The safest foundation design for partly water retaining structures

Significantly more information required and your local context will also be a major input.

We have a few R/C concrete water tanks, up to 20ML (That's not 20ml) and for our locations, neither option would be suitable.

Do you have earth tremors there?

How far down to solid foundation material?

Is the ground water charged?

What compaction is possible?

etc. etc.

Consult local civil engineer with experience!

Hi,

No earth tremors here. First depth of 4.5m, silty CLAY strata with SPT -N value gradually increase from 0 to 50 ,

Next depth 4.5m till 13.5 m, silty SAND, SPT N value >50. no soil record after 13.5m depth

Ground water may vary during dry and rainy season.

Tank is sunken 1.2 meter below groud level and base slab is below water level during wet season.

It will still be best to call a local engineering company for save results.

Great! That plus the information in the original post is a good starting point for the local Civil Engineering Consultant, who needs to be hired locally to progress the design. Next stop? Yellow Pages. Good luck with the project.

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Piled.

Consult a local engineer.

There is no one way of tackling your job. All foundation solutions are tailored to the specific site and structure. When you consult a structural or civil engineer they will probably not come back with the "safest" solution as that would be prohibitively expensive. They will come back with a solution that is both adequate and cost effective. Knowing where to strike that balance is why these engineers can charge so much for their services while still providing a solution at the lowest overall project cost.

You've provided very limited information, so it's not possible to provide you with specific recommendations.

First, you need to consult with a Registered Geotechnical Engineer.

Second, you need to consult with a Registered Structural Engineer.

The two engineers can work together!

Isn't a little too late to worry about settlement and cracking of this tank? That should have been addressed before the tank was constructed. Anything that you do now is likely to be very expensive and be like putting a small Band-Aid on a large wound, the cost of which may actually exceed the cost of constructing a new water tank.

Seriously weigh all your options, and come up with several of them to ascertain the least costliest and the most feasible.