Building cellular structures rather than retaining walls. Piles are interlocked into a closed ring, usually circular, and the cell is filled with granular material. The fill provides the weight and internal resistance, and the piles hold it as a tension ring. The structure resists lateral load by its own mass, without anchors or props, which is the reason for using it.

Because they are not loaded in bending. In a filled cell the piles carry hoop tension, transferred from pile to pile through the interlocks, rather than spanning vertically as a retaining wall does. The governing structural property is therefore interlock tensile strength, and straight-web sections are designed to maximise it while remaining flat enough to form a ring.

A series of large filled cells, circular or built as diaphragm cells linked by arcs, forming an enclosure that can be dewatered and worked inside. Each cell is stable once filled, so the enclosure needs no external anchoring or bracing. It is the standard method for constructing in rivers and open water, and for creating large dry working areas where conventional propping is impossible.

The permissible angle of deviation between adjacent interlocked piles. Interlocks allow a limited swing, and that angle determines the minimum radius the ring can turn and therefore the achievable cell diameter for a given pile width. Cell size is designed around that limit together with the required stability, the fill properties and the retained height.

Granular material, typically sand or gravel, and it matters a great deal because the fill provides the structural resistance. Its shear strength, drainage and compaction govern the stability of the cell, and poorly drained or fine material can generate internal pressures the piles were not designed for. Fill specification and placement are structural activities on a cellular structure rather than backfilling.

Quay walls and breakwaters, lock and dam construction, mooring and berthing structures, and permanent gravity retaining structures in open water. Anywhere a self-supporting enclosure or gravity wall is needed and anchoring behind the structure is impractical, whether because of water, because of what lies behind, or because the works must be free-standing during construction.

Yes, and cellular cofferdams are frequently temporary works with the piles extracted and reused afterwards. Extraction depends on the fill being removed first and on interlock condition after driving and service. Because the interlocks are the working element, their condition after extraction determines reuse more than the pile body does, and it should be inspected rather than assumed.