Axial capacity & settlement
Side friction and end bearing for compression and uplift, the load–settlement curve, and the factors of safety that govern design.
Service transfer follows pile installation: driven/displacement piles use the API RP2A-WSD (2002) reference t-z/q-z profiles, with the sand branch also checked against FHWA’s reproduction of API (1993). The driven-clay residual ratio 0.9 is a declared reference-profile assumption. Bored/drilled piles use FHWA/Reese–O’Neill curves from FHWA-NHI-06-089 Figures 9-50 through 9-53. Vesić settlement remains a separate estimate at fully mobilized resistance.
An axially loaded pile carries its load two ways: shaft (side) friction down the length of the pile, and end bearing at the tip. PileCalc adds them with the NAVFAC DM-7.02 static method — the same hand-checkable formulation behind most foundation-design spreadsheets — and traces the full load–settlement curve so capacity can also be read off at a settlement limit. This page explains the method and every input on the axial tool.
How axial capacity works
The ultimate downward capacity is the sum of the resistance mobilized along the shaft and at the tip:
Qs is the side resistance integrated over the shaft and Qp the tip (base) resistance. In uplift the sign flips — there is no tip resistance to call on, so the tension capacity is the shaft friction plus the pile self-weight W, Quplift = Qs + W. Below the water table W is the buoyant (effective) weight; for a battered pile only the side term is projected onto the vertical (W + Qs·cos β) — the weight always acts vertically.
How each piece is computed depends on the soil. In clay, side resistance is an undrained adhesion and end bearing scales with the shear strength:
In sand, both pieces are driven by the effective vertical stress σ′v — friction along the shaft and a bearing-capacity factor at the tip:
Nq grows steeply with the friction angle φ, and a driven displacement pile earns a higher Nq than a bored pile because driving densifies the surrounding sand. NAVFAC DM-7.02 tabulates Nq only for φ = 26° to 40°, so a sand layer the tip bears in must have φ in that range; the request is rejected otherwise rather than extrapolated. Because σ′v drives every sand term, the water table is a first-order input: below it, buoyancy lowers effective stress and trims both friction and bearing.
Capacity is only half the story — the tool also builds the load–settlement curve. Side and tip resistance mobilize at different movements, captured by t–z (side) and q–w (tip) load-transfer curves. PileCalc solves that response at the explicit unfactored service load; the Vesić (1977) fully mobilized estimate is retained as a supplemental check. Reading the mobilized load off that curve at your allowable settlement often governs the design.
Side mobilizes before tip
Pile inputs
The pile is a uniform elastic member; you give its geometry, stiffness, and self-weight.
The embedded length of the pile below the head.
Why it matters. More length means more shaft surface area and therefore more side resistance, and usually reaches a deeper, stronger bearing layer at the tip. Length is the primary lever on axial capacity.
The pile width or diameter.
Why it matters. It sets both the shaft surface area (side resistance scales with the perimeter, ∝ B) and the tip bearing area (∝ B²). It also sets the movement needed to mobilize end bearing.
The elastic modulus of the pile material.
Why it matters. It controls elastic shortening of the shaft under load, which is part of the settlement at working load. A typical concrete pile is on the order of 30 GPa.
The unit weight of the pile material.
Why it matters. It sets the self-weight W, which resists uplift and so raises tension capacity — the buoyant (effective) weight is used below the water table. Reinforced concrete is about 24 kN/m³.
Steel, concrete or timber.
Why it matters. NAVFAC DM-7.02 ties two defaults to the material: the sand interface friction angle δ (Figure 1 continued: steel 20°, concrete and timber ¾φ) and the clay adhesion Ca (Figure 2: a lower column for steel). If you leave it unspecified, the lower value of each table is used. An explicit δ, δ/φ or layer α overrides the default.
Source: NAVFAC DM-7.02
Soil types
Each layer is assigned one of three behavioural classes. The class is the most important choice in the layer editor: it selects the resistance method, and the strength parameters that appear adapt to it.
| Type | Resistance method | Key parameters |
|---|---|---|
| Cohesive (clay) | Undrained adhesion fs = α·Su; end bearing qp = Nc·Su | Su, α |
| Cohesionless (sand) | Effective-stress friction fs = K·σ′v·tan δ; end bearing qp = Nq·σ′v | φ |
| Rock | Not accepted within the pile length (see qu); analyse a cast-in-place socket with the drilled-shaft tool | qu |
Clay resistance is undrained and largely independent of depth except through the strength profile; sand resistance is governed by effective stress and so by the water table. A rock layer may be declared below the pile tip, with an explicit joint condition, but rock within the pile length is rejected. For rock sockets see Drilled shafts.
Soil parameters
Below is every parameter the soil methods use. Each layer also carries its top and bottom depth, which must tile the profile without gaps; the deepest layer should extend to or past the pile tip.
The total unit weight of the soil layer.
Why it matters. It builds the effective vertical stress profile σ′v that drives both sand side friction and sand end bearing. Below the water table the buoyant weight is used. Typical total ~18–20 kN/m³.
The undrained shear strength of the clay.
Why it matters. Side resistance in clay is the adhesion α·Su and end bearing is Nc·Su, with Nc rising from 6.3 at the surface to 9 at four diameters of embedment — stronger clay carries more axial load both ways. 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 kPa.
Source: NAVFAC DM-7.02
The ratio of pile–soil adhesion to soil cohesion, Ca/Su.
Why it matters. Clay does not stick to the pile at full strength; α reduces it. When you leave it empty, the adhesion comes from the NAVFAC DM-7.02 Figure 2 table: Ca = Su up to 250 psf (12 kPa), then 480 / 750 / 950 / 1300 psf for timber and concrete or 460 / 700 / 720 / 750 psf for steel at Su = 500 / 1000 / 2000 / 4000 psf, interpolated between those limits. The steel column is used when the pile material is not specified.
Source: NAVFAC DM-7.02 Fig. 2, p. 7.2-196
The drained angle of internal friction of the sand.
Why it matters. Side friction is K·σ′v·tan δ and end bearing is Nq·σ′v — both rise steeply with φ, so a few degrees materially change capacity. A bearing layer must have 26° ≤ φ ≤ 40°, the range of the NAVFAC Nq table. For orientation, AASHTO (2014) correlates SPT (N₁)₆₀ = 4 / 10 / 30 / 50 with 27–32° / 30–35° / 35–40° / 38–43° (FHWA GEC 12 Table 5-5, p. 123).
The unconfined compressive strength of the rock.
Why it matters. In a cast-in-place socket, side resistance grows with √qu, fs = αE·0.65·√(Pa·min(qu, f′c)) (FHWA-IF-99-025 Eq. 11.24), and tip bearing with qu. The axial analysis does not accept rock within the pile length: there are no rock t-z/q-z service curves, and FHWA GEC 12 gives no static method for piles driven into rock. Use the drilled-shaft tool for a cast-in-place socket.
Massive/intact rock must not carry a token RQD. Jointed rock requires an explicit closed or open/gouge-filled condition and RQD between 20% and 100%; poorer rock is outside the published FHWA table.
Groundwater & options
These settings fix the effective-stress profile and the installation method, and they tell the tool where to read settlement-governed capacity.
Whether the pile displaces soil (driven) or removes it (drilled / bored).
Why it matters. Driven displacement piles densify the surrounding sand, earning a higher end-bearing factor Nq than bored piles. Use driven for impact / vibro piles, bored for drilled shafts.
The depth to the water table below the ground surface.
Why it matters. Below the water table, buoyancy lowers σ′v — reducing both sand side friction and end bearing. For a dry profile, omit it (untick Include groundwater in the app, or leave waterTableDepth out of an API request); there is no need to enter a very large depth.
The unit weight of water used for buoyancy.
Why it matters. The app displays 9.81 kN/m³ or 62.4 pcf; the coherent US API value is 0.0624 kip/ft³. The field is mandatory whenever a water table is present — see the water trap.
The explicit unfactored working load for the released settlement solution.
Why it matters. Settlement is load-dependent. PileCalc brackets this load on the t-z/q-z response, solves it by bisection, and fails closed if the state or the fine-versus-half-mesh check does not converge.
A settlement limit at which to report the mobilized capacity.
Why it matters. Capacity is often governed by tolerable settlement, not the ultimate plunging load. This reads the load off the load–settlement curve at your limit. Often 10–25 mm, or a fraction of the diameter.
Factors of safety
Side and tip resistance carry separate factors of safety, applied before they are summed into the allowable load: Qallow = Qp/FStip + Qs/FSside. Uplift uses two more: Tallow = W/FSweight + Qs,up/FSuplift. NAVFAC DM-7.02 Fig. 1 (B) (p. 7.2-193) takes Tallow = Tult/3 + Wp, so the tool defaults are FSweight = 1 and FSuplift = 3; API requests state all four factors explicitly.
Factor of safety on tip (end-bearing) resistance.
Why it matters. End bearing mobilizes only at large settlement and is less certain; the tool default is 3, the NAVFAC permanent-load factor. NAVFAC DM-7.02 Fig. 1 applies one FS to the total, 2 for temporary and 3 for permanent loads, and not less than 2 even with a load test (p. 7.2-193).
Factor of safety on side (shaft) resistance.
Why it matters. Side friction mobilizes at small movement and is more reliable than end bearing. The tool default is 3, the NAVFAC DM-7.02 Fig. 1 (A) permanent-load factor (p. 7.2-193); 2 applies to temporary loads only. The request is rejected below 2, with the NAVFAC reason shown under the field.
Reading the results
The tool reports the capacity summary, the tip/side split, and the load–settlement curve.
Capacity summary
- Ultimate down — the plunging capacity Qs + Qp, before any factor of safety.
- Allowable down — the factored compression load, Qp/FStip + Qs/FSside, the load you can actually apply.
- Ultimate & allowable uplift — the tension capacity Qs + W and its factored counterpart; this governs for wind / seismic uplift and tie-downs.
- Settlement at the declared service load — a converged t-z/q-z solution with the side and tip load split, often the governing serviceability check.
Side vs. tip split & the curve
A bar shows how the ultimate capacity divides between shaft friction and end bearing — a friction-dominated pile behaves very differently from an end-bearing one. The load–settlement curve plots load against movement, with the mobilized load at your allowable settlement marked on it. The app auto-runs a worked example on load so you can see all of this immediately, then adjust the inputs and re-run.
Related tools
Optional serviceBaseDiameter specifies the effective toe diameter for service-transfer normalization in the request’s length units. It must be finite and positive. Circular toes normally use their physical outer diameter; driven noncircular solid toe geometry uses an equivalent-area diameter from the explicitly declared area. Positive driven annular toe resistance requires an explicitly justified settlement diameter. A zero-resistance toe contributes no toe spring. This field does not change the capacity-bearing area.