Expansive Soils Under A Constructed Water Tank, I Need Your Advise!!!! 4

[Koyote]

Hi every one,

About a year ago I was part of the design team of a potable water treatment plant. One of the elements of the plant was a clear well tank (about 30 ft diameter and 30 ft height). Soil properties studies were conducted in order to determine the best location for the tank. After the construction process ends (two or three months)the tank bottom slab begins to crack and water from the outside of the tank (infiltrated) started to fill the bottom of the tank. Starting a research we found two situations: fist there was a layer of expansive soil under the tank, the other one, at a relative short distance of the tank was located (existing) an underground a water holding tank(cistern) abandoned which we thik that contributes to our problem. We tried to apply chemicals under the tank to fill the pores and avoid any infiltration to the tank and any exfiltration. It doesn't worked. Now we are thinking about other posibilities to seal the bottom of the tank. Maybe some of you have seen a similar situation or hear about it. If somebody have suggestios about how can i work this situation, it will be very welcome. This is an actual situation that i need to solve.

Thanks in advance for the help you can bring be in this matter.

Koyote

 

[Focht3]

Where is the site?

What has the project geotechnical said?

What chemicals were injected beneath the tank, and by whom?



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[ashjun]

Hi,
I refer to your observation of expansive soil underneath the tank.
Is it possible to install lime columns under the tank and penetrating deep into the expansive clay; and then treat the tank base as you probably have done. Or have you already thought of such a solution? I suspect the uplift pressure of the clay (since it now has additional water from some source) may still have not reached its zenith.

Regards

 

[gandersen]

Koyote:

I agree with ashjun that the heave that you are experiencing may not have reached the maximum. If you cut off the source of the water (by lining the tank bottom and draining and filling in the buried tank) the heave will continue but I would suspect that it would at a reduced rate.

One of the things that concerns me is that you apparently had movements prior to cracking the bottom of the tank. In other words, the water from the tank may be a secondary effect. Differential movements were occurring before water started leaking from your tank.

In circumstances like this, the least expensive thing to do may be to try to cut off all sources of water and then monitor the performance to see that the movements are slowing down.

Good luck

 

[JedClampett]

You're talking about a relatively small tank (160,000 gallons). My estimation of the cost is maybe $80,000 US. My point is for something of that cost, you could spend a lot on engineering, investigation, construction and time and still never be sure you've actually fixed the problem. I'd tear out the tank, do whatever is necessary to stabilize the soils underneath and rebuild it. Propose that the owner pay 1/3, the geotechnical consultant 1/3 and the design firm 1/3 for the demolition, repair and reconstruction.

 

[Focht3]

While [blue]JedClampett[/blue] has provided an approach that makes sense for the owner, it may not make sense for the designers.

We don't know what the project's history has been, and what key decisions led to the current circumstance. For instance, why was the clear well tank placed near the abandoned cistern? It seems likely that the owner kept sloppy records, or was lazy and did not provide the presence of the cistern - critical information - to the design team. That appears to be the owner's mistake - and may turn out to be the problem. Assuming this scenario is close to correct, I see no reason for the design team to pay for 2/3 of the cost of the new tank. I think the owner should pay the whole bill -

I am well aware that the presence of expansive clays and free water together have a strong potential to do damage. (After all, I live and work in San Antonio - we have a well deserved reputation for expansive clay problems.) But I'm not sure that the expansive clays are the proximate cause of the observed distress. They may be, but I'm as yet unconvinced.

And we haven't even touched on other common problems that occur during the design process: inadequate design fees, refusal to allow design professionals to collaborate during the design process, changes to the project layout that occur post-geotechnical study and the geotechnical engineer et al aren't paid to study the changed layout, etc., etc., etc.

And we haven't heard from [blue]Koyote[/blue] again on the issues I raised yesterday. [blue]JedClampett[/blue]'s comments caused me to ask another question: has the clear well tank's movements been surveyed, and were those survey measurements tied to a (truly) stable reference? This will be key to understanding what has occurred -

By the way - Welcome to [green]Eng-Tips[/green], [blue]Koyote[/blue]!



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[IJR]

To add a line pals

Though this one does not apply to a tank already in place, someone told me that you can cut down swelling by applying coarse gravel directly on top of the clay base.

Makes sense to me, but have never tried it before.

Does anyone want to comment on this practice for the rest of us

respects
ijr

 

[gandersen]

IJR:

I practiced geotechnical engineering for several years in an area where expansive soils were prevelant. We saw multiple instances where the construction involved the undercutting of a portion of the expansive soils and replacement with a granular soil (primarily sand or clayey sand). What generally occurred in these circumstances was a "bathtub" effect where the more permeable soil would transmit water directly to the expansive clays and hold it there in the undercut hole. Needless to say, the results were disasterous.

Not knowing the specifics of the recommendations that you are quoting, I cannot say for sure whether or not it would work, but on the surface it would appear to provide a conduit for water to get to the clays.

The standard of care in Jackson, Mississippi on expansive clays was to undercut where necessary to get a minimum buffer of nonexpansive soils and to use silty clays with much lower permeabilities so that the water would not readily travel to the underlying expansive clays.

I hope that this helps.

 

[Focht3]

[blue]ganderson[/blue]'s practice is consistent with the practice along much of the Gulf coast - areas with moderately to highly expansive clays, a net positive moisture balance (on a long term basis) and limited quantities of locally available low cost crushed rock fill. Houston, Corpus Christi, Brownsville, Beaumont / Port Arthur / Orange, Lake Charles, Baton Rouge, Gulfport, Mobile - all do essentially what [blue]ganderson[/blue] has described. And, done properly, it works quite well.

[blue]ganderson[/blue] is also correct in his bathtub analogy - an analogy that I also use.

Other areas follow a different course. In Central Texas (San Antonio, Austin, Waco, Temple), we have an abundant supply of relatively cheap crushed limestone. It's largely inert and very easy to compact well. But it is much more permeable than a low plasticity clay (i.e. silty clay, sandy clay.) And we have very limited supplies of locally available low plasticity clay fill. The rainfall rate is much lower in these areas - so the clays are harder, more dense and drier than the "coastal" clays of similar plasticity. Yet we are able to successfully use crushed limestone as a suitable replacement fill in cut-and-fill operations to reduce damage from expansive clay soils. We can use these "less desirable" fills because we remove more of the offensive materials, reducing the expected swell of the site.

The real key to using a granular fill is making sure that the depth of the removal and replacement is deep enough to reduce the future swell to an acceptable level -

And it can be done.



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[JoeTank]

A slightly different thought to consider. Welded steel tanks have ability to handle bottom deformations and remain leak tight. You may wish to look a a steel tank as part of your fix.

Steve Braune
Tank Industry Consultants

http://www.tankindustry.com