Uplift Capacity?? 2

[dtsk]

Hi all,

Any rule of thumb when estimating uplift capacity of piles?

We have piles (300 dia, between 2m and 3m deep) embedded in a mixture of cohesive and granular soils. Su in cohesive range between 70kPa to 150kPa. DCP readings of between 2 & 3 in granular (50mm increments).

We are doing pullout test at construction stage to confirm but want an estimate for design purposes.

Cheers

 

[Ron]

Typically 25 to 50% of load capacity of the pile. Depends on depth and soil conditions

 

[Okiryu]

For driven piles, in Japan, I heard that some people assume 30% of the pile capacity.

 

[SlideRuleEra]

I've seen the value of 2/3 of the friction capacity of a pile in sand (Note: The 2/3 does NOT include point bearing, see below). IMHO, this does not contradict the 25 to 50% range that Ron and Okiryu have offered (percentage of the total pile capacity... including point bearing). A displacement pile, either cast-in-place or driven is going to have significant point bearing capacity in granular soil.

What I do consider important is the length of the pile, from the description I can't be sure.

If the piles are 2 to 3 meters long, I would expect uplift resistance to be a very low percentage of the allowable bearing (say, << 25%).

If the piles are "long" (perhaps 8 to 10 meters), with the last 2 to 3 meters embedded in good soil, then uplift resistance may be much higher (say, > 30% of total allowable bearing).

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[dtsk]

Thanks all,

A value less than 25% does sound reasonable in this case. Would this be the allowable uplift pressure or un-factored?

I came across this equation (see attached), have you guys used this before?

 

[Terratek]

Not sure about piles, but for drilled piers I have been taught that uplift can be calculated by using the skin friction value (in compression) with a 15 percent reduction. No-one has ever been able to sufficiently explain to my why the skin friction value isn't the same in tension vs compression for drilled piers. I could see a reason for the reduction in a driven pile.

 

[LRJ]

The Poisson effect is one cause for reduced shaft friction in tension compared to compression. This is particularly the case for longer piles. The premise of this is that when you compress a pile, the pile will slightly bulge outwards and further compress the adjacent soil. In contrast, when you pull a pile, the pile will become more slender and the soil would slump towards the pile. Soil is stronger when in compression than in tension, hence the difference in shaft friction for the different loading.

 

[Terratek]

Thank you LRJ! That makes a lot of sense.

 

[bdbd]

Remember not to apply any FS to weight of the piles.