Pile foundations under Liquified soils 1

[eshwaaar]

Hi,
I am assigned a job to design the pile foundations for Engine generating sets which are supported over the springs. The proposed pile cap is 2m thick and the soil beneath is soft clay upto 3m and hard to stiff clay upto 6m. The soil from 6m to 19.5m is prone to liquefaction and from 19.5 to 21 soft rock and then the bed rock.

Our soil consultant has recommended a pile length of 27m.
Now, my worry is how to take this profiled layer into effect and how to calculate the coupled mode of vibration under seismic considerations.

I have done some hand calculations for a 2mass spring system as per Arya Oneil, and calculated the eigen values also.
I have also modelled the same in staad using some linear spring supports (1300KN/m as proposed by our soil consultant) for the pile elements.
The frequencies are quiet different. Also, the stiffness of the pile group calculated as per Arya Oneil is showing quiet high values..

As the piles are composite and also due to heavy loading Iam unable to proceed further..
Any help in this regard is highly appreciated.

Thanks..

 

[garrettk]

I'm not sure how it will effect the vibration analysis, but have you considered drilled shafts as opposed to piles? The dynamic response of the shaft would likely be different than a pile.

 

[eshwaaar]

Hi, Garettk,
Since, the pile dia. is only 330mm, hence, iam feeling that it should be treated and analyzed more like a pile foundation.

But how to proceed further is what is the need of the hour.

Thanks,

 

[civilperson]

With a close spacing of piles around the perimeter of the pile cap, (2.5 Diameter of pile or less), the mass of the soil interspersed within the pile group will act as a unit and the mass of that soil block can be lumped with the foundation system. The liquefaction potential will deduct a small percentage from the total mass acting under dynamic loading, (negligible).

 

[eshwaaar]

Hi,
Is there any literature which I can claim as support for my argument, as i need to convince my boss and the client also.

If at all I have to lump the soil mass along the length of the pile, then the whole system will become a single degree of freedom system which will be quiet simple to solve.

But, how could be the effect of liquefaction in this situation is my worry.

Hope I get some good advices.

Thanks

 

[dgillette]

There has been extensive research on piles in liquefied material by Ross Boulanger at U of California at Davis, Kyle Rollins at Brigham Young University, and others. Search their websites.

DRG