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Slab on ground with high reactive soil

Slab on ground with high reactive soil

Slab on ground with high reactive soil

I review the drawings for footing construction of a residential building. The strip footing/piers o shale was designed. The piers were 2.5m spaced and strip footings are spaced at 6m max. to support brick walls above. The concrete slab on ground is 150mm thick with 2 layer of steel reinforcement mesh top and bottom.
I believe that the strip footing should be no problem when they are supported by mass concrete piers bearing on shale.
However, the slab on ground will experience the high reactive soil movement even though the standard soil compacted is provided ? That's why the slab on ground need 2 layers of steel ?

RE: Slab on ground with high reactive soil

This sounds like you need to check with a local geotech for confirmation or change of your plan.

RE: Slab on ground with high reactive soil

Is the geotech report available? I think that you need to look at it.

RE: Slab on ground with high reactive soil

According to geotechnical report, the existing fill, the topsoil, and colluvium soils is not suitable for the founding to footing. The soil is Class H (high reactive soil). Due to high potential differential settlements of the existing underlain soils, it is recommended that the building is on pier to take the building load on low strength shale.

I think the strip footing is fine. Not sure about the slab on ground is conservative designed or not ?

RE: Slab on ground with high reactive soil

I think you need to research what is done locally, both design and in-place experience.
You mention slabs on a 'highly reactive soil' and sends alarms to me. I deal a lot with moderately to highly expansive soils & formations and with slightly to moderately collapsible soils. In my experience of owner complaints, slab problems FAR outweigh the foundation problems.

RE: Slab on ground with high reactive soil

Can you excavate the unsuitable soil and replace it?


RE: Slab on ground with high reactive soil

Sounds like the structure is going on foundation elements that bear below the topsoil/colluvium. That makes sense. Sounds like the lightly-loaded floor slab doesn't need the same support. In that light, then the engineer says you can just pour a slab on grade.

I would not pour a slab on grade if there was topsoil or loose-density native soil (i.e., colluvium). If that were the case, I'd remove the topsoil, compact the prepared subgrade and proofroll with a fully-loaded 10-t dump truck. I'd then restore any portions of the subgrade that rut and be done.

Then again, would have first prepared the geotechnical engineering report.


ípapß gordo ainÆt no madre flaca!

RE: Slab on ground with high reactive soil

We have some highly active shale-derived clay soils in my area. I would never recommend using normal strip footings to carry the wall loads- ground beams supported on the piers would be recommended. If you must use strip footings, they should be founded deeper than usual (say 3-4ft) since heave tends to be most problematic at or just below surface.

That brings us to your floor slabs...I would imagine that some excavation and replacement of the poor soil is justified.

Last word- geotech. guys love to exaggerate the "highly active" nature of clays. Some clays are violently active, others are not...check this with a professional who has good knowledge of the local soils.

All the best,

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