precast concrete water storage tank 3

[PSUengineer1]

I am looking for information on the construction of precast concrete water tanks. I have a 6000 gallon tank constructed using three, 4-foot high sections(7'x17'x 12' deep). Cracks have developed at the corners of the tanks and the long walls bulge upon filling the tanks with water (no backfill present when water placed in tank). I am interested in gathering any industry-accepted code information that may be out there; specifically can anyone point me to code references regarding backfilling prior to filling tanks with water? thanks.

 

[JedClampett]

Hopefully, the tank was designed by a structural engineer. A 12 foot tall tank tall tank needs to be engineered. If the corners are cracked and walls are bulging, it tells me either it wasn't a designed or designed poorly.
There's no code prescribing backfill procedures. But a water tank should not need backfill to resist internal pressures. A correctly designed tank can resist soil pressures without liquid and liquid without soil pressures.

 

[Brad805]

Load test procedures for a a watertight tank design usually require them to be completed prior to backfilling. This is done to account for the potential for a change in site conditions. The backfill pressure should be used as bonus to resist the water pressure, not a means to reduce the quantity of rebar. The permissible crack width will depend on whether or not a liner was required. If it has a liner, then you are limited to the width for a general wall exposure. If there is no liner the design should limit the crack width to approximately 0.12mm. There are many references for watertight concrete structures. I believe the clauses you are looking for are in the ACI chapter for Environmental Engineering Structures (ACI350R). I do not believe there is a specific code for a precast watertight tank. ACI224.3R-95 Joints in Concrete may be of use for the joint design requirements.

I agree with Jed. Bulging visible to the eye suggests a fundamental problem.

 

[hokie66]

Some further detail might help. How were these precast sections connected? Is it post-tensioned?

 

[PSUengineer1]

@hokie:
the precast sections were connected shiplap style with conseal joint sealant. I have no detail from precaster at all, no engineering design drawings, specs, etc. I don't even know f'c.

 

[PSUengineer1]

@ all: see one attached photo
to be more specific about the cracks: the cracks are vertical and are typically located at the ends of the long wall at the shorter return wall. I do not have a measurement of crack widths, some are filled with sealant. The cracks are only located at the middle panel and I can only see a small amount of the top portion of the cracks for the tank walls are partially backfilled. The cracks are near the tank corners and located on the intersecting shorter wall. cracks resulting from bulging? I could not measure bulging due to backfill soil.

 

[JedClampett]

Technically, you have what is known in the industry as a piece of crap.
The precaster either didn't know or could care less that this is being used to hold liquid. Hopefully he was told this when this was ordered. I'd call him up, tell him his tank is failing, and ask him what he's going to do to make it right.
A lot of times these sections are not used to hold water, but for vaults and low pressure manholes. So they mostly resist outside (soil) pressures. And those are much easier to design for.

 

[hokie66]

Agree with Jed. What you have is rubbish, not a tank, but some blocks of concrete stacked to form an enclosure.

 

[LittleInch]

Absolute gem. It took me a while to work out this seems to be three rectangular boxes stacked one on top of another.

No idea how thick or what re-inforcement etc, but looks like a disaster in the making to me....

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[CELinOttawa]

You don't need a code to declare this unfit for use. It is unfit, and should be drained, demolished, and replaced.

I do not believe repair is a viable option.

Listen to Jed, Hokkie, and LittleInch. Cuddos to Brad for answering the actual question, but you've asked the wrong one OP. There is no need for technical information beyond fundamental design theory: It may not have collapsed yet, but to satisfy SLS the structure must be fit for use.

For once I think we are all signing from a common hymn book for very good reason.