Embedment of Piles and Frost Questions

[tdawgui]

This is a lot to read and may be slightly confusing... I apologize beforehand, but thank you for any help you may provide.

I'm trying to design a chart giving required embedments for 1.66" outside diameter piles supporting light weight residential tramways in Michigan. I want to calculate the required embedment for varying loads ranging from 800# to 1200# and various different soil types.

I used the USCS as my basis for selecting my soils (Gravels, Sands, Low Compressibility, and High Compressibility) and found a chart which gives effective friction angle (phi) for the USCS soil types. I've selected 31 for gravels, 31 for sands, 28 for low compress and 19 for high compress. I also found a chart which gives a range of maximum dry densities (gamma) for the different soil types and selected the low end to be conservative giving me 115 pcf for gravels, 105 for sands, 95 for low compress, and 70 for high compress.

Now that all that info has been gathered... I used the skin friction equation where my resistance force = my perimeter * skin friction coefficient * my embedment length. I calculated my skin friction coefficient = adhesion + (gamma*15*Dia)* tan( 2/3 * phi). The 15*dia is the critical depth where the lateral earth pressure is constant.

After calculating all this, I get a required embedment of 55 feet to resist a mere 1200#. I understand that the outside diameter of this pipe is only 1.66", but I thought that at most it would be around 15-20 feet... and I was hoping for around 10-15 feet. Anyone have any idea what I may have done incorrectly or another way to decrease the amount of embedment required? I was also considering having them use real data out in the field gathered during the pile driving process and correlate with the Standard Penetration Test to determine the actual amount of skin friction applied. This would help decrease the number of uncertainties that definitely do not help in keeping the required embedment low. Basically I would like to give them a required number of blows per displacement that they need to reach before the pile is sufficient to support the 1200#. Does anyone have or know where I can get information regarding that?

The reason the title is Embedment of Piles and Frost Questions is that I decided that it would probably be better to add a concrete collar at the top which would take care of the bearing load. However, the frost depth in Michigan is 42" and I really don't want to require the installers to dig 42" down into the soil where in some cases this might be extremely challenging.

This is why I am also wondering:

-Is frost depth for slopes different from flat surfaces where water level below the surface will be rather constant?

-How can I calculate the frost heave forces pushing up on the pipe and concrete collar and, if this force is greater than the resisting, how do I calculate frost heave displacement?

Thanks again! I hope you enjoyed reading all that!

 

[Teguci]

First, I will apologize because I am not going to answer your question. However I was intrigued by the idea of using small pipes for piles. This seems a lot like mini piles.

For minipiles, the pipe is driven in and then pulled back out a few feet leaving a void at the bottom. The pipe and void is then filled with concrete. A re-bar is included into the grout which bonds the concrete bulb at the bottom to the pile.

If you could do something like this, you could increase your bond area at the bottom and potentially reduce your embedment length. Testing would be appropriate.

As for the frost heave problem, the heaving action is acting against any downward load on the pile and, your pile sounds like a friction pile which means it should have considerable uplift resistance. Any heaving friction would be proportional to the circumference of the pile and would probably have the same friction on the pipe as the soil does - ie your soil friction resistance without the safety factor. As long as the pile is embedded more than 3 times the frost depth you can probably ignore the frost heave friction issue (pipe won't move because of soil resistance below frost depth and loading capacity won't be affected up or down).

Of course I am not speaking from experience, but given a small, non-life threatening project with lots of redundancy I might be tempted to give it a try.

 

[civilperson]

Your soil calculations are giving you 50 psf for usable skin friction. I would run pile load tests in a variety of soil types to see if the assumptions of adhesion and shear match the true load capability.