- Case study
- La Reunion New
- France
Insitu Concrete Foundations for 48 Marine Bridge Piers,Nouvelle Route du Littoral, La Réunion
Forty-eight bridge piers standing in cyclone-exposed water off the coast of La Réunion, each one bearing directly on the rock beneath it. Pier head height sets the alignment of the precast deck above it, so every base had to reach full bearing the moment it landed and hold it, with no settlement and nothing to adjust afterwards.
Location
La Réunion, Indian Ocean
Client
VINCI Construction
Designer
5.4 km offshore viaduct, 48 piers
System
Foundation Grout Bag System
Period
2016
Results at a glance
Marine pier foundations
Every pier of the offshore viaduct, consistent and repeatable
Cumulative foundation area
Bearing interface formed insitu, free of voids and washout
Grout placed from the surface
Batched onboard and tremie-pumped, no diver placement
Contents
- Six grout bag compartments were fitted beneath each 20 m or 23 m diameter pier base in the casting yard, fed by filler sleeves run up the pier shaft to deck level.
- The formwork took up to 900 mm of bed variation, so no fine-grained stone levelling and no underwater concrete pour was needed.
- Five distinct panel designs were developed, each 24 m by 5 m, zip-connected along top and bottom edges into a continuous apron.
- Grouting ran 12 to 16 hours per pier, controlled from deck level with no divers in the filling operation.
- Bearing was achieved on filling, so the precast deck could be erected to alignment without waiting on settlement or adjusting the pier head.
The constraints
The Nouvelle Route du Littoral replaces the cliff-hugging National Route 1 on La Réunion with an alignment carried out over the sea. The island’s coastal infrastructure has long been vulnerable to closure and damage from tropical cyclones, heavy wave action and seismic risk, which is the reason the road was moved offshore in the first place.
The route includes 5.4 km of elevated bridge built between 80 m and 300 m from the shoreline, which makes every pier a sea bridge foundation rather than a land one. The viaduct carries precast concrete deck segments on 48 marine piers, each founded on rock strata at seabed level. Table 1 sets out the conditions the foundation interface had to work within.
diameter
requirement
loading
seismic
The solution
Proserve was commissioned to supply an engineered insitu concrete foundation using its foundation grout bag system, pre-attached to the underside of each pier base. Six independent grout bag compartments were fitted beneath each unit in the casting yard, in factory-manufactured woven fabric that is grout-tight but water-permeable, then filled in place once the pier was landed.
The design team required a foundation interface that would ensure intimate contact between the precast pier base and the gravel bed or rock outcrop beneath it, accommodate irregularities in seabed elevation without additional underwater concrete levelling, eliminate long-term settlement so pier head height and alignment hold across the viaduct, enable fully surface-controlled installation and grouting to avoid diver activity in high-flow marine conditions, and provide structural durability in a zone subject to wave impact, current scour and potential seismic acceleration.
Grouted fabric formwork was engineered to deliver a tailor-formed concrete base insitu through remote-controlled grouting. The formwork was sized for differential seabed conditions of up to 900 mm, which meant the contractor did not need to lay fine-grained stone beyond what was required to enable a moulded concrete interface.
The precedent mattered on a project of this exposure. The same system had already been used beneath the caissons of the MOSE flood barrier in Venice and at the Second Severn Crossing, so the filling method and the vent control were proven before La Réunion adopted them.
Constructability and installation
Everything that could be done dry was done dry. Each pier base, either 20 m or 23 m in diameter, was cast in a controlled yard environment, and the six compartments were attached beneath it during fabrication. Each compartment was then condensed and folded for transport using break ties and protective sheathing, connected to dedicated cast-in filler sleeves running up through the pier shaft to deck level, and fitted with perimeter vent sleeves carrying grout sensors for pressure monitoring and fill confirmation.
That arrangement is what removes the diver from the critical operation. The filler sleeves were cast in and run up through the pier shaft to deck level, and the perimeter vent sleeves carried the grout sensors, so filling was both delivered and monitored from the surface.
Cast and fit at the yard
Each pier base, 20 m or 23 m in diameter, was cast in a controlled yard. Six independent grout bag compartments were attached beneath the unit during fabrication, in grout-tight but water-permeable woven fabric.
Condense, fold and sheath
Each compartment was condensed and folded for transport beneath the pier using break ties and protective sheathing, connected to cast-in filler sleeves running up the pier shaft to deck level.
Position offshore
The piers were transported out and positioned by gantry crane working off the temporary construction trestle, landing onto a pre-prepared gravel working layer on rock outcrop.
Hold on jack legs
Four external jack legs gave temporary vertical and lateral stability until grouting was complete.
Grout and monitor
A high-flow sand:cement micro-concrete, batched onboard, was tremie-pumped through the prefixed filler sleeves. Each operation ran 12 to 16 hours, with bag inflation watched continuously on the surface control panel fed by the vent sensor array.
The order of filling mattered. Grouting followed a fixed three-stage sequence, designed to seat the base axially first and then work outward.
Begin with the central compartments, to ensure axial seating
Progress outward radially, to minimise potential lift or rotation
Halt automatically once full contact pressure is detected at each vent
That controlled sequence allowed both macro tolerance, meaning bed undulation, and micro tolerance, meaning gravel layer thickness, to be compensated within the foundation footprint. Grouting took 12 to 16 hours per pier, with bag inflation monitored continuously on the surface control panel fed by the vent sensor array, and the four external jack legs holding the unit until the fill was complete.
Outcome
The system gave consistent, repeatable foundation construction across all 48 pier locations, despite highly variable sea conditions and difficult site access. It delivered a cumulative foundation area of over 17,000 m², approximately 3,500 m³ of grout placed entirely from the surface, fully monitored compartmental filling for every pier, and load-bearing interfaces free from voids, washout or settlement potential.
Eliminating the large-volume underwater concrete pours and the diver-placed levelling significantly reduced risk in the offshore work zone. The system also provided long-term stability, contributing to the accuracy of the bridge deck alignment and the durability of the structure in a high-energy marine environment.
Why it suited this project
- Adaptability to complex bed profiles
- Surface-only installation, enabling rapid and safe execution with no divers in the filling operation
- Controlled uplift and internal pressure, preventing rotation of the base during grouting
- A high-performance contact interface, with no reliance on compaction or blinding layers
- Seismic and hydrodynamic resilience, supporting the design intent in a cyclone-prone zone
The same combination applies to marine foundations generally: structures that need precise placement, reduced marine risk and engineered reliability from a foundation formed in place.
Who did what on this project
Delivered the offshore viaduct section of the Nouvelle Route du Littoral.
Engineered insitu foundation using grouted fabric formwork, pre-attached beneath each pier base, and the surface-controlled grouting method.
Continue
MOSE flood barrier, 719 grout bag foundations
The precedent cited when this system was selected for the NRL: 20,000 tonne caissons bedded to gate-alignment tolerance on a seabed undulating by up to 900 mm, filled entirely from the surface.
Read the case study →Foundation grout bag system
The system in general: compartmented bags, cast-in filler sleeves, vent sensors, and where surface-operated filling replaces diver work.
Read →Sea bridge foundations
The NRL is an offshore viaduct on discrete pier bases. This is the same problem set out generally: founding bridge piers on the bed as found, offshore.
Read →Founding bridge piers offshore? Send us the base geometry and bed survey.
Send us the pier base geometry, the bed survey and the alignment tolerance you have to hold, and we will come back with an outline approach: compartment layout, fill and vent arrangement, and the grouting sequence. If a conventional bedding layer is the better call on your project, we will say so.
- Base geometry, diameter and weight
- Bed conditions and survey, including variation
- The placement and alignment tolerance
- Sea state, access and programme constraints