How to preventwater from excavation side from under water table excavations

[Nasir1956]

I am executing a project to construct a RCC water reservoir sized 26 MTR long x 10 MTR wide and 4.50 MTR deep
water table is at 1.90 MTR deep
construction site is very near to sea. I reached to the required depth and poured the lean concrete thickness varies from 250 MM to 180 MM thick. I am dewatering around the newly poured lean concrete.
Now I want to start the structural RCC construction for new reservoir. Water stopped from the bottom after lean concrete but I have big issue of water seepage from sides of the excavation. I have extended the floor lean concrete from edge to edge of excavation.
I was thinking today, to pour lean concrete to the sides of excavation up to water table level.
Will it work and will stop water from sides of excavation ??
Secondly, under these circumstances, its very hard to install forms and pour the concrete for walls in single pouring. Can I pour the walls first up to height of water table, and then after backfilling to do the second poring up to required top level ??

 

[oldestguy]

If you are down below the water table and the submerged weight of the concrete there now is less than possible uplift from the water table (now or at high tide), be prepared for that concrete to be lifted. As to your side of excavation stability being questionable, it would seem that you need pumping from well points, possibly even connected to drains under the slab, as well as in a "line" surrounding the excavation, to be able to complete the job without significant problems. A perimeter ditch may do the job, but I doubt it.

 

[SlideRuleEra]

+1 to oldestguy's comments. The only reason the bottom lean concrete has not lifted already (blown out) is the the water flow from the sides relives the uplift pressure on the bottom. If the side were successfully "sealed" with lean concrete... the bottom would likely "blow out" then.
Suggest getting a geotech engineer to investigate. If well point's won't do the job, a cofferdam may be required.

For your second question, concrete can be placed under water but constructing a formed properly reinforced structure underwater is not practical.

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[jike]

The amount of water seepage depends on the type of surrounding soil and the water table level. I agree that you most likely will need well points and continuous pumping. Continuous pumping is will be needed to prevent blow-in from the bottom and sloughing from the sides. Pouring concrete against the excavation slope may prevent sloughing but not blow-in from the bottom unless you pour a very thick mat underwater to counteract the potential uplift and seal it properly to the sidewall.

Is there a geotech on the job that can give you advice?

I once did a deep excavation next to a wide, flowing river with sandy soil. With the properly designed and installed well points the excavation was completely dry during the construction process.

 

[dcarr82775]

wellpoints or coffer dam. I have done several shoring walls extending 20 feet + below water,they were all coffer dams.

 

[PEinc]

A sheet pile cofferdam will not stop the inflow of water through the base of the excavation unless the sheet piling is toed into some impervious layer. Without this impervious seal, the sheet piling, if deep enough, can prevent the subgrade from becoming quick but there will still be seepage that needs to be pumped.

http://www.PeirceEngineering.com

 

[oldestguy]

As to sheet piles, remember the S.F = 1 as the limiting seepage water gradient. Otherwise you have a quick condition inside the excavation. Keep it closer to 2.0 due to unknowns. That gradient is seepage length divided by the water head from outside to inside.