Pile Design (point of fixity)

[KkEngineer]

Dear all,

I have an instance in a pile design where the vertical load is very low such that the point of fixity (which is primarily governed by the diameter of the pile, calculated as per Tomlinson) is deeper than what is required by bearing capacity check. I need a minimum pile diameter in order to practically support the platform above it.

My question is how do i determine where to stop driving my pile in such a case. There should be a practical depth below the point of fixity in order to achieve that point of fixity right?

Ex.

Pile load: 190 kN approx. (SLS)
Pile dia approx. 900 mm
Sea bed level: -8 m
Top of pile: +4 m
Depth of point of fixity below sea bed: 7.5 m
Pile depth required as per bearing capacity check: 6 m below sea bed.

Thanks
VK

 

[Ron]

What is the lateral loading on the pile?
Have you considered the sea bottom "fluff" in your fixity calculation?
Is there any uplift on the pile?

Have you used L-Pile?

 

[KkEngineer]

Ron,

I tried to consider the lateral load of the pile in order to determine the depth but that again is very small (around 25 kN).

I am sorry but I am not sure what fluff means. There is no uplift. I used hand calculations using Tomlinson.

Thanks

VK

 

[BigH]

VK - the fluff Ron is talking about concerns a thin layer of underconsolidated sand or clayey "muck" - what we in Canada call "Loon Shit" . . . There are times that the upper portion of the seabed is very soft or loose and not indicative of a metre or so below.

 

[BigH]

By the way - have you taken, too, into account lateral forces of, say, a small boat (or a bigger boat) hitting the platform? I'd think that the lateral force would then be greater than 25 kN (2.5 US tons).

 

[KkEngineer]

Hey,

Oh yes, I should have just guessed that. I have assumed that the pile is unsupported upto 1 metre below the sea bed level.

I have not considered any impact loads. That is a fair point.

In any case my doubt still remains. That there are these two independent analyses,

1. Point of fixity calculation
2. Bearing capacity check

It seems more probable (considering my very short experience) that the bearing capacity check will always provide a depth below the point of fixity.

What do we do when it does not?

Thanks.

 

[Lomarandil]

If you can provide fixity at the top of the pile (which can be very difficult), your structure may be stable without developing fixity in the embedded length.

Otherwise, I believe you need to provide the larger of the two embedments required. As Ron suggests, you may be able to refine your point of fixity calculations using L-pile or an equivalent spring analysis with the guidance of a geotechnical engineer.

 

[SlideRuleEra]

VkEngineer - I've sketched my understanding of your project, see below. If this is correct, something must be wrong with the math, the results are not reasonable. Fixity is used to analyze lateral loads. As BigH and Ron have determined, your lateral load is very small for a 900 mm pile. Also, a pile must be much longer than the depth to the assumed point of fixity. Soil properties have got to be considered to determine fixity, but you don't mention them. A depth of 7.5 meters to fixity is very deep. With that said, soil properties must not be too bad, 6 meters of embedment for bearing is "low".

A time tested "rule of thumb" from the early 20th century predicts that, in good soil, pile embedment = pile projection to resist lateral loads. This indicates (and that is all) that the pile tip should be at approximately Elevation -20. Even that would not be deep enough to ensure fixity at Elevation -15.5.

Things do not add up, recheck your calculations. Also, download "How To Determine Lateral Load Capacity of Piles" from this page of my website:

http://www.slideruleera.net/TreatedWood.html

This paper talks about estimating fixity using hand calculations. Don't let the fact that the paper address wood pile deter you, its the same math for other pile materials.

Here is my sketch:

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net