Sheet pile walls: cantilever versus anchored
Net pressure diagrams, Blum's simplified cantilever method, free earth support for anchored walls, and why embedment, bending moment and anchor force trade off against each other.
A sheet pile wall is held up entirely by the soil, through passive resistance in front of the toe and sometimes an anchor or prop near the top. Design looks for an embedment depth that gives moment equilibrium with an adequate margin, and then the maximum bending moment and anchor force for selecting a section.
Blum's method replaces the reversed pressure below the pivot with a single force at the toe. Taking moments about that point gives a cubic in the embedment D0, and the result is then increased by about 20 % to allow for the simplification. Commonly a factor is also applied to Kp, or factored strengths are used in limit-state terms.
Adding an anchor
An anchor near the top changes the structural system. In the free earth support method the wall rotates about the anchor and the toe is not fixed. Taking moments about the anchor gives the embedment, and horizontal equilibrium gives the anchor force. Embedment and bending moment are usually much smaller than for a cantilever of the same height, but you now need to design the anchor, its waler and its fixity behind the active wedge.
- Water: unbalanced water pressure often dominates. Model it explicitly instead of relying on submerged unit weights.
- Surcharge: construction plant behind the wall adds a uniform lateral pressure of Ka·q.
- Deflection: limit-equilibrium methods do not give deflections. GEO5-style subgrade-reaction or FE analyses are needed when nearby structures are sensitive.