Caissons are used in breakwater construction to reduce the amount of marine construction required in high wave climates and when rubble mound breakwaters are not suitable. Initial and permanent stability of the caissons is essential as they could be subjected to design wave heights and forces even during construction.
Stone bedding layers are often used under the caissons in combination with porous rock armour in front. This combination allows wave impact pressures to not only act on the front face of the caisson, but also the bottom which can cause instability if the weight of the caisson is insufficient for the conditions.
When on a rock or clay bed, an impermeable grout bag foundation can be used in place of bedding stone to remove the uplift pressure from underneath. It also eliminates the risk of washout of the bedding stone from under the caisson, resulting in greater stability and or a possible reduction in caisson size.
In addition, impermeable concrete mattress scour protection can be used around the foot of the caisson to prevent underscour of the grouted foundation or loss of bedding stone. These mattresses can be up to 1.2m thick and mechanically connected to the structure to resist any uplift forces. This modest thickness can completely remove the requirement for rock armour protection on the seaside. See this Wave Protection for more detail.
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
- Caisson level tolerance
- Initial stability requirements
- Shear transfer
- Uplift limits
- Transport distances
- Working environment
- Installation windows
The formwork and grouting pipes are prefixed to the concrete element in the casing yard. Once the element is submerged and positioned on jacks, the bags are pump filled from a grouting barge or from land. The formwork controls the grout, filling the void between the element and the bed surface in a carefully designed sequence. This guarantees void filling and full contact with controlled uplift pressures.
Foundation grout bags allow increased construction reliability and performance of the permanent works by:
- Providing a foundation that cannot be washed out or scoured, especially during construction before other forms of protection are placed
- Reducing caisson sizes by reducing wave forces on the structure through eliminating uplift pressures
- Absorbing large bed tolerances which can occur in dredged rock beds
- Even load distribution over the base of the caisson (eliminates differential settlement)