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    Calculates locally in your browser · equivalent raft settlement
    Geotechnical / settlement

    Pile group settlement

    Capacity is usually not what governs a pile design — settlement is. The equivalent raft method replaces the group with a single footing at the depth where the load has spread out, and computes the elastic settlement of that footing in the soil beneath. The load spreads at 1 in 4 from two-thirds of the pile length.

    VerdictOverRaft6.5 mDepth10.0 m
    Total settlement
    48.4 mm
    above the 25 mm limit
    Soil settlement
    47.0 mm
    97% of the total
    Pile shortening
    1.33 mm
    elastic compression of the piles
    Raft pressure
    142 kPa
    at 10.0 m depth
    The equivalent raft
    The group is replaced by a footing at two-thirds depth, where the load has spread to a 1-in-4 slope.
    48 mm
    The equivalent raft
    QuantityValueNote
    Group load6,000 kN9 piles
    Group footprint4.0 × 4.0 mas entered
    Pile length15.0 mas entered
    Raft depth10.0 mtwo-thirds of the pile length
    Raft size6.5 × 6.5 mload spread at 1 in 4
    Raft pressure142.0 kPaload over the raft area
    Soil modulus20 MPabeneath the raft
    Soil settlement47.0 mmelastic, flexible area
    Pile shortening1.33 mmQL/(2AE)
    Total48.4 mmthe number that is checked
    Limit25 mmtypical for a framed structure

    Total settlement is 48 mm, above the 25 mm usually allowed for a framed structure — the group is capacity-adequate but serviceability-limited, which is the normal outcome and the reason this check exists.

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    Settlement against pile length
    Longer piles push the equivalent raft deeper, which usually reduces settlement — until the pile compression takes over.
    Settlement against pile length0.00013.8227.6341.4555.2769.086 m6 m10 m10 m15 m15 m20 m20 m25 m25 mTotal, mm
    Soil against pile
    The soil beneath the raft does almost all of it; the pile compression is a rounding error.
    Soil against pile0.00011.1022.2033.3044.4155.5147.04SoilSoil1.33PilePilemm
    Canonical equations
    The exact relationships implemented by this engine.

    (2/3) L · Where the load has spread.

    B + 2(L/3)(1/4) · 1-in-4 spread.

    q B(1−ν²) I / E_s · I = 1.12 for a square.

    Q L/(2 A E_p) · Uniform shaft friction.

    engineering methodology

    A clear path from inputs to results.

    Capacity is usually not what governs a pile design — settlement is. The equivalent raft method replaces the group with a single footing at the depth where the load has spread out, and computes the elastic settlement of that footing in the soil beneath.

    Step-by-step methodWorked exampleAssumptions explained

    How to use

    1. Enter the group load and footprint.
    2. Set the pile length and diameter.
    3. Add the soil modulus and Poisson’s ratio.
    4. Read the soil settlement, the pile shortening and the total.

    The formula

    s=q B (1−ν2) IEs+QL2AEps = \frac{q\,B\,(1 - \nu^2)\,I}{E_s} + \frac{Q L}{2 A E_p}

    Reference: Equivalent raft method (Tomlinson), with the load spreading at 1 in 4 from two-thirds of the pile length.

    Worked example

    6,000 kN on a 3 × 3 group of 15 m piles

    Inputs
    • 6,000 kN, 4 × 4 m footprint, nine 400 mm piles 15 m long.
    • Soil modulus 20 MPa, Poisson 0.3, pile modulus 30 GPa.
    Calculation path
    1. 1

      The equivalent raft sits at two-thirds of 15 m, or 10 m.

    2. 2

      The load spreads at 1 in 4 over the remaining 5 m, so the raft is 6.5 m square.

    3. 3

      The raft pressure is 6,000/42.25 = 142 kPa.

    4. 4

      Elastic settlement is 142 × 6.5 × 0.91 × 1.12 / 20,000 = 47 mm.

    5. 5

      The piles shorten by 1.3 mm, giving 48 mm total — above the 25 mm limit.

    Result

    47 mm soil · 1.3 mm pile · 48 mm total.

    Interpreting the result

    The soil beneath the raft does almost all of it, and the pile compression is a rounding error. That is why the soil modulus is the input that matters, and why a settlement estimate is really an estimate of the modulus rather than of the geometry.

    Assumptions & limitations

    • A rigid group load spread uniformly.
    • Homogeneous elastic soil beneath the raft.
    • No consolidation settlement in clay.
    • No pile-to-pile interaction beyond the raft idealisation.

    Frequently asked questions

    Why two-thirds of the pile length?

    Because that is where the load has transferred to the soil in a friction pile. An end-bearing pile on rock would use the full length instead.

    Does this work in clay?

    The elastic part does, but clay also consolidates, and the consolidation settlement is usually larger and slower. A one-dimensional consolidation calculation is needed for that.

    How does a pile group differ from a single pile?

    The group settles more than a single pile at the same load per pile, because the stress bulbs overlap. That is what the equivalent raft captures.

    Why is 25 mm the limit?

    It is a common serviceability limit for framed structures. Masonry and cladding are more sensitive, and some codes use 50 mm for isolated foundations.

    What if the modulus is unknown?

    Use a correlation from SPT or CPT and treat the answer as a range. The modulus is often uncertain by a factor of two, and the settlement scales directly with it.

    Methodology last reviewed 2026-10-22. Results are informational and not financial or engineering advice — see our disclaimer.