By dividing the structural roles. King piles at intervals carry the bending moment and vertical load; intermediate sheet piles span horizontally between adjacent kings and retain the soil. Because the intermediates only have to span the king spacing rather than resist the full wall moment, they can be much lighter, and the heavy steel is concentrated where the demand actually is.

When the retained height, water pressure or surcharge produces bending moments beyond what any plain sheet pile section can economically carry, or beyond what is available at all. Deep quay walls, large basement excavations, heavy port and industrial structures and walls carrying vertical load from a deck above are the standard applications.

Either fabricated box sections, commonly formed by welding sheet pile sections together, or rolled H sections, and occasionally tubular sections. Boxes give the highest bending capacity about both axes and a sealed section; H sections are simpler and cheaper where the bending demand is more moderate and predominantly about one axis. The choice follows the loading and the water conditions.

Where the king is a box made from sheet piles, it carries interlocks on its outer faces and the intermediates thread directly into them. Where the king is a plate box or an H section, welded connectors are attached to its flanges to provide the interlock. The connector detail is a designed item, since it transfers the load from intermediates into the kings.

Kings first, driven accurately to line, level and verticality, since everything else references them, usually through a guide frame. Intermediates are then threaded between adjacent kings and driven. Because the intermediates are constrained on both sides, king accuracy is critical: kings driven out of position or out of plumb make it difficult or impossible to install the intermediates between them.

By balancing the two components. Wider spacing means fewer, heavier kings but demands more from the intermediates, which must span further. Closer spacing does the reverse. The optimum depends on the bending moment, the available sections and the relative cost of kings against intermediates, and it is settled during design along with the connector detail rather than chosen on site.

Usually not. Kings are driven to the depth required for bending capacity and vertical bearing, which is often substantially deeper than the intermediates need. Intermediates typically stop above the toe of the kings, at a level determined by the soil below excavation and by seepage requirements. That shortening of the intermediates is a further source of economy in the system.