Ice Uplift Force on Piles 7

[PEStruc]

Hi Everyone,

I have a project in lake Michigan and it’s basically a marine dock fixed on piles in lake Michigan. I’m trying to calculate the uplift force on the piles due to the ice. The historical ice thickness is 30”, pile dia. is 12”, pile height above the lake bed is 14’ and it has an embedment of 24’, so total length = 38’ . I used AASHTO eq.3.9.5-1 for determining the vertical force due to ice adhesion as follows:

Fv= 80 t2 (0.35 +0.03R/t0.75)

where:
Fv= Vertical force in kips
R= radius if ft.
t= ice thickness in ft.

Since the above equation doesn’t address the type of the material, so I’m not sure whether shall I use the ice uplift force from the above equation times the coefficient between the ice and pile material (steel) in order to determine the ice uplift force in the pile design? Or shall I take the force as is from the above equation without multiplying it in friction factor. There will be a huge difference between the two approaches.

Also, I checked great lakes small craft harbor and structure design for ice conditions : An engineering manual by Allen Wortley, university of Wisconsin sea grant institute, however it has the same problem as it doesn’t address the used material for the determined ice uplift force and I’m not sure if I shall use the friction factor times the uplift load or not. I would appreciate if anyone has an answer for the above question.

Thanks

 

[oldestguy]

As I recall some discussions with Al Wortley when he was active in this subject, the exact gripping force was pretty hard to determine or predict. Some jobs he worked on got around this by installing bubbler systems.

 

[oldestguy]

I did a Google search for Alen Wortley and two places came up that may be still active. it may be worth a call to Al, although a few years back a a funeral I understood he was experiencing some old age problems, but seemed OK. Sometimes he used C. Allen Wortley.
There are two numbers on the search one in a suburb and the other in Madison, WI

 

[chicopee]

Contact your local Corps of Engineer. Also here is a link pubs.usgs.gov to this publication "Frost Heaving of piles with an Example from Fairbanks, Alaska". Some of that content is applicable to your situation.

 

[OldBldgGuy]

Consider sleeves (pvc or poly pipe) over the piles at the active area and reduce the uplift to a negligible amount. Of course if the ice comes up under the dock it won't matter much.

 

[PEStruc]

Thank you oldestguy,chicopee and OldBldgGuy for your responses. The project has a limited budget,so it will be hard using bubbler system . Also the concern of using the sleeve, poly pipe, epoxy coating,... that the uplift force will be reduced, however the reduction cannot be quantified, so I'm not really sure how much will be the final uplift which the pile embedment shall be designed upon.

 

[CarlB]

Do your calculations show there is the potential for uplift, using the worst case (no reduction for friction factor)? I'd think that 24' embedment can easily resist 30" ice adhesion. Rule of thumb for arctic pile embedment is to have twice the embedment below the active layer, as the full active layer can adhere and uplift with frost heave. Probably the reason those publications don't mention a reduction for various friction factors is there is not much difference in adhesion (adfreeze) bond strength between typical pile materials.

 

[dik]

CarlB:

That's the rule of thumb in Winnipeg, too... typically a minimum of 20' friction piles...

Dik

 

[PEinc]

Excerpts from The Marine Environment and Structural Design, by John W. Gaythwaite, PE, Van Nostrand Reinhold, 1981:
[ul]
[li]Ice fails by cracking and not by slippage at the ice-pile surface.[/li]
[li]The radius of load distribution is greater than the pile diameter (approx. 6" greater for steel piles and 3" for timber piles) and is characterized in actuality by the formation of an ice collar or a zone of ice of thickness greater than the ice sheet adjacent to the pile.[/li]
[li]Wortley's method is a thin plate analysis which may not be as accurate when the ice thickness is greater than the diameter of the pile plus the thickness of the ice collar.[/li]
[li]The method shows that the uplift force increases rapidly with the ice thickness but does not increase significantly with pile diameter.[/li]
[li]Assuming an ice flexural strength of 200 psi would give minimum uplift forces for a 12 inch diameter pile on the order of 9, 30, and 70 kips for ice thicknesses of 12, 24, and 36 inches respectively (slightly higher forces for steel and lower for timber piles)).[/li]
[li]By contrast, the more exact uplift forces on a steel pile in 24 inch thick ice would be approximately 33, 37, and 42 kips for pile diameters of 12, 24, and 36 inches, respectively.[/li]
[li]Corresponding adhesion values (applied to the pile perimeter x ice thickness) are typically less than 50 psi, but for very thick ice and smaller diameter piles it is possible that the pile would slip before the ice failed.[/li]
[li]Although traditional methods of estimating ice uplift by applying an adhesion value such as indicated in Table 6.2 may have been conservative, Wortley cautions that the method described gives minimum values for design and maximum uplift loads could be up to several times greater, depending upon the ice strength and exact failure mechanism.[/li]
[/ul]

www.PeirceEngineering.com