Pull out force calcs for casing borehole 1

[farhadsh]

Not sure if this is right place to put this question.
I have undeground borehole with casing pipe with grout to hold it in place. I look for basic formula to see if grout is right to hold pipe in place. I did some search looks I have to calc pull out force in this case first and then calc downward force which will be weight of pipe and media inside.
If this is correct I need to know bond strength between grout and ground rock/ soil and bond strength between grout and steel pipe.
As long as following is valid It should work: weight of pipe and media < external area by length by bond strength

Need some help.

 

[oldestguy]

Start by having a test boring made by geotech firms, if possible. Most testing. drilling labs have many ways to measure shear strength of the soil. Some of them include tests down there at various depths and other methods require taking a sample (as undisturbed as possible) and running tests in the lab to determine shear strength characteristics. Using that figure is not a simple thing without knowing more, such as ground water table, effect of density, sensitivity to disturbance, grout strength variations relative to water content, different soils can have significantly different shear resistances (cohesion, friction angle), etc. It also might be necessary to run some lab tests of shear strength of grout against the casing. Also tell the geotech (who does the testing) what your need of this well thing is for anyhow. Simple static shear strength tests may not be sufficient, because it is common when installing or pulling test boring casing that it is moved by a combination of static and dynamic loading.

Finally there may be some factor for lab shear strength tests that may have to be applied in calculating the in-place shear strengths.

 

[farhadsh]

Thanks for input. If I understand it correctly by testing we can get shear strength of grout to soil and grout to casing? and using above formula we can check if grout strength is enough to hold casing and media in place or even strong enough to add additional end loads.

 

[oldestguy]

It may be that some slight modification of standard test procedures might be needed to get the values needed in a calculation. However, if we knew more of what you are wanting to do, we can help. So far it sounds like something we usually don't do any calculations for.

 

[LRJ]

I think the analogy with a drilled and grouted pile is a good start.

In the absence of test data: Kraft and Lyons (1974) state that flush (i.e. no shear keys) drilled and grouted piles have been observed to have a grout-steel friction of between 20 psi and 35 psi. The lower bound value (= 138 kPa) may be an appropriate value if the other factors are fairly similar to the case studies in that reference. These recommendations are also adopted in the offshore standard API RP2A WSD (note that API RP2A LRFD uses a value of 248 kPa for flush piles).

For the grout-soil friction, you'll be able to find countless references for the appropriate α or β factors to apply for different interfaces.

However, as stated above by oldestguy, it's always preferable to have test data.

 

[farhadsh]

Oldestguy requested further info: I have 400 meters 6 inch steel pipe in a ground borehole with dip angle of 70 degree. Pipe is supposed to be secured in place by adding grout between pipe gap and hole. Pipe OD is 168 mm and drill hole is 260 mm. How to ensure grout is going to secure pipe in place? I know weight of pipe and media. Assume pipe and media has force of F downward How to ensure when we have flow going through the pipe, it wont move or slide down? Down at the end of borehole there will be horizontal piping connecting to borehole pipe. What type of grout should be specified in regard to strength, shear forced etc?
Thanks

 

[oldestguy]

So far we don't know (at least I don't know) what this installation is. If it is one pipe 400 meters long, that is one thing where pipe strength will control what happens. As to making sure you have solid grout and all cavities filled, you need some means to be sure of that, such as pumping down into the pipe and seeing flow come out along side the pipe. From the little I can understand about this, it seems you have no need to be concerned about the grout. Instead the mechanical connections, such as couplings, pipe wall thickness, etc. will govern what ever you do. Sounds most unusual, such as a mining job?

 

[miningman]

It most certainly is a regular job in the mining industry. If dip is 70 degrees , is the pipe centred in the hole or does it sit on the footwall. When grouting the biggest challenge is to avoid air pockets.
Best method is to introduce grout at the bottom and push grout to the top of the hole, Any regular 3000-6000 psi will work but microfine product makes for easier grouting.

 

[PEinc]

What you are trying to design is similar to micropile design. FHWA's micropile design and construction manual has info on bond of cement grout to various soil and rock types and for grout to smooth and deformed steel.

Check this DFHWA publications web site:

https://www.fhwa.dot.gov/engineering/geotech/library_listing.cfm

https://issuu.com/anchorsystemshawaii/docs/fhwa-micropilie-design-sa-97-070

http://www.PeirceEngineering.com