Concrete Slab Within Fill to Protect Underlying Pipe? 1

[DerekYY]

I am working on a landfill project in which an existing HDPE pipe is beneath the existing landfill. The pipe strength is adequate for the existing conditions. It is proposed to expand the landfill with a great amount of additional fill over the pipe. It has been determined that the pipe is unsuitable for the additional fill. The client thinks that if we put a reinforced concrete slab, whatever width needed, along the length of the pipe, within the proposed waste fill it will take some of the future waste loading off of the pipe. Is this possible? I've been researching foundation designs, and I can't see how the slab will distribute the above loading, when the sourroung waste on all sides of where the potential slab will end has the same load. There really isn't a concentrated load to distibute. It just seems that the slab will add weight over the pipe. Any thoughts are appreciated.

 

[SharkswithLasers]

I'm not a real Geotechnical Engineer, but I play on on the forum...

Seriously, I agree that it seems placing a piece of concrete over the pipe (separated by some thickness of soil) adds additional weight over the pipe and nothing more. Let's think about it, when you place a slab (spread or pad) footing, and place a load on that footing, the load is dispersed throughout the soil (at approximately 2:1). So it would seem that the load above the slab (i.e., the additional fill) would be dispersed in the area of the pipe. That seems true, I can live with that. But there is still the fact that the weight of the slab, which does weight something, that acts upon the pipe.

Does the additional weight of the slab counter balance the additional fill above the slab enough to reduce the forces acting on the piping?

Seems safer to me to design a slab over the pipe that transfers the load of additional fill via soil screws to the material below the piping. Check out the fancy drawing:

Additional fill up here

*********************** (slab)
* *
* O *
* *
soil screws; pipe in the middle; pea rock pipe bedding. Transfer the load of the additional fill to below the piping.

What do you think?

Jim

 

[Ron]

If you have soil beneath the slab and above the slab, you will accomplish almost nothing with the slab over the pipe...load will still be transferred, though distributed slightly differently than will soil. If you do not have soil between the pipe and slab (bridging), the slab will protect the top of the pipe but increase the lateral load on the pipe. Damned if you do, Damned if you don't!

You could accomplish the same things by placing a geotextile over the pipe and using a higher modulus material layer over the geotextile. This will likely do more than a limited width slab. The geotextile system would have to be designed based on loading and lateral transference of the load into the fabric.

Have you considered slip lining the pipe and grouting the annulus to increase the pipe strength? If you can stand the hydraulic capacity reduction, this would likely be the most viable solution.

 

[DerekYY]

Shark,

Thanks for the response. I think the weight of the slab wouldn't hurt too much. There would be approx. 225 ft of 65pcf material over top of the slab. By soil screws, do you mean like columns or supports beneath the column. I don't thik this is an option since the pipe actually sits beneath a geosynthetic liner system which I'm told can't be penetrated.

Ron,

Thanks for the response. Slip lining was actually the original plan that will work. The pipe system is actually an 8-inch SDR17 pipe inside of a 12-inch SDR17. We planned to slipline a 10-inch SDR11. The client is not comfortable with this because the the buried end of the pipe (part of a leachate collection system) is welded to a liner system. There are scared to punch through.

The problem is that the pipe has only one exposed end and is buried beneath 25 feet of waste and soil fill, not to mention a liner system. Therefore, exposing the pipe is not an option. I still feel that sliplining is the only viable option.

 

[Focht3]

The concrete slab approach is a waste of time - unless you create a relieving platform (ala SharksWithLasers screw anchor supports.) However, constructing a relieving platform will require penetrating the landfill's liner and leachate collection system. Not a good idea.

Stick with the sliplining - or put the trash somewhere else!

(I have done work for landfill owner / operators - had to throw that one in!)

 

[VAD]

Concrete slabs have been advocated and used as relieving platforms/slabs over gas pipe lines and culverts where protection is needed against traffic loads because of too litle cover. Your problem is an interesting one that requires looking at the concepts of behaviour of buried structures. As another concept,it may be possible to create an arch structure over the existing pipe and attempt to provide a solution. This may be costly though. The text Buried Structures: Static and Dynamic Strength by P.S Bulson, would be a good reference to start with.

 

[DerekYY]

Update, we are working with two options right now.

1.
Sliplining a 10" Stainless Steel Pipe between the HDPE pipes. Note that the worries with the sliplining is that both the outer and inner pipes have weld beads on the inside and outside of each 20' section welded together. It's a matter of whether we can work the pipes while sliplining to get between the beads. Also, is the compressive strength of steel pipe just a function of its ultimate strength, Fy. I've yet to find info on stainless steel pipe.

2.
Use the same reinforced slab concept, with the slab extending beyond the pipe ends, but with beams under the slab. The idea is that the beams would extend some distance out from the slab transferring the load on the slab out to areas not above the pipe. Still working through this possibility. Not too sure on it. What do you, think.

Cost will definately play a part, in which case the sliplining will be much cheaper but also risky.

Thanks for your thoughts guys. This is the most interesting/difficult problem I have encountered in my career. Any design will most likely be custom (nothing straight by the book to go by).

 

[Focht3]

Definitely not a textbook problem!

The problem with the slab - put another way - is a difference in stiffness (the inverse of compressibility.) Landfills settle a lot - some as much a 40 percent of their thickness - after they are completed. Only God knows how much settlement occurs during active filling! The rubbish is, on average, quite compressible and not stiff at all.

The pipe, on the other hand, is quite stiff - incompressible in comparison to the rubbish. The relieving platform (stiff) will be supported by rubbish; since "load seeks resistance", where do you think the additional weight will bear? [rubbish -> platform -> pipe!] In fact, one could argue that the relieving platform could make the problem worse...

I'd stick with the sliplining; you can have the welds on the pipe exterior ground flush since the pipe won't be under pressure.