Water Tank Pressure Relief

[miecz]

I'm designing an in ground concrete tank for a fish hatchery. The water table is high and the tank would need a 54 inch thick mat to resist buoyancy. Is it feasible to use a pressure relief valve for this type of application?

 

[SteveWag]

Clow lists a "F-1492" for floor mounting and a "F-1494" for wall mounting. Unless the floor is reinforced to resist uplift (designed as an upside down beam), use the floor mounted valves. I guess the normal W/L in the tank is above max ground water, if not the tank will just fill with ground water.

Steve Wagner

 

[JedClampett]

Make sure the end user understands that they'll not be able to lower the level in the tank when the groundwater table is high. You don't want them pumping the tank dry to work in it. Also, the contractor will need to provide some groundwater control during construction. Workers have been known to put rocks on the groundwater relief valves to keep the tank dry.
Also that you might get some groundwater in the tank during normal operation. Will that bother the fishies?

 

[kslee1000]

Maybe I am confused. How the pressure relief valve (alone)can solve the uplift problem? I guess the floatation occurs when inside is dry (inflow of water is not desired), and the ground water is high. If that is the case, you will need a drainage/dewatering system (consists of pumps ,valves & free water flow chennels - drainage backfill or drainage fabric) around the sides and the bottom of the tank to lower the groundwater table, thus reduce the uplift pressure.

 

[JStephen]

Seems like you'd need to install some sort of gravel and drain system to make sure ground water flow into the tank was adequate.

You might also consider how water-tight the valve would be after many years of operation in mud and silt.

 

[miecz]

Thanks for the responses.

SteveWag- Those were the valves I was looking for.

JedClampett- Good ideas. I've just learned that the design water table isn't as high as I had feared, so the problem goes away.

kslee1000- Not sure I understand your question. I was thinking that, in the event of the ground water reaching the design level when the tank is empty, the valves would allow ground water into the tank to offset the buoyant force. No drainage would be needed to lower the groundater during operation.

 

[SteveWag]

All most 40 years ago I had a 45' dia. steel tank with a 12" slab on grade base float in about 16" of water when a local stream got out of the bank during a hard rain. Be very careful about ground water and the level it can reach. I assume the tank is very close to a stream if working in a fish hatchery.

If frequent draining of the tank is required, I have installed a sump to accept a pump suction and put a foundation drain around the tank. This will allow ground water to be lowered so the tank can be drained. If ground water is above the valves the water in the tank will be at the same level as the ground water. I usually use three 4' valves in a 50' tank and provide a stone water way (ditch filled w/ stone under the base slab) from the valves to the foundation drain

 

[cvg]

I would second stevewag - if your design life for the tank is 20 years, can you be sure that 20 years from now - the water table won't be high enough to cause flotation? The drain valve is useful but flotation is still possible with them.

Especially since global warming will have melted most of the polar ice caps? LOL

 

[kslee1000]

Jmeic:

For your original question, as pointed out by stevewag, the answer is "yes", it is one feasible option. I was only trying to point out that it is more involved, and there is more than one method, to manage the high ground water situation.

In addition, no matter what type of system is adopted, early warning (on system failure) and emergency set-ups are equally important. We had a case in Michigan, the concrete water tank (emptied for maintenance) was pushed up and the structure failed after a rain storm. The reason for the excessive uplift pressure was not a big surprise - clogged drains in conjunction with failed pumps.

 

[ahunt]

In ohio the prv's freeze up. We've seen tanks float in the early spring when the prv's are froze and large rain events take place.

We have used uplift piles and rock anchors, and in Florida, we just use mass (thick members).

 

[miecz]

Thanks, everyone. I can see that I've got lots to think about. There is a stream nearby. That's what promted my original post. I then learned that borings taken after a wet spring show the water table at 6 feet. I'll have to wait for the geotech's recommendations. Freezing valves really worry me.