system against floating pipe 1

[carmoch]

Hello,

we have to install 2 pipes (100 inches each other) in a swamp area (soft clays with water level 1.5 m below ground). The trouble is that the client wants to install a layer of gravel or crush rock above the pipes for avoid the buoyancy. I don´t believe in that system because it could produce an important settlement in the foundation of the pipes. What´s your opinion? Could be possible a system with screw anchors? The thickness of soft soils is 10 meters.

Thanks!!!

 

[BigInch]

Set-on weights, or saddle bag weights, or screw anchors and concrete coating have all been used to solve that problem for pipelines crossing swamps and high water table areas. Weights or coatings are sized and distributed along the pipeline such that they nearly balance buoyant forces, to have a net pipe-coating specific weight relative to water of 1.2. The downward load from weight/coating & pipe will not be so great as to cause any further sinking below the installation depth. Screw anchors also work well. Install weights and screw anchors at center to center distances such that buoyant forces do not cause excessive bending on the pipe span between them. Calculate bending moment on the pipe at the centerline of span = 1/10 * w * L^2.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[bimr]

Installation of pipe in marshy or swampy soils represents one of the most demanding applications. In certain situations, the high water table that is characteristic of these soils can result in significant buoyant forces that may raise the pipe from the trench in which it has been installed. When this possibility presents itself, a ballast system may be designed which can prevent or minimize pipe flotation.

You have not mentioned the type of pipe that you are using. However, design information is available at the following sites:

http://plasticpipe.org/pdf/chapter10.pdf

http://techserver.hancor.com:8080/technical_notes/H5.05-Pipe_Flotation.pdf

 

[carmoch]

Thanks for your answers! I forgot to say that pipe is made of steel

 

[BenJohnson]

I had that problem before, and we used a concrete slab (sort of like a foundation slab). We put one at every pipe joint and used a steel strap from the slab around the pipe. The slabs gave the pipeline support against settlement plus the steel straps kept the pipe from floating.

 

[MarGwy]

Carmoch,

Are your pipes 100 inches in diameter or 100 inches apart from one another? Where is your project located (country/state)?

 

[bimr]

Here is a design guide for steel pipes:

http://www.steeltank.com/Portals/0/pubs/Welded Steel Pipe 9 13 07 Low resolution.pdf

 

[rconner]

There may be some perspectives also on page 24 in the document at

http://www.americanlifelinesalliance.com/pdf/Update061305.pdf

as well as AWWA Manual M11.
[Also, while not saying it is necessarily a problem with what anyone has done or is doing, I'll make a general comment as well about laying lots of buried pipes deliberately on top of at least discontinuous slabs or blocks (at least where pipes are not fully encased). Many decades ago I believe there were common bedding practices in some areas of laying pipes of many materials deliberately on periodic, permanent wooden timbers/blocks or mounds of earth. I think it was eventually found that this can result in some cases/locations at least in non-uniform axial support and loading, as presumably the earth and pipe where there is not underlying blocking had a tendency in the long-term to settle differently than the portion directly at the support. This resulted in localized beam effects not always accounted for in load factors or determinations of required pipe wall thickness etc. For those reasons, unless the pipes were/are purposefully and specially designed for said non-uniform loading, I believe the bedding condition of laying pipes on blocks underground were removed long ago at least from most standards for laying pipes.]

 

[bikerben]

Another option that I've seen used in in soft soil is a "soil correction" where the excavator will remove 2-6 ft of material below the pipe and place gravel to prevent settlement issues. It is a cheap and easy way to gain piece of mind about the foundation. It is much less complicated and cheaper than helicals since I assume 100" diameter pipe will already have some very large machinery on-site for the install.

I've seen horseshoe shaped reinforced concrete pipe anchors for individual pipelines or double-W anchors (think of two concrete anchors shaped roughly like a W - one on bottom and the other on top then bolted together with long bolts/studs) for double barrel applications.