How bridges get built · Ground improvement · Scour protection: riprap, mattresses and collars
Also called: rock armour, gabion mattress, articulated concrete blocks, scour countermeasures
How it is done
- Excavate or dredge a bed pocket around the pier to the top of the filter layer.
- Place a geotextile or granular filter, then riprap of the computed stone size (often 0.3-1 m) to a thickness of two stone diameters, extending two pier widths from the face.
- Mattresses are placed by crane from a barge; collars are cast at the estimated scour level.
Where it fits
- Existing bridges with scour vulnerability, river piers where deep foundations are already fixed, bank protection at abutments.
Where it does not
- Relying on riprap alone for new piers: design the foundation for scour and treat the armour as a second line.
Plant, pace and money
PlantExcavator or barge crane, dump trucks or hopper barges, divers for inspection.
Productivity500-2,000 t of stone per day depending on access.
CostUSD 60-150 per tonne placed.
Risks and controls
What goes wrong
- Undersized stone washing out; edge failure where the armour meets the bed; winnowing of fines through a missing filter.
Quality assurance
Stone grading certificates, post-placement bathymetry, inspection after the first flood.
Examples
FHWA HEC-23 countermeasure matrix; Korean river bridges after the 2020 floods.
Related methods
Further reading
FHWA HEC-23 · CIRIA C683 Rock Manual