Gabion Mattresses for Bridge Pier Scour Protection

Mattress, not a gabion box

A gabion mattress is a wide, shallow cage, often a few hundred millimetres thick, divided into cells so the stone cannot bunch at one end. A gabion box is the deeper unit used for a wall or a heavy toe. At a pier, the mattress is there to armour the bed and to fall into a scour hole as the bed drops, rather than to stand as a wall.

Thickness, cell size and stone are a hydraulic choice. This page will not give you a thickness. The designer uses the velocity, the depth, the pier shape and the bed material. A product name does not replace that.

Stone, filter and edges

The stone has to stay inside the mesh and stay stable under the design flow. Larger stone and a thicker mattress are the usual response to faster water. The smallest stone still has to be too large for the opening. For polypropylene composite mesh, the nominal opening in supplier data is 65 ± 3 mm. Check the grading against the opening on the drawing, not against a wire mattress you built last year.

A filter under the mattress stops the bed soil pumping up through the stone. The edges and the toe need a detail so the carpet does not roll up. Units are tied to each other. A gap between mattresses is a place for the bed to leave.

Underwater placement needs a method and a way to see that the mattress landed where it was drawn. A lighter empty polypropylene panel is easier to handle in the dry. The filled mattress is still stone. Do not plan the crane from the empty mesh weight.

Which gabion mesh

Steel-wire mattresses rely on the coating once they sit in the river. Debris and placement can cut galvanizing or PVC. Polypropylene composite mesh has no steel-wire rust mechanism, so it is considered in water that is hard on coatings. Abrasion from sediment, UV on any exposed edge, and the strip connection still apply.

Guidance written for wire mattresses, including FHWA scour countermeasure notes such as HEC-23, describes the countermeasure. It does not approve a different mesh or a different joint. If the mattress changes from wire to polypropylene, the designer accepts that assembly. Do not copy the wire lacing pattern onto polymer strips.

A brochure example, and what it is not

The May 2024 brochure describes an earlier proposal that used plastic stretched geogrid as a gabion around a construction-bridge caisson, to hold stone during placement. That is background on why containment matters. It is not a record of the current composite mesh, and it is not a completed pier-scour design you can copy.

For a live pier, send the survey, the pier shape, the water levels and the mattress schedule the designer has already accepted. Ask for drawings of the panel, the connection and the stone. Separate the mesh supply from underwater installation.