How do I anchor an underground tank?
How do I anchor an underground tank?
(OP)
I have a large underground concrete tank that is designed to be water tight. The water table could be above the lid resulting in extremely high bouyancy forces. To help get an idea the tank has about 3,000 ft^3 of volume - I would need a 7' thick x 20' x 12' concrete deadman to anchor it too to resist the bouyancy. There is no bedrock nearby to anchor to either. One thought was to pour a skirt - concrete wall that extends around the tank such that the soil above this wall helps keep the tank from floating - this skirt would be 8" tall and as wide as needed to resist the force - but will this work? If I have to assume the water table is above the tank than will there be any soil to help resist the uplift?
Any thoughts?
thanks
Any thoughts?
thanks






RE: How do I anchor an underground tank?
RE: How do I anchor an underground tank?
RE: How do I anchor an underground tank?
Concrete is cheap, anchors and fancy design details are expensive. Plus it always works.
RE: How do I anchor an underground tank?
On the deadman idea, keep in mind that you're just basically pouring concrete in a hole, so it's not as expensive as you might imagine. Possibly use flowable fill to backfill and consider the weight of it?
RE: How do I anchor an underground tank?
The lip should help. 3000 sq. ft is ~190,000 pounds of uplift. If you have a 20'x20'x7' deep tank and add a 2' wide skirt, you have 1120 cubic feet of soil, which even at a buoyant weight of 50 pounds per cubic foot is 56,000 pounds. If the tank is buried several feet in the ground, you can also use the soil weight on top of the tank.
You should talk to the project geotech about this.
RE: How do I anchor an underground tank?
I have always considered the weak tension link in helical piers to be the single bolt connecting shaft segments. The tension capacities never seem to work for the bolted connection I have seen.
RE: How do I anchor an underground tank?
RE: How do I anchor an underground tank?
RE: How do I anchor an underground tank?
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: How do I anchor an underground tank?
An interesting side light on this. The lead company didn't want to do this anchoring, and they questioned our use of a 1.5 safety factor. We agreed that it seemed a little high but was normal. The lead then called a conference of chief engineers from local peer companies. They all agreed that 1.5 was high and that something less would work. Things stuck there until my boss went around the table asking what factor each individual would use. They were unanimous at 1.5 because there was no published tests or recommendations for less.
When I was researching for hold down methods, I found that one engineer had poured a base slab on grade, laid out steel ingots on it in a pattern, drilled dowels in between and then completed the concrete placement to above the ingots.
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: How do I anchor an underground tank?
On a few where it has been warranted, we have gone the route of drilled piers but as I mentioned before we typically go with the mass concrete anchor.
RE: How do I anchor an underground tank?
Assuming your tank has a mat foundation, I think just throwing extra yardage at it can be problematic.
You can't have any cold joints so the second you start pouring you've committed. Not sure where you are located but where we do many of our tanks, in the rural adirondacks, batch plants are often an hour away with only a few trucks. The second there is a hang up - say a broken pumper - your trucks stack up, go over the time limit, and can't roundtrip in time to avoid the previous placement setting up before the pour continues.
Only reason I mention it is because you're doing a small 3000cf tank so I figured you may be in the stix like me. I've been present for the pours this week and last week on a 4000cf tank and we've continuously dealt with hang ups that have made getting these modest pours accomplished without cold joints pretty challenging. IMO the smaller the pour the less can go wrong. Plus - correct me if I'm worng, but with a giant mat arn't you still responsible for meeting ACI 350 crack criteria and would have to fill it to the brim with bar? 50cy of $180/cy mix is $9k extra but that could be exceeded by steel and labor.
RE: How do I anchor an underground tank?
RE: How do I anchor an underground tank?
If you have ever seen CDF poured, it is very fast for each truck to empty. Common time per truck is under 5 minutes for a 9 to 10 yard load, then move on to the next. And there are no concerns with construction joints as it is done so quickly. No consolidation, no finishing. No worries.
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: How do I anchor an underground tank?
RE: How do I anchor an underground tank?
RE: How do I anchor an underground tank?
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: How do I anchor an underground tank?
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: How do I anchor an underground tank?
RE: How do I anchor an underground tank?
msquared - like paddingtongreen said, we call it flowable fill or CLSM. Yes, I've seen it poured. In our area its $90/yd... barly cheaper than 3000psi, so it rarely is used. No kidding, its that expensive.
My point- which may be moot - was that the less yardage and complexity in anything that must be monolithic the better. 5 minutes to unload 10 yds is fast but if it will be a couple hours before he can come back with 10 more we can have issues.
RE: How do I anchor an underground tank?
You do not have to waqit for the first poir to set, but can make the second, third, etc immediately after, depending on the batch plant and the pumper capacity...
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: How do I anchor an underground tank?