A closed section formed by welding two or more sheet pile sections together along their interlocks, or by fabricating a box from plate. The closed form gives high bending capacity about both axes, high torsional stiffness, and a sealed interior, none of which an open H section or a single sheet pile provides.

When bending demand, stiffness or axial capacity exceeds what an H section can deliver, or when a sealed section is needed. Typical cases are king piles in deep combined walls, mooring and berthing dolphins, fender piles, monopile-type foundations and heavily loaded foundation piles. Where the requirement is straightforward axial capacity with low displacement, the H-pile is simpler and cheaper.

The box piles are installed at intervals as king piles and take the vertical load and most of the bending from the retained soil and water. Lighter intermediate sheet piles span horizontally between them and retain the ground. The result is a wall that can be far deeper and more heavily loaded than plain sheet piling, with steel concentrated where the bending demand is.

Because a box built from sheet pile sections retains interlocks on its outer faces, so it can be threaded directly into a sheet pile wall and engage the intermediate piles either side. That is what allows it to serve as a king pile in a combined wall. A box fabricated from plain plate has no interlocks and must be connected by welded intermediate connectors instead.

Yes, and it is common where high axial capacity is required. Filling raises compressive capacity substantially, adds mass and stiffness, and protects the interior from corrosion. It also increases weight, which affects handling and driving, so filling is normally carried out after driving rather than before, and the design should confirm composite behaviour is being relied upon correctly.

As a single unit, usually by impact or vibratory hammer, with the section fabricated and welded before delivery or assembled on site for very long piles. Because they are heavier and stiffer than an H-pile of similar capacity, driving equipment must be sized accordingly, and driving stresses through the welded joints should be considered where the box is fabricated from sheet piles.

The exterior is treated like any steel pile, with allowances by exposure zone and the splash zone as the most aggressive. The sealed interior is generally not exposed to oxygen once the section is closed and driven, so internal corrosion is minimal, and concrete filling removes the question entirely. Weld integrity matters here, since a leaking box admits water to the interior.