- Case study
- Venice New
- Italy
719 Grout Bags Under the Flood Barrier Caissons, MOSE, Venice, Italy
Venice’s flood gates only hold if the caissons housing them sit level, and each caisson weighs around 20,000 tonnes. They had to be bedded onto a dredged seabed undulating by up to 900 mm, under tidal flow constraints, with limited diver access, and the works were not reversible. Manual levelling and conventional concrete infill were both ruled out.
Location
Malamocco & Lido San Nicolò inlets, Venice
Client
Grandi Lavori Fincosit
Designer
Technital
System
Grout Bag Foundation System
Period
2012 – 2014
Results at a glance
Grout bags fully filled
Across both the Malamocco and Lido San Nicolò inlets
Seabed undulation absorbed
Local variation taken up by the bag, with no prior levelling
Per caisson supported
Units up to 60 m long and 48 m wide, held to gate tolerance
Contents
- MOSE closes the three inlets of the Venice lagoon with mobile steel gates housed in precast caissons up to 60 m long, 48 m wide and around 20,000 t.
- The gates only hold if the caissons sit level, and they had to be bedded onto a dredged seabed undulating by up to 900 mm. Manual levelling and conventional concrete infill were both ruled out.
- Remotely grouted fabric formwork was fixed beneath each caisson and filled from the surface, expanding to meet the bed as found.
- 719 bags were filled across the Malamocco and Lido San Nicolò inlets, with vent feedback confirming uniform contact and no overpressure or blowout.
The constraints
MOSE is the flood defence protecting Venice and its lagoon from tidal surge. The city has long suffered acqua alta: the 1966 flood reached 194 cm above mean sea level, and by the early 2000s tides over 110 cm were occurring more than 60 times a year.
It closes the lagoon’s three inlets with mobile steel flap gates housed in precast concrete caissons, each up to 60 m long, 48 m wide and around 20,000 tonnes.
The gates only hold if the caissons housing them sit level. Each had to be bedded onto a dredged seabed undulating by up to 900 mm, under tidal flow constraints, with limited diver access, and the works were not reversible. Manual levelling and conventional concrete infill were both ruled out.
Proserve’s remote grouted fabric formwork was selected against four requirements. Table 1 sets out each, and what the system does to meet it.
bed as found
of up to 900 mm
contact
grouting
involvement
filling
sensors, real-time
Together those give a low-risk, high-control foundation method suitable for large-scale marine structures, with the filling both operated and monitored from the surface.
The solution
The grout bag units were fabricated from grout-tight, permeable woven fabric, pre-fixed beneath each caisson and protected within PVC sleeves. Internally compartmentalised, the system allowed controlled expansion and conformance to the seabed.
Each unit was equipped with prefixed filler sleeves and upstanding vent pipes containing embedded sensors, so surface teams could monitor grout rise and pressure throughout the fill.
Full-scale development trials were carried out by the parties in 2009: bag expansion under pressure in simulated trench conditions, grout delivery verification with a neat cement mix, and CFD modelling of tow and launch forces to guide the restraint design. Fig. 2 is one of those test fillings.
Constructability and installation
The preparation was done in the dry. The bags, their prefixed filler sleeves and their vent pipes were fitted beneath each caisson at the Malamocco yard, held hard into their recesses and protected within PVC sleeves. The grouting itself then ran fully from the surface, which is what eliminated diver involvement.
Cast and fit at the yard
Caissons were cast at the Malamocco yard and the grout bags fitted beneath each base, held hard into their recesses and protected within PVC sleeves.
Launch and tow
Each unit was launched using a ship lift and towed out to the inlet. CFD modelling of tow and launch forces had guided the restraint design.
Position on the trench bed
The caisson was placed within its pre-prepared trench bed and held on temporary jacks while the base was grouted.
Grout each compartment
Surface-mounted pumps delivered neat cement grout into each compartment in a controlled sequence, filling the internally compartmentalised bag progressively.
Confirm and shut off
Vent sensors triggered shut-off once full contact was confirmed. A typical caisson was filled over a two-day period, with no diver intervention at any stage.
The design of the fill allowed uplift to be controlled and washout to be avoided, and assured full base contact across complex seabed profiles. A typical caisson was filled over a two-day period, without diver intervention. Vent sensors triggered shut-off once full contact was confirmed.
Outcome
The system performed reliably across all installations at both the Malamocco and Lido San Nicolò inlets, with 719 grout bags fully filled. Full grout contact was achieved beneath each caisson, and vent feedback confirmed uniform filling with no sign of overpressure or blowout. The foundations supported precise caisson placement and held the tight tolerances the gates need to stay in alignment.
Since the MOSE system was activated in 2020, Venice has remained protected during several high tide events that would historically have flooded much of the city. In November 2021 and again in December 2022, tides exceeded 130 cm but were fully held back.
- Full surface-based installation, with no diver dependency
- Foundation tolerances met across multiple large-scale units
- Long-term structural stability established in difficult conditions
- A continuing contribution to the protection of Venice from tidal flooding
The project reinforces the capability of Proserve’s automated fabric formwork to deliver repeatable, engineered foundation solutions in some of the world’s most sensitive marine environments.
Who did what on this project
Delivered the caisson works at the Malamocco and Lido San Nicolò inlets.
Named as engineer on the project.
Grout bag foundation system, development trials and surface-operated grouting method.
Continue
Al Faw Grand Port, 1.75 km piled jetty
The same fabric-formed principle at the other end of the scale: 430,000 m³ of imported rock armour replaced by 26,500 m³ of locally batched concrete, and scour protection taken off the critical path.
Read the case study →Grout bags
The foundation grout bag system in general: fabric, compartments, vent control, and where surface-operated filling replaces diver work.
Read →Caisson foundations
MOSE is a caisson set into a trench bed. This is the same problem set out generally: bedding a large precast unit onto an unlevel seabed to tolerance.
Read →Bedding a heavy precast unit onto an unlevel seabed? Send us the unit and the bed survey.
Send us the unit size and base detail, the bed survey including undulation, and the tolerance you have to hold, and we will come back with an outline approach: compartment layout, fill and vent arrangement, and the filling sequence. If a conventional bedding layer is the better call, we will say so.
- Unit size, weight and base detail
- Bed conditions and survey, including undulation
- The placement tolerance you must hold
- Diver access, tidal window and programme