Ground Water Pressure Relief Valves in Tanks

[jjezmarlo3]

On our below-grade tanks, we generally try to use the self weight of tank walls and slabs to prevent tank uplift. However, for the project I am working on now, self weight isn't going to economically work. I was looking into ground water pressure relief valves, and it appears there are two main types, wall-type that goes through the tank side walls, and floor-type that goes through the tank base slab. From what I can tell, the wall-type valves are slightly more reliable than the floor-type, but they also allow more head of water to build up (as they are further up on the tank wall) and would not seem to work as well as the floor-type in basins with a large base slab, such as a large clarifier. I am curious to know what other firms spec (wall-type or floor-type), and if they have worked well. Thanks.

 

[hokie66]

The only experience I have had with pressure relief valves is in swimming pools, not tanks. The ones in swimming pools are located in the bottom. They do require maintenance, generally replacement after 10 years or so. When a pool starts losing water more than from evaporation, the pressure relief valve is the first suspect.

 

[cvg]

just discussing these the other day and the design teams concensus was that a) they are un-reliable b) can't easily be inspected or maintained c) they tend to be used only as a safety factor on somewhat lower risk applications such as clarifier tanks. Additional concrete is much more reliable. For our project we are using rock anchors

 

[Ron]

Agree with cvg...anchor the tank through ground anchors or concrete dead weight. Have designed a lot of partial bury tanks and anchoring was often necessary.

 

[Hemifun]

Our firm designs a lot of below grade tanks for wastewater plants and we quit using the ground water relief valves over 20 years ago after we actually had a valve malfunction and a large clarifier floated about 2 feet out of the ground. We now use the tank weight exclusively to counter act the ground water lift forces. Don't forget, by extending the bottom slab past the walls you get additional slab weight plus the weight of the soil above the extended portion of the slab.

 

[JedClampett]

Our company discourages the use of "pop-up" or other check valves for the reasons stated above (clogging, malfunction, etc.). They're also messy during construction and maintenance. But sometimes, we don't have a choice. In those cases, we use both wall valves and pop ups in the floor. But before you start down that path, figure out what it's going to cost to ballast the tank. It's slab on grade concrete, very cheap.
Funny story. One time we had a contractor who got tired of the water seeping up from a pop up in a floor. So he (probably one of his laborers) put rocks on the pop up to stop the leak. It worked so good that it failed the floor.