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Gabions, rock mattresses and rip rap for scour protection

Scour is water removing the ground from under or around a structure, and the answer is rock that water can pass through but cannot move. Gabion baskets stacked into a wall retain a bank while letting water through; laid flat as a mattress or blanket they armour the bank or channel bed so flow cannot lift the soil. Rip rap, loose rock sized to the flow, protects outlets, toes and transitions. The specification is about velocity: the faster the water, the larger and heavier the rock and the heavier the mesh. Filter fabric under the rock stops fines being pulled out through it. Because the whole system is flexible, it settles with the ground instead of cracking, which is why it is the standard answer along drainage lines, creeks and culvert headwalls where a rigid wall would be undermined.

Walls, mattresses and blankets

A gabion wall is a gravity retaining structure of stacked baskets. A rock mattress is a shallow, wide basket laid on a slope or bed as armour. A rock blanket is loose or lightly contained rock over a filter layer. Each has a role: the wall retains, the mattress and blanket protect the surface from flow.

Specifying rock and mesh

Rock size is set from the design flow velocity so the water cannot move it. Mesh is galvanised, and PVC-coated where the water is aggressive. Filter fabric goes under and behind everything. We deal directly with basket manufacturers and quarries, so the specification and the supply are settled together.

Installation

Baskets are assembled on site, placed, filled with rock and closed; face rock is hand-placed where the wall is visible. Mattresses are laid on a prepared, fabric-covered slope and filled in place. Our own plant handles the rock.

Questions we are asked

How is a gabion wall different from a crib wall?

Both are flexible gravity walls that drain through their faces. Crib is precast concrete units with rock fill; gabion is wire mesh baskets of rock. Gabions suit wet ground and creek lines; crib suits batters and corridors.

Can gabions be planted?

Yes. Soil pockets and vegetation establish in the rock over time, and planting can be designed in.

Gabions Crib block Stacked rock

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