Footing partially in a water table, what precautions 2

[IJR]

I have never considered this one before

Time to consider now, a footing supporting a light framed building, where bearing capacity is no big deal, but geotech reports a water table very close to the surface. I am not involved in the design but I would like to know if there are simpler solutions to the problem

Respects

IJR

 

[KAM_imported]

This is the case of many structures built in Florida. Main issues will be relatively low available bearing capacity, and constructability. Buoyancy effects should also be considered. Contractor should be aware that dewatering will be required for footing construction. Depending on required depth of drawdown, well point dewatering better than pumping out of a sump because the latter can result in upward seepage pressures, which loosens sandy soils. Have you considered placing fill in building area to lessen water problems?

 

[IJR]

PREFACE: THE KNOWLEDGE THAT KAM IS THERE AT FND ENG IS VERY RELAXING

Yes it did occur to me that water must be removed and filling must be made. However I am not familiar with this kind of practice.

A friend suggested I put a geotex at base level all over and cover it with a thin lean concrete layer stuffed with mesh to prevent water flow upwards and the mesh to protect the geotx.

Thanks Kam

IJR

 

[Ron]

IJR...in addition to KAM's comments, you mention that the footing is lightly loaded, so you might consider over-excavating the footing bottom and backfilling to footing bearing level with graded aggregate, then placing your concrete. That is assuming the water table is only right at the bearing level. If higher, you would need to do some specific dewatering and KAM has covered that well.

One other consideration is dilution of the concrete. If you place concrete into a water laden footing, you will dilute the concrete, since there is likely an inadequate volume to displace the water as is done in shaft placement. Make sure the concrete is not diluted as this occurs mostly at the bottom where the concrete needs to have both good strength and good durability (protection for the rebar).

Ron

 

[KAM_imported]

Besides using footings, you might want to consider short, helical compression piers. A grade beam or cap would still be required. Both the AB Chance Company and Atlas Systems, Inc. make these foundation elements, and have web sites.