Because their interlocks sit at the outer flange edges, at the extreme fibres of the wall, rather than on the neutral axis where horizontal shear is greatest. The wall therefore acts as a continuous section without relying on shear transfer across the interlocks, and the full theoretical section modulus is available. That removes both the design reduction and the need for crimping or welding.

Generally yes on a steel-weight basis for a given bending capacity, because the full section modulus is available without reduction. Whether that translates into lower total cost depends on availability, on driving conditions, and on whether the extra robustness of U-section interlocks is needed. In easy to moderate ground where bending governs, Z sections usually win; in difficult driving they may not.

Singly as standard, though they can be supplied interlocked in pairs for handling efficiency. Driving as singles gives flexibility when threading around obstructions or correcting alignment, and makes replacing a damaged pile or extracting selectively more straightforward. It also means more individual pitching operations, which is a consideration on long walls.

Their interlocks are lighter than the substantial U-section equivalents, so they are less tolerant of hard driving into dense material, cobbles and obstructions, and declutching is a greater risk in those conditions. Good ground investigation, careful pitching, panel driving and control of verticality all matter more with Z sections, and in genuinely difficult ground a U section may be the safer specification.

Deep retaining walls, quay walls and jetties, flood and coastal defences, basement and deep excavation support, and permanent works generally, particularly where the wall is highly loaded in bending and steel efficiency matters. They are the common default in many markets for permanent retaining structures where driving conditions are known to be manageable.

As with any sheet pile wall, through interlock treatment rather than the interlock itself. Sealants can be applied to the interlocks before driving or welding carried out on the exposed face afterwards. Because Z piles are usually driven as singles, every interlock in the wall is a working joint, so the number requiring treatment is higher than on a wall built from crimped pairs.

On section modulus per metre of wall and on mass per square metre of wall, after applying any reduction factor to the U section for its interlock position and support conditions. Comparing catalogue section modulus without that adjustment flatters the U section. Then weigh driveability against the ground investigation, since a more efficient section that cannot be installed is not the cheaper option.