[Australia] Uplift in Shallow Piers

[jrbaus]

Inspired by a recent post on this board, I would like to ask for some more opinions regarding "post/pole" and bored pier footings.

In my location (Queensland, Australia) it is common to see houses and portal frame buildings supported on shallow pier footings. Mostly the soils I deal with are sandy/silty clays so there is enough fines that I can assume a reasonable amount of cohesion. There are however a few instances of sandy sites coming in which I have not dealt with extensively in the past.

I have seen standard drawings by other consultants which show 300mm dia x 900mm deep piers required for houses on sandy sites. The drawings specify that this is suitable for supporting up to 11 square meters of roof N3 wind conditions (118 square feet / 112 mph wind gusts for my imperial colleagues). Given there isn't much mass in a footing of that size, I assume the consultants have considered skin friction in their calculations. Looking at my references attached, there doesn't appear to be any guidance in any current Australian standards. The one standard cited in the attached reference is from the 1979 piling code which is so old I haven't been able to locate a copy. My question - how are other engineers designing shallow piers for uplift in sandy sites?

My first instinct is to size the footings large enough such that no skin friction is assumed in the design. However, given that builders in the area are use to seeing drawings with relatively small footings for sandy sites, I will likely be put in a position to defend my designs. I want to further investigate what (if any) skin friction should be considered in the design of such footings.

Also, if anyone has any good references on determining suitable cohesion (cu) values I would appreciate the advice. I found that soil testers do not like to offer any advice on this, so I generally adopt 20 - 30 kpa depending on the depth of the footing and soil classification which I determined as a "reasonable" estimate in consultation with another structural engineer. The reference attached gives typical values for clays, but I am not sure what reduction should be made for sites with significant quantities of sand, silt and clay (i.e loamy sites). I did manage to see an extract from the old piling code which indicates that the shaft adhesion for uplift design may be taken as equal to the un-drained shear strength (cohesion) where cu < 40 kPa.

Looking forward to hearing your opinions on the above.

Regards Jake

 

[asixth]

I can't say I'm experienced with this but generally I see geotech's specifying pile skin friction around 10% of the bearing capacity but this is mostly for rock and cohesive soils.

 

[jayrod12]

Where I do the majority of my design the uplift skin friction of a pile is noted as 50% of it's downward skin friction by most geotechs. We are in clays predominately and 20kPa is on the high end of an allowable downward skin friction for us so our uplift is roughly 10kPa.

I don't have much data in sandy soils to comment about a conservative value.

 

[jrbaus]

asixth, jayrod12, thank you for your comments. The 20-30 kPa in my previous post for undrained shear strength did not have any FOS applied so will correlate roughly with the 10 kPa shaft adhesion that you mentioned.

For residential footings, Australian Standard AS2870 Clause G5.3 states "No side adhesion or friction should be assumed to exist to a depth of 0.75Hs for down loads". Given that Hs (the depth of suction change) is at least 3m in my area, I usually don't consider any skin friction/adhesion for downwards loading. That said, downward loading doesn't usually govern the design for single story clad frame construction (in my experience).

Regards Jake