Offshore grouting

[LRJ]

Something I've always wondered is how the water content of offshore cement grouting is specified. Surely the grout is surrounded by water, so there is relatively little control regarding the mix? Are there any references for this? I'm guessing the cement can only take so much water in the reactions, so is that what is controlled to ensure sufficient strength?

 

[London]

The cement is mixed offshore on the vessel. Seawater may be specified instead of freshwater for the mix. The grout is pumped into the void in a controlled manner so that raw seawater does not contaminate the grout except at the immediate interface. The pumped grout pushes out the seawater from the bottom of the void upwards. Once the chamber is full, the grout is overfilled to displace the contaminated grout, leaving the correctly mixed material in the void.

Having the right admixtures to ensure pumpability is important, but even so a strength of up to 85MPa can be achieved after 36-48 hours (according to Sika)

 

[LRJ]

Thanks for clarifying how control is ensured.

Do you know how reliable the grout pours are in terms of strength? I would presume that, despite the measures you describe, this is a lot less controlled than onshore grout pours, so the reliability must be less also.

 

[BUGGAR]

As an aside, when pumping grout through a tube underwater, we sometimes put a plug of excelsior or similar in the tube and pump this plug ahead of the grout to keep it from mixing with the water in the tube. A certain amount of grout is contaminated and must be "wasted" when doing this.
As London notes, pump the grout from below so only the surface of the grout can mix with the surrounding water.
See "Tremie Seals"

 

[LRJ]

Thanks for the additional information.