket scales with the square root of the rock strength, normalized by atmospheric pressure Pa:
qu is limited to the shaft concrete strength f′c, which the request must supply whenever rock lies along the shaft, and αE is the joint factor (1 for massive rock). End bearing is 2.5·qu for a base socketed at least 1.5 base diameters into massive rock (Eq. 11.5); FHWA gives no value for a shallower socket, and PileCalc then takes qu as a conservative convention.
The layer must explicitly select massive/intact, closed-joint, or open/gouge-filled rock. Jointed rock uses the appropriate FHWA Table 11.4 RQD column over its published 20–100% range. Jointed rock is allowed for side resistance, but a jointed-rock bearing stratum is rejected until a compatible base-resistance and displacement-compatibility method is released; brittle side and base maxima are not simply added.
FHWA side-resistance exclusion zones
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 (half the unconfined strength bands of FHWA NHI-06-088 Table 4-2) — soft 12–25, medium stiff 25–50, stiff 50–100, very stiff 100–200 kPa. The method accepts Su/Pa ≤ 2.5 (about 253 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. FHWA NHI-06-088 Table 4-1 (after Peck et al. 1974): loose 4–10, medium dense 10–30, dense 30–50; the released range is 0 < N ≤ 50.
The unconfined compressive strength of the rock.
Why it matters. It sets the rock-socket side resistance fs = αE·0.65·√(Pa·min(qu, f′c)) and the base bearing 2.5·qu or qu. Default 5000 kPa.
Also declare the rock joint condition. Closed or open/gouge-filled joints require RQD from 20% through 100%; values below that table range require project testing or another qualified method.
Source: FHWA rock socket
Groundwater & factors
The water table and its explicit unit weight set buoyancy in the effective-stress profile. Separate factors of safety turn tip, side, self-weight, and uplift resistance into allowable loads.
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. If the table is supplied, waterUnitWeight is mandatory; there is no hidden 9.81 kN/m³ or 62.4 pcf default in the public contract.
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 compression friction times FHWA's factor Ψ (Eq. 11.30, p. 288): 1.0 in clay, 0.75 in sand and 0.7 in rock. With a bell, side resistance is zero from the bell base to 2 bell diameters above the top of the bell. The weight term is the shaft volume, including the bell (FHWA-IF-99-025 p. 287; the bell is taken as a cone frustum from the shaft diameter to the bell diameter, a lower bound on its volume), times its unit weight — buoyant (reduced by the water unit weight) below the water table — which is why a heavier shaft and a deeper embedment both help in tension. For the allowable tension load, the side and bell terms are factored by FS uplift (default 2.5) and the dead weight by FS weight (default 1.5). These are PileCalc defaults, not FHWA values; FHWA-IF-99-025 notes ASD factors of safety for drilled shafts typically range from about 2.0 to 3.5.
Reading the results
The tool reports four capacity figures, a side-vs-base split, the uplift breakdown, and—when requested—settlement at an explicit service load. Every response also includes the declared method and fine/coarse numerical-convergence evidence.
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 — the factored tension capacity (side + bell over FS uplift, weight over FS weight). The tension load you can apply.
Service-load settlement
In capacity-and-service mode, the reported settlement comes from the released t-z/q-z load-transfer model at the requested service load. It is not an ultimate-load displacement estimate. For a uniform circular socket in one homogeneous massive-intact rock layer, select the complete rock socket model. It uses the Kulhawy–Carter elastic/full-slip approximation reproduced in FHWA 2018 Appendix C.3.1. The shaft starts at the rock surface with full base contact; bells, layered or jointed rock, tip-area overrides and tensile loading are excluded. Enter measured or justified side/base rock moduli, Poisson ratios, and concrete–rock interface cohesion, friction and dilation. The model rejects loads above nominal resistance or incompatible side/base load sharing and reports its active branch and transition load. This predicts response to applied head load only; construction settlement, self-weight settlement and preload are not computed. Head displacement remains continuous at the elastic/full-slip intersection, while the separate asymptotic side/base load partitions can have a small jump. These parameters require site-specific engineering judgment; the published example is a calculation benchmark, not validation for a project. Capacity-only rock analysis remains available.
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.