Dense/weak layers 1

[ANSIF]

Dear Colleagues: I have a dense sand layer (N>50 blows) with a thickness from 2.5 to 4 m, underlain by a weak sand (N 10 to 20 blows). In the case of using a piled foundation, Meyerhof (1976) recommends to reduce the pile tip resistance of the hard stratum according to a proportional relation between the thickness of the dense sand, H, and a critical thickness of 10B (B, wide of the pile); another proposal suggests to verify the bearing capacity of the weak layer, considering an enlarged loaded area defined as a function of the hard layer thickness. Do you know some other approach or criteria to take into account the influence of the weak layer?

I thank your comments.

 

[BigH]

Are you driving your pile into and stopping in the very dense sand? (i.e., assuming the sand is not the surface layer) What is the material above the very dense sand? - or are you driving your pile through the very dense sand into the compact sand?

If the former, it would appear that you might have a problem of "stopping" in the thin layer - only 12ft thick - you will have to be careful. (I did this once - stopping in a compact sand underlain by soft sensitive clay). I would estimate how much "room" you have below the pile tip - and see if the pressure bulb (see the classical failure surfaces) extend below it. If you are okay, I wouldn't likely reduce the capacity on a theory - but would likely be conservative on my final estimated allowable capacity.

If you are driving through the very dense sand into the compact sand, the very dense sand wouldn't have an effect. Check Poulos and Davis pile book on densification of sand due to pile driving.

Now, keep in mind the effect of piles - and how the group effect may go into the lower layer. In my case above where I did it before, we used wide pile spacings so it was all "single" pile and no group loadings.

 

[ANSIF]

BigH, over the dense sand I have another weak sand layer about 10 to 12 meters. It is a good idea to make piles working as an isolated element, avoiding they work as a group.

Thank you very much.