designing house piers in extremely reactive clays

[flyingdebris]

can someone point me in the right direction of how to design house piers in E class soils with AS2870? I have heard of people sleeving piers to create a "frictionless" surface between soil and pier to limit movement. Is this essential? Is the other option to make the piers deeper to effectively lock them in to the soil so their movement is limited? how do you calculate this? is there a pressure value based on the Iss or ys movement of the soil exerted on the pier, which you have to resist with an uplift calculation of the pier?

 

[flyingdebris]

I should add that the house is timber, fully supported on stumps.

 

[rowingengineer]

generally the depth of the black soil out back of qld is about 1.2m. so we noramally sleeve this depth and have a pier under. you coyld however design for uplift and not sleeve. we normaly calc deeper pier with this method

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"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[flyingdebris]

Im afraid i only have approximately 1.5-2m to bedrock, and maybe not enough depth to achieve this uplift resistance. rowing, where can i find the formula to calculate the uplift force/pressure caused by the soil swelling on a pier? Csnnot find in 2870.

 

[flyingdebris]

Hi Rowing, ignore last post. I have found the relevant formulas.

 

[rowingengineer]

I have only seen it in a few locations the main being from the army corps TM5-818-7, basically it is the max shear strength by the depth of the soil for the movement zone, others I have seen divide this by 1.5 for the change in likely moisture content over the movement zone.

I ahve seen no real direction from the Australian standards in this regard.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[flyingdebris]

for a raised old queenslander timber house on steel stumps supporting only by piers (no slab), are you connecting all piers (being concentrated loads) as described in as2870 3.10.1? I know of a lot of engineers that are not doing it and think it is not required. turns a simple house raise into quite an expensive project. your thoughts??

 

[rowingengineer]

No I wouldn't connect all the piers, I don't see the advantage, as piers are very low movement items if design appropriately.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."