Uplift on Underground Slab

[ahunt]

I have a theory question I hope you folks can help answer.

We often have slabs under uplift conditions due to high groundwater (out to out of exterior walls footprint under uplift) . We extend the edges of the slab outward to engage the soil as part of the "righting" gravity load, this soil is assumed to be buoyant (120 pcf - 62.4 pcf = 58 pcf). There is some soil friction (granular backfill) that can also be taken into account isn't there? The effect of uplifting the slab results in an additional "wedge" of soil that must also be overcome due to this soil friction.

Do geotechnical engineers take this friction into account under uplift conditions? Or do they just suggest to use only the soil overtop of the slab directly? In the past, we have assumed a 2v:1h slope on this wedge to represent the soil friction. Can you point me to a good text book or research? I feel the way we've been doing this is somewhat unconservative.

Thanks,
A-

 

[Gimbli]

1) Why bouyant? Is it saturated?

2) If I understand your point well, you are saying that granular backfill will be providing a reaction to the slab subjected to uplift forces. If so I understand the soil friction that may be accounted occur directly through a vertical line located on top of the slab edge up to the surface and may be calculated as effective overburden pressure times tangent of phi. Basically, the total resistance is obtained by the friction previously mentioned plus the wight of slab and weight.

Bowles book provide a good explanation together with names of researchers on the subject and suggested equations.

 

[kslee1000]

The internal friction angle for fully indundated material is not well defined as well as the friction against wet concrete surface, thus it is a risky business to include the soil wedge and friction force in your calculation for a ledged box-type structure against uplift.

 

[fattdad]

Determine the weight of the soil that is present in the "wedge" of soil that extends upward from the edge of the footing using a 1/2H:1V projection. Use bouyant unit weights as appropriate. If you use the wedge, do not consider the shearing resistance. If you want to use a vertical projection and the shear forces, I guess you could. I just do it the other way.

f-d

¡papá gordo ain’t no madre flaca!