anchoring a RC tank base to rock

[renko]

Hello,

I'm wondering if there's an easy and cheap way to anchor a reinforced concrete base of a tank to underlying rock,to resist flotation.

I'm designing a Washwater Recovery Tank. The Site Investigation shows a water table upto 1m below ground close to the site. Using this as my level for flotation, I have to increase the weight of the tank.

The Contractor says he is excavating down at least 4m before he gets any ground water. And of course he doesn't want any extra concrete on his bill just for flotation.

Any suggestions for a cheap anchoring system please?

 

[csd72]

Have you considered drilling and epoxy grouting galvanised rebar into the rock?

These would need to be tested though.

csd

 

[civilperson]

Nothing is cheaper than excess thickness of concrete to prevent floating.

 

[csd72]

yes, on second thoughts civilperson is absolutely right.

csd

 

[drile007]

What above drilling and grouting with expansion mortar?

Drile007

 

[BigH]

Civilperson has a point if the tank is relatively small. If the tank is large, like 100 ft in diameter, I think that the concrete would become a bit expensive. You should then consider rock bolts (tie down bolts). There are established design methods for these.

 

[renko]

Thanks to all,

I was looking at grouting rebar into the rock. Unfortunately the Contractor and I disagree on the likelyhood of the water table rising above 4m below ground. But it's his bill and his risk. And I know nobody will get hurt if the tank lifts. Still, I don't like the idea of a tank I've designed setting sail sometime in the next few years, all for the sake of a few extra loads of concrete.

Anyway, I will be instructing a good drainage material for backfill so the water has a choice to move on if possible.

 

[Ussuri]

Have you considered adding a skirt to your tank? This will allow you mobilise the weight of the soil on top of the skirt to resist uplift.

 

[renko]

Yep,

I've included a 'skirt' except I call it a toe, some others call it a heel.

It does help, but the tank is 20m x 15m (about 66' x 49') on plan so this leaves the base resisting uplift in bending and shear. I've got this to work with a 475mm base and some hefty rebar.

 

[jgailla]

Just because the contractor is hitting groundwater at 4m does not mean the water table will not be higher in the future. If your geotech says groundwater is at 1m, you are opening yourself up to a lawsuit if you ignore the geotech recommendation and something happens.

 

[Gumpmaster]

Can you provide relief valves in the tank to let groundwater in?

Can you provide a groundwater monitoring well and in the O&M manual state "do not de-water the tank when groundwater exceeds XX elevation"?

 

[dbuzz]

Rock bolting is actually quite expensive. I'd just add more mass in concrete.

 

[hokie66]

I think it depends on whether there is a drilling contractor in your area who wants to do rock anchors at a reasonable price. For hydrostatic uplift, you only need passive anchors, so grouting in reinforcing bars will work.

Relief valves have a history of failing, thus causing your tank to leak out into the soil, and if you drawdown the tank quickly, can actually contribute to uplift forces.

I think anything other than mass or rock anchors is getting too technical.

 

[Ussuri]

Heavyweight Concrete? It may prove cheaper to use less volume of a more expensive material than more volume of a cheaper material.

But, as they use some weird and wonderful materials for the aggregate(metal based typically)you would need to be certain that it was applicable for tank containing liquid.

Alternatively consider a hybrid where the heavyweight provides mass only and does no structural work?

 

[vincentpa]

Go with the geotech over the contractor. The contractor has done it 1000 times that way over a 30 year career and nothing has happened. How many times have you heard that one? Sometimes contractors can be as bad as used car salesman to save or earn a buck. We are fortunate that we have geotech's in the office. I would ask them for an estimate to anchor the tank to rock and then I would give the contractor the option between using a thicker base or anchoring the tank into rock with the stipulation, take it or leave it. I'm getting tired of contractors trying to save a buck at my expense. They try to put you back on your heels to do something you are uncomfortable doing or you know is risky. Safety and servicibility should always come first, not their bottom line. If they underbid the job, we can work with them only to the extent that we do not jeopardize the safety and servicibility of the structure. That should be understood at the beginning of any contractor "negotiations". If they lose money because they underbid the job and your design passes value engineering, too bad for them. That's a lesson in business. If they don't learn it, they will go under.