Pile Settlement Calculation

[RFreund]

What theories/equations are commonly used for immediate pile settlement?
Poulos and Davis?
Bowels?

This question probably seems vague so I can provide more information but I'm not sure what information to provide...

EIT

http://www.HowToEngineer.com

 

[GeoPaveTraffic]

I don't have a specific reference since this isn't something that comes up very often; but the two you list are certainly a good place to start.

In general there will be two components to immediate pile settlement: elastic compression of the pile and elastic compression of the soil/rock the pile is bearing in/on. If we ignore the case of a pile bearing on rock, the real question is how the load is transferred from the pile to the soil since that determines the amount of load in the different materials. Frankly that question is hard to answer with a high degree of confidence in most cases. To further complicate the situation, as any long term settlement or consolidation of the soils take place the amount of load transferred along the pile also changes causing additional elastic settlement.

Here is a rough outline of how I would approach the problem.

1. Develop a FOS=1 load curve for a single pile versus depth.
2. Use the actual load to be applied to the pile and the curve above to estimate the amount of load in the pile at various depths.
3. Estimate the modulus of elasticity of the pile and the soil at various depths.
4. Calculate the strain and then the settlement of the pile at various depths.
5. State the result as a range of possible values.
6. Expect the rest of the design team to be unhappy with the size of the stated range of possible values and that if field measurements are taken expect the measurements to be of low accuracy and of little use in determining how much movement actually occurred.



Mike Lambert

 

[RFreund]

Mike thanks for the response. Your approach seems very reasonable. I especially like #6.

Next step... What if you consider only end bearing? I am thinking of a helical pier situation. I suppose that could be an entirely different thread...

EIT

http://www.HowToEngineer.com

 

[GeoPaveTraffic]

If the pile is really only end bearing, and I expect most helical piers are (at least if they only have one disc); then you could assume that 100% of the load goes to the disc. Then the load in the steel is easy and so is the compression. As for the soil, you would have to assume a stress distribution below the disc and then apply that load to modulus of elasticity.

Given the relatively low capacity of most helical anchors, I would expect the elastic settlement to be small.

Mike Lambert

 

[RFreund]

Thanks Mike.

One other question:
This has been brought up before Here, Here and Here.
Does anyone know where to find:
Fox Embedment Factors from volume 1,pp 129-132 of the 2nd icsmfe

EIT

http://www.HowToEngineer.com

 

[Okiryu]

RFreund, because your piles are end bearing, perhaps settlements are not a big concern: you have stiff/dense soils so settlement may be minimal.

But, for helical piles, I would check also for the torsional stresses and the structural capacity of the pile. In my area, helical piles have small shaft diameters so we have to check them for torsion and buckling. The pile installer may have a torque vs. pile capacity graph so you can check against torsion. This is more critical if the bearing layer is rock so you can get high geotechnical parameters.

 

[geotechguy1]

In our areas most companies will do one of two things: say that settlement will be less than "x" mm if designed using their parameters, or they might use a high tech correlation like assume 0.5% to 1% of the pile diameter for friction piles.

For most projects the above methods are probably suitable given the limited scope of most investigations, limited amount of testing, variability in testing, variabilty in correlation used to determine parameters.

Eg: I do some SPT tests in 4 boreholes, where the tests have a variability in the test results of +-30% depending on the company and operator. I then convert the SPT N value to a shear strength using some correlation of questionable applicability with a variation of +-30%, or a correlation for SPT N vs soil modulus parameter that was determined by drawing a linear line of best fit through a scatter plot of data...data obtained from 100 tests in a very limited geographic area with questionable applicability to our local soils and limited information on the physical and chemical composition of the soil to compare it to our local soils.

Unless you have a huge amount of data it's probably not worth it to do detailed calcs. I've seen a few projects where it was, but we had half a dozen boreholes, a dozen CPTs plus sonic holes within the footprint of a storage tank which was used to create a 3d model for settlement. I think they got the final settlement whithin 35%.

 

[BigH]

RFreund - Tomlinson's Foundation Engineering and Construction book has the Fox chart. Page 77 in the 6th Ed and Page 79 in the 7th Ed.

 

[kmart30]

Did you do any CPT's? Cones are a good tool for getting all your design criteria for when you are designing piles. Obviously double check your soils with SPT's and other methods but I think the FHWA has a good report on CPT data correlating to pile design.

 

[Ajay_Shastri]

Correct me if I am wrong but if we are talking about immediate settlement why not just A-Pile, you can set the it custom discretization (for greater accuracy). It does provide load vs settlement graphs directly (the procedure is based on RFreunud's procedure)