By section modulus per metre of wall, not per pile. Piles are supplied in different widths, so a wide pile covering more wall must provide proportionally more capacity to match a narrower one. Weight per square metre of wall is the corresponding cost measure. Comparing individual pile properties, or pile weight, will produce the wrong answer whenever the candidates differ in width.

Because it decides whether the wall acts as a single continuous section in bending. Interlocks at the extreme fibres of the wall, as on Z sections, are in the region of highest stress but do not need to transfer shear between piles for the section to work. Interlocks on the neutral axis, as on U sections, must transfer shear for the pair to behave as one, and if they cannot, capacity is reduced.

A factor applied to the theoretical section modulus of U-section walls to account for the possibility that the interlocks on the neutral axis slip rather than transferring full shear. Its value depends on the soil, the wall support conditions and whether interlocks are crimped or welded. Crimping or welding the interlocks in pairs allows a higher factor, at additional cost, and is common on demanding walls.

Separation of the interlock between adjacent piles during driving, usually caused by obstructions, by piles deviating out of plumb, or by driving into ground that is too dense. It leaves a gap in the wall with no continuity, which compromises both retention and water tightness, and it is difficult to detect from the surface. Careful pitching, driving in panels and monitoring verticality are the standard controls.

Interlocks restrict flow but do not seal. Where seepage must be controlled, interlocks are treated with swelling or bituminous sealants applied before driving, or welded on the exposed face after installation. Complete water tightness is rarely achievable in practice, so designs normally combine sealed interlocks with a drainage or dewatering strategy rather than relying on the wall to exclude water entirely.

A cantilever wall relies only on passive resistance in the soil below excavation level, and the required embedment grows rapidly with retained height, so the practical limit is modest and depends heavily on soil strength. Beyond that, walls are anchored with tie rods and walings or propped across the excavation, which reduces bending and embedment considerably and is usually far more economical than deepening a cantilever.

Yes, and it is normal on temporary works. Piles are often hired rather than purchased for cofferdams and excavation support, extracted at the end and reused elsewhere. Reusability depends on the ground, on interlock condition after driving and extraction, and on corrosion. Damage expectations and condition on return are normally agreed in the hire terms before the work starts.