E' for Pipe Bedding

[moe333]

I need to recommend an E' (Modulus of Soil Reaction) value for pipe bedding for a 96" steel pipe. Bedding is 10" below pipe and up to springline. Pipe depth varies from 20 to 40 feet. I'm thinking of either crushed rock or 2 sack sand cement slurry.

USBR recommends E' of up to 3,000 psi for crushed rock, and similar and lower values for SW/SP depending on the compaction. I have seen 1,500 psi used in many cases.

Intuitively, I think E' should be a function of the pipe depth, trench width, adjacent soil modulus, depth of bedding relative to the pipe, etc. Does anyone have a good reference that accounts for some of these factors?
I heard AASHTO may have tables that take into account some of these factors. Specifically looking for a value for a 2 sack sand cement slurry.

Thanks

 

[Ron]

moe333...my apologies, but I misunderstood what you were asking in the Terzaghi forum...I thought you were asking for the elastic modulus, since you were asking for a parameter of a concrete rather than a soil.

E' is a somewhat nebulous term and concept. Typically, we look at the stiffness properties of support soil structures, which in pavements would be a "k" value (modulus of subgrade reaction) or resilient modulus (a tested or correlated parameter).

With cementitious materials, the parameters change. We look specifically at the elastic modulus, E, and unconfined compressive strength. Given that, I have attached a correlation graph from TM5-822-13, a military training manual on the elastic layer analysis of pavements, that gives a correlation between unconfined compressive strength and cracked section modulus. I believe you can use this parameter for your "2 sack" mix parameter.

Ron

 

[moe333]

Thanks Ron, I thought we were talking about two different things earlier. The figure you attached provides some good information, but I was looking for something more specific to a pipe bedding application. I would think that the effective modulus would be related to the depth of the pipe and the thickness of the bedding both vertically and horizontally, relative to the pipe. I believe AASHTO may have something on this.

 

[fattdad]

I'm just struggling to imagine 215 tsf as an appropriate soil modulus for aggregate backfill (i.e., 3,000 psi). I'd think it would be 350 tsf or better.

I'm not going to offer much help on the big-picture problem, but how will you use the E' value once you figure out an appropriate value to use?

f-d

¡papá gordo ain’t no madre flaca!

 

[moe333]

I am not using E' for any of my geotechnical recommendations. I am providing the E' value to the civil engineer and they use it to check pipe deflections and ultimately to design the pipe size/type/wall thickness etc. I assume they use Spanglers equation (Iowa Eqn) or various other code based equations which may or may not be a modification to the Iowa Eqn for this evaluation.

 

[fattdad]

O.K., are you using "The Design of Reinforced Pipe?" This book has Spangler charts that show you what type pipe is needed for varying bedding conditons.

f-d

¡papá gordo ain’t no madre flaca!

 

[moe333]

No, I don't have that book. As I stated, I am only providing E' value. There are a number of parameters in the pipe deflection equation, most of which I don't have the experience to evaluate. So I am trying to provide an E' value for varying bedding conditions based on the bedding material, pipe depth, and the geometry of the bedding (height and width around the pipe). The civil engineer uses these different E' values iteratively with the other parameters in the equation to come up with an appropriate design.

I understand that AASHTO has tables of E' for different bedding materials, compaction levels, pipe depth, and other geometrical bedding parameters. This is the sort of information I am looking for.

 

[CarlB]

API Recommended Practice 1102, "Steel Pipelines Crossing Railroads & Highways" provides a table with following values
for E', Modulus of Soil Reaction:
Soft to Med Clays, Plastic- 0.2 ksi
Soft to Med Clays, Low Plasticity, and loose sands & gravels - 0.5 ksi
Stiff clays/silts, & med dense sand & gravel-1.0 ksi
Dense sand & gravel - 2.0 ksi

 

[bpattengale]

fattdad would those tables apply with steel pipe? I'm referencing The Concrete Pipe Design Manual, but the E' values of the bedding don't come into play, just the bedding type. I had to check myself, but steel is designed as a flexible pipe.

moe333, just for your reference the civil might also request the E' values for the native soils that the excavation is in. The civil will use it to determine a soil support factor for use in the Iowa Formula to check the deflection of the pipe. (Just finished some calcs for a 24" PE sewer)

 

[moe333]

bpattengale,

It's my understanding that the E' value in the Iowa formula would be for the bedding material and not the native soil. To me, this would depend on the thickness of the bedding, the width of the trench, and whether or not the bedding comes up to the springline of the pipe. If the trench was narrow, the bedding was relatively thin, and it didn't come up to the springline, then I would tend to provide the E' value for the native soils. If it was the other way around I would provide the value for the bedding.

This is essentially my question... how wide and thick does the bedding need to be so that its E' value controls? The other questions are what E' value is appropriate for a 2 sack sand-cement slurry? And what effect does the pipe depth have on E'.

 

[Ron]

moe333...you're continuing to mix up the E' value of soil conditions with the much superior bedding of a cementitious material. They are two significantly different parameters and have little or no relation to each other. Pick one and go with it...do you want soil/gravel bedding or do you want a stiffer cementitious material bedding... E' is undefined for cementitious materials. It is poorly defined for soil conditions.

 

[moe333]

My mistake, I should have used the term modulus or cracked modulus for the cementitious material. It's not really a question of "pick one and go with it". The intent would be to be able to evaluate either condition.

Whether you use a cementitious or soil/gravel bedding, the resistance to pipe deflection will depend on the depth and thickness of the bedding, and the width of the trench (and bedding), and whether or not the material comes up to the springline of the pipe. This was the focus of my question.

 

[bpattengale]

moe, yes the E' are for the bedding material, but E' for the native soils of the trench are also important. The following in from the PE Pipe Handbook, yes it not steel, but its still flexible pipe.

"Soil Support Factor, FS
Ring deflection and the accompanying horizontal diameter expansion create lateral earth pressure which is transmitted through the embedment soil and into the trench sidewall. This may cause the sidewall soil to compress. If the compression is significant, the embedment can move laterally, resulting in an increase in pipe deflection. Sidewall soil compression is of particular concern when the insitu soil is loose, soft, or highly compressible, such as marsh clay, peat, saturated organic soil, etc. The net effect of sidewall compressibility is a reduction in the soil-pipe system’s stiffness. The reverse case may occur as well if the insitu soil is stiffer than the embedment soil; e.g. the insitu soil may enhance the embedment giving it more resistance to deflection. The Soil Support Factor, FS, is a factor that may be applied to E’ to correct for the difference in stiffness between the insitu and embedment soils. Where the insitu soil is less stiff than the embedment, FS is a reduction factor. Where it is stiffer, FS is an enhancement factor, i.e. greater than one."

The depth of the pipe shouldn't affect the E' of the bedding material, with the exception of shallow bury depths. At shallow depths the confining presure on the soil may not allow the full E' to develope.

As to the bedding strength, this has some good information

http://www.plasticpipe.org/pdf/chapter07.pdf

starts on page 10 of the pdf