Water occurring under slab

[kansascity]

We have recommended a 2 ft over excavation and backfill with crushed limestone aggregate w/fines under a slab on grade for a building. However, there is either ground water or drainage occurring within the in situ soils. The soil is a fat clay/expansive.

I would think we would need to divert the water somehow, or possibly put some open graded rock(gravel) under the aggregate base. Thoughts?

 

[fattdad]

How will the slab be used? If there are any floor finishes, you need a capilary break of open-graded aggregate.

f-d

¡papá gordo ain’t no madre flaca!

 

[kansascity]

It will be a slab for a library. I believe it is going to be carpeted.

 

[fattdad]

without a doubt you must include an open-graded moisture barrier and ALSO a sheet of visqueen to further reduce vapor transmission. You also need to make sure that the relative humidity in the slab is less than about 75 percent prior to applying flooring materials. This can take some time.

f-d

¡papá gordo ain’t no madre flaca!

 

[theCorkster]

If you do have such shallow groundwater, you will want to provide drainage, and I would strongly suggest using a designed vapor retarder as well (belts and suspenders).

 

[oldestguy]

With your open graded materials and drainage, don't forget the principles of filtration. I prefer to use graded layers from fine to coarse (sand to gravel), but some swear by filer fabric. You don't want to undermine your slab by taking silt and fine sand along with the water.

Lastly that "vapor barrier" had better be a good one. Ordinary Visqueen is not good enough, especially with stakes driven thru it.

 

[msquared48]

I would recommend under the slab lateral drain pipe under the capillary break (gravel)to carry the water away. Otherwise, you will get seepage, let alone, moisture.

Mike McCann
MMC Engineering

 

[dik]

I would install underslab drainage and separate the drainage layer from the concrete using a PEVB. You may require added sumps if the amount of water flow is great enough...

Dik

 

[fattdad]

Just a caution: Be careful with the use of open-graded aggregates beneath a concrete slab on grade. If you are working in a rainy area and your working without a roof, 100 percent of rainfall will infiltrate an open-graded aggregate and you'll have no where to get rid of the infiltrating rainwater. If the contractor proceeds with construction in the days following a rain event, the subgrade will turn to mud and you will end up with a lousy slab on grade.

We recommend dense-graded aggregate below all ground-supported slabs. It is less of a vapor barrier (i.e., capillary break), but much better for construction.

f-d

¡papá gordo ain’t no madre flaca!

 

[escrowe]

I frequently recommend a perimeter underdrain consisting of slotted PVC embedded at the base of the perimeter footing, draining to either a sump and punp system or by gravity to daylight.

Also, someone on the project needs to consider the impact of vapor intrusion on floor coverings. I believe greater than 3 to 5 pounds per 1000 square feet per 24 hours is excessive for most varieties.

The completed slab can be tested for vapor intrusion...