WWTP tank slab reinforcing

[HeavyCivil]

Hi,

I am designing a very large septic tank (or series of tanks) for a small-town WWTP. I have reinforced the 8" walls based on PCA moment tables keeping steel stress at or bellow allowable per ACI 350 10.6.4.2. So, I am not too worried about cracking in the walls. The slab is a different story.

Most tanks I have done have had Mat foundations. This site, however, has good soil and to keep cost down by saving concrete and an the extra layer of bar, I would like to use a slab for the tank bottom which is poured continuously with wall ftg's (ftg's were sized like retaining walls, resisting overturning and sliding so slab would not be subjected to any compression.

Obviously it is very important to keep crack widths to very very small widths. I was planning on using a 6" slab with #4's each way @8".

I was not planning on using control joints, mostly because I do not understand them in function enough to be sure that I won't be causing cracks larger than desirable. I am hoping that using that much steel (Rho=.004) and designing a good mix will eliminate undesirably large cracks and the need for control joints.

Is this an accurate method for achieving the water-tight goal??

 

[JedClampett]

It's almost a certainty your tank will leak. 8 inch walls and a 6 inch slab will not be leak tight. You might be saving your client some money, but at the expense of a leaky tank.
You can't waterstop 8 inch thick walls. You need to increase them to at least 10 inches, with 12 inches being better. The footing portion of the slab should be slightly thicker that the wall. At some distance away, you can decrease it, but I wouldn't go less than 8 inches.
If you gave the size of tank, I could comment more intelligently. A Rho of .004 is not that much, especially with no movement joints.
Concrete is pretty cheap. Pouring slabs on grade is very inexpensive with thicker slabs only costing the increase in materials. And a 12 inch wall doesn't cost much more than an 8 inch wall. Once again, once you build the formwork, the materials are the only variable. If the wall has any height at all, it will be hard to vibrate well at that thickness.

 

[HeavyCivil]

Jed-

Total volume is about 11,000 cf and is divided into three tanks.

I wanted to avoid two layers of bar in the walls (I think 10" is still okay to use one layer per ACI but is the maximum). Tank walls are only 8' tall. It is a fairly small structure and because I have #6's @ 8" o.c. in walls keeping Fs under 20,000 psi, I didn't anticipate much cracking. I also have never done such a design so my basis for anticipation of cracks is purely theoretical.

I was also unaware that you cannot water-stop an 8" wall. Is this in practice or per code?

Again, I designed foundation walls/ wall footings to resist all lateral earth and hydrostatic pressure without support from the slab so by all accounts that I have read (code, aci publications) I thought this would help minimize cracks.

Again- it sounds like you have a great deal more experience on the subject than I do. Could you elaborate more why large thicknesses are a minimum for water-retaining structures?

 

[JedClampett]

As far as the waterstop, there's just not a lot of room to place reinforcing and waterstop in an 8 inch wall. We sometimes do it for containment walls, but not for water or wastewater retaining structures. This is suggested in ACI 350-89, Section 2.5. You should get a copy of that edition as it is in a more informative type format than the newer ACI 350's.
As I said, I would use two layers of bars and not worry about the cost. All the money you save the client will be forgotten if the tank leaks. I'm a little curious about your footing/slab design as your post suggests that they are independent of each other.
Two more things to consider is that you need to design the walls for the two cases of liquid and no backfill and backfill and no liquid. Also, if there might be groundwater, has flotation/submerged soil pressures been designed for?
The reasons that thicknesses are larger for water retaining structures is that it reduces the chances that cracks will be all the way through the thickness of the wall/slab. Plus concrete is slightly porous, so the more mass the less leakage that way.

 

[HeavyCivil]

Jed- okay thanks. I understand.

The footing and slab, as it is right now (and will probably change per your recomendation) are NOT independent of one another. It is a rigid connection. The footing is simply wide enough to resist overturning and sliding with std FOS of 2 and 1.5 for both.

The wall is indeed designed to resist case 1, 2 and 3 (Leak test prior to backfill, back fill prior to water fill, and axial/ wall loading + moments with no lateral support from backfill. That is why the walls are designed as retaining walls (for moment with maximized base fixity and assumed cantilever conditions) but also footings are designed to prevent any translation so that the slab will not be responsible for restraining the walls.

So what I have is retaining walls designed to deal will the worst case scenario loading in all directions and slabs rigidly connected between all walls to form a water-tight tank. Footings and slab will be poured monolithically (if possible) with const joints at the bases of walls.

However, as I understand your post, this structure, due to thin walls and slab, may not be as water tight as I think, so I will accordingly increase the slab to 8" thick between wall footings and increase the wall to 12" thick and add an additional later of reinforcing.

Thanks a lot for your help. It is appreciated.

 

[BAretired]

It has been several years since I designed a water reservoir, but I do remember several waterproofing products used on the interior surface of the walls and floors of reservoirs. The names I recall are Vandex, Xypex and Krystal, but I imagine there are a whole host of other products available today. If you have not already done so, I would recommend that you review the various products available.

As stated by Jed, you have to carefully consider the situation where the tank is drawn down and the groundwater rises. I recall one circular water reservoir in Alberta which was empty or nearly empty when a sudden rainfall occurred. Storm runoff saturated the granular fill around the walls. Water pressure on the underside of slab was sufficient to rupture the slab in several places, allowing storm runoff to flow upward through the slab and into the tank.

The importance of good drainage and good grading away from the reservoir cannot be overemphasized.

BA

 

[JedClampett]

You've made some good changes. As you may have guessed, I've done a lot of water retaining structures. The amount you might save in a lighter design is nothing compared to the lifetime of one of these tanks. I've done work on clarifiers contructed in the 1940's. I doubt the designer had any idea they'd be in service for so long.