Slab on Grade Near Water Table 3

[NBRY1]

How close can the capillary water break below a slab on grade be to the water table for optimum performance?
I'm assuming as long as the capillary water break is not overwhelmed, it would be okay.

 

[r13]

Please explain "capillary water break" and its relation to frost-heave action. If the latter will not occur, and a moist floor is not a concern, then it should be fine.

 

[JAE]

If the water table gets up into the granular material, and there is an adequate moisture barrier above, AND there is a way out for the water (say a serpentine perf drain to a sump) then it may be OK as long as the water table is perhaps in the lower half of the granular material thickness.

I'm not sure I'd want to live in a house, or reside in an office/business with that high of water, though as moisture barriers aren't always perfect.

Another more aggressive option would be to use a waterproof barrier (Volclay panels) or some such waterPROOFING system, then have a very thick (18" to 24" perhaps) uniformly graded granular capillary break below that to a sump with a perforated drain. But even then I'd not be totally comfortable. With very high water tables it sometimes pays to elevate the building/slab system higher than surrounding grade.

 

[oldestguy]

You sound OK to me. My water table is below basement slab at times maybe 6" and no negative interior condition is observed. Part of site is clean sand and part very silty sand. Otherwise, no special barrier.

 

[r13]

It is a good site to consider design a swimming pool to take advantage of the free water, once filtered

Joking aside, for ground floor in a relatively humid region, when south wind bellows, not a good combination. Raise the building/slab with coarser fill is good suggestion. I did specify water/vapor barriers for SOG, but always doubt its value given so many factors can render it a waste.

 

[Ron]

NBRY1...I see you are in Georgia and presumably from your water table question, somewhere in south or coastal Georgia.

A capillary break needs to have a fineness modulus of somewhere above about 2.5. Fine gravel is even better. With that in mind, extend the capillary barrier a few feet beyond the slab on grade and you'll be fine with a shallow water table. I practice in Florida and Georgia and have seen this many times.

If your slab is confined by a footing or stem wall, you obviously can't conveniently extend beyond the slab, so increase the fineness modulus or use fine gravel. If the water table gets to the bottom of the capillary barrier, you'll be ok. If it goes higher, it's no longer a capillary rise issue but a hydrostatic pressure issue.