anchoring a rc tank base to rock

huangyhg

anchoring a rc tank base to rock
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?

have you considered drilling and epoxy grouting galvanised rebar into the rock?
these would need to be tested though.
csd
nothing is cheaper than excess thickness of concrete to prevent floating.
yes, on second thoughts civilperson is absolutely right.
csd
what above drilling and grouting with expansion mortar?
drile007
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.
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.
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.
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.
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.
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"?