Septic Tank Anchoring

[NFExp]

I'm looking for ideas to anchor a septic tank. The tank is within a VE zone and therefore subject to buoyant force. The project is in florida (no seismic).

I have heard of using auger cast piles. We specify piles for pool shell holddowns all the time.

The contractor is sugesting using "long screw anchors, hand installed, with metal straps attached to the tank".

Also the scour elevation accoding to DEP is 5.2. Grade elev = 13'. Does the achoring system need to go beyond the scour depth? I have heard that if we design the tank to sink, I can use a shorter anchoring system. It makes sense to me, what do you think?

Thanks

 

[par060]

depending on the boyant force....can't you just add more mass(concrete) to keep it from floating?

 

[msquared48]

Yep, CIP slab with straps tied to the vault to hold it down.

Mike McCann
MMC Engineering

 

[Ron]

Wait a minute! If the septic tank is that close to the water table, a drain field won't work! You are required in Florida to have a minimum of 48 inches of separation between the bottom of the drain field and the top of the high seasonal groundwater. Drain field has to be at least 18 inches deep, so you need a groundwater depth of at least 66 inches. Why would you install a tank below that depth?

Next, a septic tank must be kept full of water all the time to work at all. If your water table is going to be as high as the freeboard height of the tank, you've got a problem that a septic tank won't solve.

Next, if you are pumping to an elevated drain field, it's the wet well you have to worry about floating, not the tank.

Outside that, use additional concrete ballast as Mike and par060 noted.

 

[NFExp]

Thanks for the replies. Ron, I think you made good points. I really don't know alot about septic tanks. I was just asked to provide an anchoring system.

 

[Spartan5]

Wait a minute! If the septic tank is that close to the water table, a drain field won't work! You are required in Florida to have a minimum of 48 inches of separation between the bottom of the drain field and the top of the high seasonal groundwater. Drain field has to be at least 18 inches deep, so you need a groundwater depth of at least 66 inches. Why would you install a tank below that depth?

Next, a septic tank must be kept full of water all the time to work at all. If your water table is going to be as high as the freeboard height of the tank, you've got a problem that a septic tank won't solve.

Next, if you are pumping to an elevated drain field, it's the wet well you have to worry about floating, not the tank.

Outside that, use additional concrete ballast as Mike and par060 noted.

Ron:
It is possible to pump directly from a septic tank to an elevated drainfield, low pressure sewer, etc. using a filtered pump vault in either a single or two-compartment tank. Orenco makes a fine product to facilitate this:

http://www.orenco.com/sales/choose_a_system/effluent_pumping_systems/index.cfm

In the event that a wet well is being used, the septic tank still needs to be protected against flotation. Yes, it's usually full of water, except for when it's not (e.g. during periodic maintenance pumping). They'll come right up out the ground like a submarine next to the pumper truck if the ground water is high and there isn't adequate cover or concrete on top of the tank.

All that said, for the original poster, your client should be sure to use a high-seam tank to reduce the chances of infiltration/exfiltration. Then just make sure the top is thick enough to provide enough weight (along with the proposed amount of cover on top of the tank) to keep it in the ground.

Vacuum test the tank on site as well to ensure it's watertight, especially if they'll be pumping out of it.