Tank Foundation Design Help 4

[Neauxla]

I am a young engineer and have been tasked with designing a small foundation/spread footing/slab. The project invilves the storage of a hazardous liquid, and i have been given two different options to design: 1)A foundation (approx 12'x20') for a skid mounted, horizontal 40000 lb (when full) tank and 2)A foundation (approx 20'x40') for two 50,000 lb (when full)tank trucks to be spotted on site.

Also, both options will need to have a containment wall tied into the foundation.

I'm looking for advice on where to start and what kind of calculations do i need to perform to design the thickness and reinforcing steel?

Thanks in advance!

 

[dik]

There are several things you must do. You have to determine the nature of the hazard. What type of hazardous material, is the material hazardous and volatile, hazardous and dangerous to the groundwater system, corrosive to concrete, etc.?

Do you need a secondary containment tank, or is a pit adequate. Do you have to coat the concrete with a material resistant to the material?

Quite often the sides of tanks are supported on a concrete foundation wall with a sand fill to provide bearing for the base of the tank... the secondary containment will have a similar support... and be sufficiently high to contain the contents plus maybe 20 or 25%, in the event there is a rain when the tank fails. How long can you store it? do you need a sump pit, does the secondary containment slope to a pit if there is one... Is the soil suitable to support the containment tank? Do you need a liner to keep the material from the groundwater? will a bentonite sheet work? EPDM? or whatever?

Just for a start... you need information and have to know your materials.

 

[chicopee]

Lacking more specifics of site and tank skid details, I would say reinforced concrete slab 10" thick with top and bottom arrays of @4 rebar spaced longitudinally and laterally 12 " apart. Monolithically, have a portion of walls may be 12" high poured at the same time, again reinforced with #5 rebar spaced longitudinally 18". Keep top of the partial walls very rough for additional pour. When concrete concrete is cured, finish remaining portion of walls with rebar and new concrete. You may want walls to be tied in with slab with 90d hooked vertical rods. 3" concrete cover over rebar is suggested. Wire tie all rebar.

 

[Ron]

Agree with the comments above and add a couple....

The apron for the tanker truck is simply a rigid pavement. Design as such. Since the stored liquid is transient relative to the slab and is contained in a tanker that must meet more stringent transportation requirements, it is doubtful that you need secondary containment for that application.

As for the stationary tank, the loading is not that significant considering a reasonable skid size and bearing area. The more difficult issue is designing the containment. As dik noted, the concrete will likely need to be coated, and you will need to design for close crack control. Pay attention to the quality of the concrete, not just the structural design of the system.

As for liners and tertiary containment if required, if the hazardous material is petroleum based, don't use EPDM and be careful with other synthetic membranes as they can be attacked by various chemicals. If bentonite sheets are used, be careful that they result in a "seamless" application. Another consideration is to use a compacted clay liner...usually very good for such as long as the hazardous material doesn't cause clay to flocculate!

 

[dik]

Ron... one of my latest undertakings is a multi-million gallon anhydrous ammonia tank...

Dik

 

[Ron]

Geez dik....that's a tank!! Lots of farms in your area?

 

[JAE]

or militias

 

[Neauxla]

I'm more concerned about the structural design of the slab itself and the the capacity of the soil.

The soil report indicates that a footing foundation has a net allowable soil bearing capacity of 1000 psf and a mat foundation has 800 psf. So if I use a Safety Factor of 3.0, does this mean that the weigth of the tank plus the weight of the foundation divided by the area of the foundation has to be less than 333.33 psf?

Also, when doing the slab design, should I design for flexure and then check to make sure the design is sufficient for shear?

Anything else that I should consider for the design?

 

[Ron]

Neauxla....the given allowable bearing capacity is low. Yes, if you must use a FS of 3.0, that is one way to apply it. Keep in mind that when you increase the area, you also increase the depth of influence on the soil. If you have strata at greater depth that will be susceptible to increased settlement, this needs to be considered.

Go back to your geotech and discuss the foundation issues with him/her. While bearing pressure and settlement are considered by some geotechnical engineers to not be greatly influenced by each other, if your soil is highly stratified in the influence zone, the bearing pressure can more directly affect the settlement.

In short, you have to balance the depth of stress influence with the delta stress given the change in bearing pressure.

 

[dik]

Check with you're geotekkie and confirm that the load capacity is service loading and not factored loading and, if service, find out what safety factor he has used. For storage of hazardous materials you may want to use a slightly higher factor of safety. 1999 psf is low and indicative of a fairly 'soft' soil and differential settlement issues may become an issue... are all your connections above grade? Is the tank insulated?... my latest one is heavily insulated for added weight <G>... actually, contents at -35deg...

Dik

 

[Ron]

One other thing you need to find out from the geotech and your client who is requesting the 3.0 FS....do they want 3.0 on gross or net allowable bearing capacity? The geotech has probably given you a net value, which already contains a factor of safety. Get that clarified.

 

[dik]

Mind out of shape, or fingers or something... "your" not "you're" and 1999 should be 1000...

Dik

 

[Neauxla]

Yes, the soil is very soft clay, and Ron I believe that you are correct in that the 1000 psf is a net value because the report states that the 3.0 safety factor is included.

 

[BigH]

If one is designing a foundation for very soft clay (or soft clay), why is one even stating the Factor of Safety when it is the settlement that governs the permitted bearing pressures? The net bearing capacity with FS (=3) might be 1300 psf but the net bearing pressure (based on settlement) might be only 900.

 

[sdz]

My 2 cents worth.
Design the bund area as a liquid retaining structure. This will normally result in higher levels of reinforcement but will reduce cracks and leaks. It also provides a better surface for any applied lining.