Axial capacity & settlement
Side friction and end bearing for compression and uplift, the load–settlement curve, and the factors of safety that govern design.
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.
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. 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, with Vesić (1977) elastic settlement at the working load. 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 is subtracted in compression but resists uplift, so it raises tension capacity. Reinforced concrete is about 24 kN/m³.
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 | Side and tip resistance scaled from the unconfined compressive strength | 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. Rock is a distinct regime where capacity scales with the intact strength. For drilled-shaft-specific behaviour 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 ≈ 9 — stronger clay carries more axial load both ways. Guide values — soft 12–25, medium 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. Soft clay ≈ 1.0, stiff clay ≈ 0.5 — typically 0.5–1.0, decreasing with strength. Set it explicitly in SI work.
Source: Tomlinson
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. Loose 28–30, medium 32–36, dense 38–42°.
The unconfined compressive strength of the rock.
Why it matters. Rock side and tip resistance both scale with qu. Stronger rock at the tip can dominate the capacity.
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. Set it very large for a dry profile.
The unit weight of water used for buoyancy.
Why it matters. It must match your unit system: 9.81 kN/m³ in SI, 62.4 pcf in US customary. Getting this wrong corrupts the entire effective-stress profile — see the water trap.
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.
Factor of safety on tip (end-bearing) resistance.
Why it matters. End bearing mobilizes only at large settlement and is less certain, so it carries a higher FS than side friction — typically ≈ 3.
Factor of safety on side (shaft) resistance.
Why it matters. Side friction mobilizes at small movement and is more reliable, so a lower FS is used — typically ≈ 2, below the tip value.
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 allowable load — shaft shortening plus tip- and shaft-transmitted components, 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.
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