Using bentonite slurry as a permanent filling material 1

[csd72]

I have a project where it is proposed to insert a PE pipe into an existing cast iron pipe which runs under a rail line. We have done this type of thing in the past but usually the PE pipe is a reasonably tight fit.

In this one the CI pipe is about 24" and the new PE is only about 12" due to the need to push it around a few bends. The managers of the line have requested engineering calculations to prove that the replacement works will not cause undue settlement of the line in the future.

My proposal is to fill the annulus between the new and the old pipe with bentonite though I am not sure about the suitability of this as a permanent filler. Does anyone know of a good reference for using bentonite in this application and also what type of mix and what type of shrinkage to expect if it does dry out.

I am planning on sealing of the end of the pipe with expanding foam but I am concerned about leakage in the future from pipe corrosion.

Any help/advice on this would be greatly appreciated.

 

[bimr]

Railroad engineering departments typically have a standard detail that shows the pipe seals on the ends of the casing as well as the casing detail.

What RR are you dealing with?

 

[cvg]

a sanded cement grout might be a better choice

 

[csd72]

bimr,

Its Network Rail in the UK. The casing in this case is the existing pipe so there is not much I can do about that. I am just trying to get some type of infill to remove the void and I need to be able to justify its long term stability.

cvg,

I cant use anything cementitious as they dont like it against the PE (corrodes it or something - have never had a clear answer).

 

[Ron]

csd72...my choice would have been cementitious as well. Bentonite would have be placed as a slurry and because it must be confined all around, would probably never dry out. Shrinkage would occur if it did.

I would consider an expanding foam soil stabilizer or similar chemical grout injected into a coarse sand filler.

 

[csd72]

My concern is with the eefect that a chemical grout may have on the PE, an unknown quantity for most grouts I would say.

 

[Ron]

Most of the grouts are urethanes or similar polymers. Check with the pipe manufacturer.

 

[ACtrafficengr]

Sliplining with HDPE pipe and grouting the void is becoming common practice in the US.

Have you considered cured-in-place liners or spiral wound liners? These could be used to rehabilitate the pipe with a much smaller reduction in cross section area.

"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail." - Wm. Phelps Eno, ca. 1928

 

[ACtrafficengr]

http://www.fs.fed.us/eng/pubs/pdf/05771201.pdf

"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail." - Wm. Phelps Eno, ca. 1928

 

[bimr]

For PE pipe, it is recommended to grout the ends with cement and allow the middle of the pipe to snake. See the Performance Pipe Field Handbook:

http://www.performancepipe.com/bl/p...ocuments/PP901 Field Handbook WEB VERSION.pdf

Page 55:

When installed in a casing, OD controlled polyethylene pipe does not require centering spacers (centralizers) for electrical isolation to a metal casing. Polyethylene is non-conductive and will not affect casing cathodic protection. Allowing the pipe to snake inside the casing can usually accommodate minor thermal length changes of the polyethylene pipe in the casing. If used, centering spacers will force thermal expansion thrust loads to the pipe ends, which may weaken or break casing end seals.

Unless groundwater pressure could cause pipe collapse (see Table 14), grouting the casing annulus is not required. The PE pipe must be protected against shear and bending loads at the casing entry and exit. When pulling PE pipes into the casing, the Allowable Tensile Load (ATL) for the pipe must not be exceeded.


Page 48:

DriscoPlex pipes that enter or exit a casing or a structure wall such as a building wall, vault, or manhole, must be protected against shear and bending loads that can develop from settlement and embedment consolidation.

A compacted foundation and compacted bedding should be installed below the pipe where it exits the casing or structure as illustrated in Figure 28. At a casing entry or exit, the pipe should be wrapped with an elastomeric sheet material; then the annulus between the pipe and the casing should be sealed either mechanically or with a cement grout. The seal prevents backfill migration into the annulus.

 

[bimr]

Here is the sketch:

 

[csd72]

ACtrafficengineer,

Do you know what type of grout they tend to use around the PE?

In place liners are not commonly used over here and are therefore more expensive that just inserting a new pipe. This also would not alleviate the rail engineers concerns about the strength of the existing cast iron pipe.

bimr,

Thanks for the great information, but unfortunately this does not help with the strength of the pipe under the railway loading which is my problem. I cannot guarantee the strength of the cast iron pipe so the simplest option is to fill it with something that will fill the void between the old and new pipes and remove the settlement risk from pipe failure. I am just not sure what I can fill it with that will not damage the PE.

 

[ACtrafficengr]

Usually a cement grout is used for a structural reinforcement, or controlled low strength material (aka flowable fill) if it's just filling space. CLSM is a self-consolidating 40 to 120 psi concrete mix. You'd have to ballast or brace your plastic pipe so it doesn't float. Also the material will erode if exposed to the elements at the ends.

I'm surprised your pipe manufacturer doesn't have information on chemical restistance for their materials.

"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail." - Wm. Phelps Eno, ca. 1928

 

[bimr]

Cement grout will not damage the PE. The manufacturer recommends cement grouting the ends of the casing.

I would be more concerned about the coefficient of expansion of PE versus the cast iron and the pipe stress that will occur as the PE tries to move.

If the cast iron is not in good condition, maybe the recommendation should be to replace the casing pipe. Cast iron has not been made for years and pipes do not last forever. With a new casing, you can install the force main as per the recommended method.

 

[cvg]

how about trying a FRP pipe instead of PE and just use standard grout to fill the annular void space

 

[Ussuri]

Would something Benefil from Uretek be a possibility? Not sure what its compressive strength is though,

 

[csd72]

ussuri,

good point, I will look into the benefil option.

 

[dicksewerrat]

CSD72,
Cured-in-place lining was started in the UK. If you need a contractor, google CIPP.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com