Load Distribution through gravel vs clay 1

[TheHayDuke]

Been working on some wind turbine crane pads for a few months now, and I swear the ground is getting progressively worse with every now pad. Generally I've been lucky enough that the bad material is close enough to the surface that the client is willing to simply remove it all and replace it with well compacted gravel but I'm looking to get ahead of the inevitable issue were we get something soft at a depth that isnt feasible to replace.

So I've been looking at different theories of pressure distribution below footings, boussinesq, westergaard, 2:1 etc but all of these methods generally assume a homogeneous material. Maybe I'm off here, but the increased load under a meter of gravel would be less than it would under a meter of a soft clay right? the increased strength/rigidity would cause the load to spread out more is what my gut tells me, but i cant find any literature to support that hypothesis.

I did stumble across a couple papers by Rocscience for a "method of images" they use in their settle3d that does seem to account for differences in rigidity across a layered soil, but they dont seem to have any public information on how exactly that works.

Any experts around have some knowledge to share?

 

[SlideRuleEra]

...get something soft at a depth that isn't feasible to replace.

Where I worked, South Carolina coastal plain, poor or layered soil are normal. Suppose there is a sophisticated method to handle this situation, but a simple way is to use a modified 2:1 approach:

Use 2:1 through good soil, then vertical thru poor soil. If the soil is layered, like this:

Calculate footing bearing pressure based on allowable bearing pressure for the "poor" soil layer.

If the soils are too "poor", a pile supported foundation is usually the next most cost effective solution.

As soils get worse, foundations get more expensive... there are no shortcuts, tricks, or ways to get around this.

 

[dik]

The big problem is that soil capacity is often determined by settlement and not by bearing value.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[SlideRuleEra]

Where I worked, bearing value is not "allowable" unless predicted settlement is acceptable.

 

[PEinc]

Drive some bearing piles or drill some micropiles?

http://www.PeirceEngineering.com

 

[geotechguy1]

I thought Westergaard assumed alternating sand / clay layers? I could be wrong, that's what I remember though.

SlideRuleEra's image is interesting. How thick is the good soil? If you have 0.5m of gravel over 5m of poo, I suspect the gravel might just fail by punching through into the clay

 

[dik]

in these environs, it's called loon sh*t...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[geotechguy1]

I've been in NZ for 4 years Dik (originally from Canada)...they made fun of me so much for calling all of the soil in NZ loon shit so much that I call it poo now

 

[dik]

Nice country... I understand it's one of the best. I wouldn't have changed the terminology... it's like BFRs... (big rocks)... one's that are too big to step over, but not big enough to warrant walking around.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[BigH]

Interestingly, Dr. Harr (Purdue) wrote a book on Particulate Medium . . . so you could,for the gravel use the stress distribution in that book - and then elastic in the clay . . . but for practical purposes, SRE's suggestion makes sense.