Help needed with Slab on Grade Design for Sedimentation Basins

[buening]

I'm in the preliminary stage of a couple Sedimentation Basin designs and am looking for a few tips. Most slab on grade designs involve vehicle loading and/or column/post loads. Sedimentation basins are normally full of water and some soils at the bottom. What is the general consensus in designing these? Do you assume a purely uniform load? I've done alot of research and most examples and books are geared towards industrial floor slabs, where columns and vehicles cause moments in the slab. And if you do assume a purely uniform load, then is the thickness of the slab determined by the moment from the CIP walls of the basin? These basins will have the walls cast in to the slab, emulating a bath tub. If you can't tell i'm a young engineer so any tips/help is appreciated! The basins are not that large, i.e. 10'x10'x6'deep and 10'x15'x7'deep. One of the risks in this design is the lack of soil information. The site is located along a major river (Illinois River), so I feel it is safe to assume sandy soils. The water table is below the slab, so buoyancy is not a problem.

BTW, this website seems like it could be a good resource for me, as this is my first post

 

[buening]

I will be ordering the PCA Rectangular Concrete Tank design book as a reference but still open for tips

 

[SlideRuleEra]

Welcome to Eng-Tips. The basins you describe sound more like sumps, and the floor is more like a foundation than a slab-on-grade. That foundation has has to support not only the water inside, but also the walls and the horizontal forces on the walls (perhaps earth pressure tending make walls fall "in", when the sump is empty, or water pressure tending to make the wall fall "out" when the basin is full).

The floor certainly should be reinforced, and fairly "thick" to allow for two rebar mats (top & bottom).

Because of the small size and number of basins, the design can be conservative without costing a lot of (extra) money.

Unless you have hard evidence that the water table will ALWAYS be as low as you state, design to resist buoyancy (rivers flood and/or basins overflow). All you may really need to do is make the bottom "really thick", which is normally very cost effective.

Get some kind of information on soils, either a new geotech report, or possibly an old report for a satisfactory nearby structure.

Another approach could be to consider purchased precast structures which are set in place.

http://www.SlideRuleEra.net

 

[Ussuri]

Generally when designing tank bases the moments generated in the slab are not from a uniform applied load.

With slabs for traffic loading, you are more often interested in the sub base material. The slabs are designed to prevent sagging at soft spots and resist punching shear, edge spalling etc from wheel loads. If the slab has a UDL on it and the sub base is uniformly compacted there will be no bending in the slab as the load is transferred directly into the soil.

For the case of tank base its different. The main cause of bending occurs when the tank is empty. The weight of the walls on the perimeter cause the base slab the hog (tension top face). There are a couple of different approaches to assessing this depending on whether you assume elastic or plastic behavior from the soil. I usually go for the elastic approach and calculate the weight of the walls, distribute it over the base and analyse it like that. I will then also check the case when the tank is full and assume a soft spot has developed in the sub base and check that condition.

A point to note, if your tank is being designed as continuous you need to look at the tank as a whole, and not the walls and base independently. This is because the moments calculated and the base of the wall will distribute into the slab depending on the relative stiffness of the wall and base. You need to a bit of moment distribution to work out what effect this has on your base slab.

 

[BigInch]

Its been a long time since I did one, but I seem to remember using a much higher As minimum ratio, or was it a maximum Fy of 40 ksi. No hairline cracking is allowed for fluid containing basins. Sorry I don't remember exactly what it was, but it made an impression on me at the time.

And no matter where the water table is, design against flotation.

http://virtualpipeline.spaces.msn.com

 

[buening]

Sweet! Thanks for the help everyone! Here are the load cases i was going to consider in my design:

Empty and full tank with backfill against walls
Full tank without backfill on walls (in case heavy rains occur before backfilling has finished)

Depending on the weight of the tank, i may extend the slab out farther than the walls to add some overburden soil weight to resist flotation.

These basins will be a two stage sedimention setup, with the smaller tank first and pipes going from the first tank to the second tank. The first one is more of a collection tank, where the second tank will have an internal wall to trap sedimentation.

I have a few more questions so bear with me

Ussuri, I know the moment cause from the lateral earth pressure on the walls will transmit to the foundation slab. Do you have any online resources for doing an elastic approach? Foundation design is not one of my strengths and with this slab fully supported everywhere I'm not sure where to start LOL. You mention assuming a soft spot in the soil and checking that condition. Can you assume a deflection and treat the slab as simply supported at the walls and calculate the moment?

 

[minorchord2000]

I have designed many of these types of basins. What I do is calculate the the vertical wall loads and moments. The moments are due to two cases: earth pushing in on an empty tank and water pushing out on a wall with no soil backfill. I then create a 2d model in RISA or STAAD and assume a soil spring value under the slab to simulate the soil properties. These springs constitute the model's support system. I locate the soil springs at about 1 foot on center. I load the tank with the full weight of the fluid and any other contents as a uniform load, apply the moments and vertical load at the node where the walls are located. Run the two cases and get you shear and moments in the bottom slab.

 

[buening]

Thanks minorchord2000. I had started a STAAD model using plates on all four sides and on bottom and called them 12" thick concrete. It was the support under the slab that i wasn't sure about. I chose to use the "Foundation" tab in the supports. I wasn't sure if i should use footing, elastic mat, or plate mat. BTW, is there a rule of thumb for determining the subgrade modulus (psf/ft)? You guys have been a great help! I'll also try the 2D model using soil springs and compare the results with my 3d and hand calcs, when completed.

 

[minorchord2000]

As a start assume 100 pounds per cubic inch (very conservative)and convert to this to a soil spring with units of lbs/ft or kips/ft keeping in mind that you will be analyzing the slab with a 1 foot trib width into the paper. Try spacing the springs at 1 ft on center (Trib Width).

Example: 100 lbs/in^3 * 1728 in^3/ft^3 * 1' TW * 1' strip into the paper = 172,800 lbs/ft = 173 kips/ft. This is your spring constant to be applied at nodes which represent you foundation slab. Try varying you spring constant and you will find that it does not vary the resulting moments all that much. You will see that the wall moments which are applied as concentrated nodal moments at the ends of the slab will be transferred to the slab.

Don't forget to increase your loadings if you use the load factor method which I recommend.

 

[Ussuri]

I dont know of any online references but there are a couple of good books available.

"The design of water retaining structures" by Roger Batty and Ian Westbrook

"The design of liquid retaining concrete structures" by R D Anchor.

The both detail the approach to analysis and design quite well. The only downside is that the design aspect is to British Standards, so that will be of no use to you.