Tie-down anchors to be installed under water

[peek]

Dear fellow Engineers,

This is my problem. I need to build a cofferdam within an existing canal. The subsurface material is rock and it is very permeable. My plan is to first construct a braced cofferdam and then construct a concrete tremie seal (3 ft)/tie-down anchors all under water. Once the tremie slab hardens, the overlying water can be pumped out. It is expected that the anchor/slab system will resist the hydostatic uplift pressures (about 1200 psf). So this is the question. Can these anchors be installed and then fastened to the slab, all under water? Any suggestions?

 

[SlideRuleEra]

Often thickness of the seal is calculated to resist flotation by dead weight alone, even if this means the seal is very thick (10 or 15 feet thick would not be unusual)

Then you avoid the type problem and safety concerns that you are seeing.

Keep it simple - cofferdam failure is catastrophic.

http://www.SlideRuleEra.net

 

[wmta]

I have design cofferdams both ways. The concept of a relative thin tremie slab is very difficult to install in practice. This is one of the most difficult cofferdam problem, i would recommend that you have someone with lots of experience in this type of work design or review the process.

If a failure (an open channel to water) occurs, the problem is extremely difficult to repair.

http://www.wmta.com

 

[PEinc]

SlideRuleEra is correct. I've seen many plans for DOT bridges in cofferdams where the engineer shows very thin tremie slabs. Usually, these thin concrete slabs are not thick enough to resist the uplift pressures. If you make the tremie thicker, the cofferdam needs to be designed for a deeper excavation. If you use the thin tremie slab, you will need additional resistance to uplift, such as tiedown anchors. However, it will be very hard to install tiedown anchors through a thin tremie seal and under water

With rock at subgrade, you should not have boiling, piping, or a quick condition due to unbalanced hydrostatic head. However, you could have a lot of leakage through the rock and under the sheets if the rock surface is uneven and if the sheets can't penetrate a little into the rock. Possibly the seepage could be handled by pumping.

You need someone who is very experienced in cofferdam design.