slab on grade supported on piles

[hockeyball]

Hi,

Wondering if anyone has any advice on designing a slab on grade supported on piles (the prefab building is located on muskeg which will be removed and backfilled with fill approx. 3.0m. Under this will be clay.)

Would you assume that the slab is structural and that it is supported only by the piles, similar to a building suspended slab? Any good links or articles that anyone knows of?

Thank-you.
JB

 

[msquared48]

A slab-on-grade implies no pile.

If you need pile for the support of the building, then a large degree of vertical settlement is usually anticipated. Unless the slab settles uniformly with respect to the exterior building foundation, you will need to have a structural slab / grade beam / pilecap / pile system to prevent the slab from becoming cracked beyond belief to the point of not being useful or, at the least, unsightly.

Unless the slab is on pile too, chances are that it will crack, and substantially, especially if it gets hung up at the exterior wall of the building.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[SteelPE]

In the US, most of the information you are looking for would be provided in the Geotechnical report. The report would state the susceptibility of the soil to differential settlement and whether the slab should be supported on piles or not.

The design would be similar to that of a suspended structural slab depending on what type of supporting systems you use (grade beams/pile caps, mat foundation etc.). However, your loads may be larger(this would be project specific) and your reinforcing coverage may be larger as well.

 

[dik]

Muskeg is not usually good 'country' to build stuff in/on. If you require a slab that will not move, then design it as a flat plate and support it on void form.

Driven piles?

Dik

 

[hockeyball]

This is my first time designing with piles, hence the questions....

Driven piles will be used, the geotech indicates the end bearing and skin friction. I have calculated the vertical capacity based on this. I haven't used void form before, how would you address this in your analysis?

The geotech has also indcated a formula for downdrag which I have calculated, but I'm not sure about checking the settlement with this.

Thanks
JB

 

[msquared48]

Your Geotech should be able to provide you with the anticipated settlement values. That is why you have him (or her).

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[hokie66]

Piles are just columns in the ground, so the slab is just like any other suspended slab. You are using the ground for formwork. The only difference would be if you have swelling material under the slab, but with 3 metres of fill, that would be unlikely. The fill can be placed without a lot of compaction, so the natural settlement of the fill will form a void under the slab.

 

[JAE]

Agree with hokie66. I wouldn't use void forms if the reason for the piling is to resist soil settlements (downward).

If there is expansive clays and the possibility of heaving soils, then a void form system would be a consideration.

 

[JAE]

..sorry -- "if there ARE expansive clays..."

 

[Preeng1]

Can I ask if the geotechs included any geo fabric in their design under the fill ?

Is the clay under the fill material what ws there to begin with and if so what sort of composition or is it part of the fill plan and thus "clean" material ?

We have seen record rainfalls in western Canada in some areas and the results have been unusually high predictions of heaving in some unstable soil areas much less muskeg regions.

Bob

MBC International

Bob
MBC International
Pre-engineered Building Systems