Slab Over High Volume Change Potential Soil 1

[Abuh001]

Hi All,

I hope you are doing well.

I have been instructed to design a new concrete ground floor for a renovation project. The ground conditions are London clay with a high volume change potential, please refer to image 1 for the borehole log. There is a 15m tall ash tree roughly 12m away from the edge of the concrete slab.

Due to the presence of very shallow existing footings amongst other constraints, the slab is to be cast directly on the clay. I have decided to support the slab with screws piles at regular intervals, should the soil shrink away, the slab would still be supported. However, should the clay swell it would apply pressure directly to the underside of the slab. How can I calculate this swell pressure? Any help would be greatly appreciated.

 

[TigerGuy]

Do you have any lab results for these soils?

 

[EireChch]

Dont bother calculating it. Include a void former beneath the slab which absorbs any swell pressures. You can get different thicknesses for different volumen change potentials. Contact a supplier like cordex.

 

[Abuh001]

Hi

thanks for the responses, still waiting for lab results, however, I have been informed that it is high volume change potential.
With regards to Cordex void former, this is a very sensible idea and something I would typically do. However, this project is unusual in that I do not have any tolerance to dig under slab, not for blinding, not for hardcore, not for any Cordex. The slab will have to sit directly on the clay while being supported by piles. The span between the piles will be circa 2m, the loading will be minimal. My concern is resisting any potential swelling forces. Is there no approx formula (even if conservative).

 

[TigerGuy]

You have to look at the piles as if they are inverted. Your uplift resistance is based on weight and shear strength of the soils. Remember that the soils are now disturbed, so your shear strength may reduced, once the piles are installed.

 

[Abuh001]

Hi

Hmm.....my confusion is mainly regarding how to estimate the pressure under the slab from clay swelling (due to intake of water).

 

[Stego1]

You can run swelling pressure tests on U100 samples in the lab.

You may also want to have a look at the desiccation state of the clay, and what the swelling/heave potential is. BRE 412 covers this kind of thing.

Also if you can get hold of 'Correlations of Soil Properties" by Carter and Bentley, Chapter 8 discusses shrinkage and swelling.

 

[ABTSGeoScience]

There are some papers out there with correlations for estimating swell pressure using index properties, but I'm not sure how accurate they are. If it were me I don't think I would go that route. Agree with Stego1, swell test is going to be best practice. Personally I don't think it is a good idea to construct anything directly in contact with a high plasticity clay, I would suggest some type of CL buffer between the slab and the CH clay, that or preswell the clay.