Because their interlocks lie on the neutral axis of the wall, where horizontal shear is greatest. For the wall to behave as one continuous section in bending, that shear has to pass across the interlock between adjacent piles. Whether it does depends on interlock friction, the surrounding soil and the support conditions, so codes apply a reduction to the theoretical section modulus unless the interlocks are positively connected.

It deforms the engaged interlocks of a pair at intervals so they cannot slide relative to one another, forcing the two piles to act as a single section. That allows a higher design factor and therefore more capacity from the same steel. It is normally carried out before delivery, since it requires equipment and access to the free interlock, and it adds cost per pair.

Pairs where the design relies on the higher section modulus, or where handling and pitching a heavier unit is acceptable and driving conditions allow. Singles where piles must be threaded individually, where obstructions may require piles to be driven separately, or where extraction and reuse is planned. The decision affects both design capacity and site method, so it is taken early rather than at order.

Z sections develop their full section modulus without a reduction, because their interlocks sit at the extreme fibres, so they are generally more efficient per kilogram in bending. U sections have heavier, more robust interlocks, drive well in difficult ground and are widely available. The choice is between bending efficiency and driveability, and both types are used extensively depending on ground and market.

Yes, and it is one of their main advantages. The interlock geometry is substantial and resists the twisting and separating forces generated when a pile meets dense material or an obstruction, so declutching is less likely than with lighter sections. Where the ground is known to contain cobbles, made ground or dense granular layers, that robustness often outweighs the section modulus penalty.

Piles are pitched into a guide frame or trestle so that each engages the interlock of the one before, then driven in stages rather than each to full depth in turn, which keeps the wall vertical and reduces the risk of declutching. Panel driving, in which a set of piles is pitched and then driven progressively, is standard practice for anything other than short walls in easy ground.

To the same extent as any sheet pile wall: interlocks restrict but do not seal. Sealants applied before driving, or welding the interlocks on the exposed face after installation, reduce seepage substantially. Because the interlock is on the neutral axis and each pair has one free interlock in the wall, the number of joints requiring treatment is lower than for single-pile walls of equivalent length.