Immersed Tube Tunnels (ITT) are often used in marine tunnel construction to reduce the amount of work undertaken in the water or when other tunnelling methods are not appropriate. These massive caissons are cast in the dry, floated into position, placed on the bed, and connected to the previous segment to create a tunnel.
Often, stone bedding layers are used as the foundation, these are inherently variable when constructed underwater, and can be onerous to level, liable to settlement, washout, and scour.
Compared to stone bedding layers, grout bag foundations are well suited to ITT foundation applications where:
- Construction tolerance is tight
- Settlement is undesirable
- There is a risk of stone washout or scour during or after construction
- When accurate stone levelling is not practical
- When working on a rock bed
- In seismic zones
Foundation grout bags are specifically developed with the designer and contractor for each project’s particular requirements. Our engineers use a range of variables, not limited to:
- Bed tolerance
- ITT element level tolerance
- Initial stability requirements
- Shear transfer
- Uplift limits
- Transport distances
- Working environment
- Installation windows
The formwork and grouting pipes can be prefixed to the concrete element in the casing yard or placed on the bed ahead of ITT element positioning. Once the element is submerged, connected to its neighbour and held on jacks, the grout bags are pump filled. The formwork controls the grout, filling the void between the element and the bed surface in a carefully designed sequence for guaranteed void filling and full contact with controlled uplift pressures.
Foundation grout bags provide control during construction by:
- Achieving tight construction tolerances and little settlement
- Absorbing large bed tolerances
- Eliminating the risk of washout
- Initial stability where required
- Even distribution over the caisson base
- Eliminating requirement for stone layers when working on rock
Performance has been demonstrated by 30 years of proven performance on some of the world’s largest marine construction projects.