Marine Pile Design Calculator

The underlying formula, q_ult = c·Nc·sc·dc + q·Nq·sq·dq + 0.5·γ'·B·Nγ·sγ·dγ, is the general bearing capacity equation for a footing bearing directly on soil — the kind used for a mudmat or gravity base sitting on the seabed, and the same shallow-foundation approach documented in DNV-RP-C212 (Offshore Soil Mechanics and Geotechnical Engineering). A driven or drilled pile instead derives most of its capacity from skin friction along the shaft and end-bearing at the tip, which are entirely different mechanisms not represented anywhere in this equation, so for actual pile foundations you need a separate axial pile-capacity calculation.

The standard Meyerhof relation for Nc involves dividing by tan(φ), which is undefined at φ = 0 — the purely cohesive, undrained clay case that's actually common on soft seabeds. Rather than producing an error or a nonsensical value, the calculator uses 5.14, which is the well-established classical bearing capacity factor for undrained (φ = 0) conditions, matching standard geotechnical practice for that special case.

Every soil particle below the seabed sits fully immersed in seawater and experiences the same buoyant uplift as any other submerged object, so its effective weight contributing to overburden pressure and self-weight resistance is reduced by that buoyancy — this is the submerged (or buoyant) unit weight, typically 7–10 kN/m³ versus 18–20 kN/m³ for the same soil above water. Entering a dry, above-water unit weight instead would substantially overstate both the overburden and self-weight terms, and therefore the calculated bearing capacity.

The depth factors dc and dq in the calculation scale up with the ratio of embedment depth to foundation width, reflecting that soil above the foundation base level provides confining overburden pressure that resists the shear failure surfaces the foundation would otherwise punch through. A foundation embedded a meter below the seabed surface therefore shows higher allowable bearing capacity than an identical footing sitting right at the mudline, all else equal — though this only applies while embedment depth stays modest relative to width, matching the shallow-foundation assumptions behind the formula.