Scour protection: riprap, mattresses and collars

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

Size the stone from the design velocity near the pier with pier shape factors (HEC-23), and check it against waves and ship propeller wash where they apply.
Design the filter (geotextile or graded granular) and the layer: at least three times D50 thick, extending at least two pier widths from the pier face all round.
Excavate or dredge a pocket so that the top of the armour sits at or below the existing bed; raised aprons cause scour at their own edges.
Place the filter: weighted geotextile panels or sand-filled geocontainers under water, or a granular filter by bottom-dump or skip.
Place the riprap by excavator, crane skip or side-dumping vessel, working outward from the pier in controlled layers; confirm thickness by multibeam survey.
Where stone of the size needed is not available, use articulated concrete blocks, grout-filled mattresses, gabion mattresses or partially grouted riprap instead.
Inspect after placement and after every significant flood, replace displaced stone, and keep a monitoring plan with closure triggers for scour-critical bridges.

Key numbers

Pier riprap (HEC-23)D50 = 0.692 (K·V)² / [(Ss − 1)·2g], with K = 1.5 for round-nosed and 1.7 for rectangular piers.
Layer thickness at least 3 D50; extent at least two pier widths from the pier face.
Typical D50 for river piersabout 0.3–1.0 m; rock density at least about 2,600 kg/m³.
Granular filter ruleD15 of the filter no more than about 4–5 times D85 of the base soil.
Costabout USD 60–150 per tonne placed.

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.

Choosing it

For new bridges design the foundations for full scour and use armour as a second line; for existing bridges on shallow foundations, armour is often the economic fix.
Choose riprap where suitable stone is available and access allows; prefer articulated concrete blocks or grout-filled mattresses where the stone size needed is not available.
Prefer flexible armour (riprap, block mats) to rigid aprons, which crack and are undermined at their edges.
For scour-critical bridges awaiting replacement, combine armour with monitoring and a closure plan rather than relying on armour alone.

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.

Origins

Rock armour is as old as river engineering; systematic US countermeasure guidance, FHWA HEC-23, was first published in 1997 after the scour programme set off by Schoharie Creek, and the CIRIA/CUR/CETMEF Rock Manual (2007) is the European reference.

Examples

FHWA HEC-23 sets out the countermeasure options and design guidelines used in the US; rock aprons and mattresses are the standard retrofit for older river bridges on spread footings or short piles in Europe and Asia.

Case studies

Schoharie Creek BridgeUSA · 1987Riprap around the spread footings had not been maintained; a flood scoured beneath a pier and two spans fell, killing 10 people.

Malahide ViaductIreland · 2009A pier collapsed just after a train had crossed; investigators found that scour had removed the rock armour and weir protecting the foundations, which had not been inspected or maintained under water.

Inn River motorway bridge near InnsbruckAustria · 1990After a scoured pier sank about 1.2 m and tilted the decks, about 38,000 t of boulders were dumped around its base, grout was injected under the foundation, and cables tied the pier to the bank.