Basement Floor Below Seasonal High Water Level 2

[Birchy]

Hi all!

My client wants to build a small residence (@ 110 sq. m footprint) in a coastal zone with a high water table. Soil borings show standing water at - 58" and a seasonal high water table at - 38". He wants a full cellar and is willing to live with a bit of dampness if necessary. Local land use regulations would put the top of the floor slab no higher than 45 to 48" below grade (otherwise the "cellar" becomes a "basement" and must be counted as a "story").

I have never designed a cellar with a floor below seasonal high water or so close to the water table. My calculations suggest that by excavating right down to the standing water and then placing a rubber membrane, 6" of coarse gravel and 6" concrete slab, we will have a slab with its top at - 46" that will not "float" under worst case scenario. I would also place two independent drain tile systems, one under the slab, and one a bit higher outside the footings, both piped to two sump pits and pumped to daylight. The foregoing, however, is purely academic. Does anyone have some real world experience in this area that they would be so kind as to share?

Thank you all.

 

[cap4000]

I have never exactly have done this for a house but more on commerical large $$ projects. Look into a "Volclay Voltex" rubber membrane waterproofing system and completely surround the slab on grade and foundation walls. Similar to a "bathtub" design,used at the once standing World Trade Center 6 stories below the Hudson River.

 

[SlideRuleEra]

Using bentonite, as in the Volclay product that cap4000 recommended is a good approach. Another product that works well, and may be easier to install is "Volclay Panel"; here is a link that describes both products

http://www.cwservices.com.au/voltex.htm

Recommend that you use water-stop in the concrete joints also. I work in an area where the water table is (correctly) assumed to be ground level - my opinion of "traditional" PVC water-stop is very low, it is rarely installed correctly. About 20 years ago started using bentionite based water-stop, here is a link

http://www.cwservices.com.au/waterstop.htm

It seems to install and perform much better than PVC.

You may want to start your foundation stone well below the "minimum" water level to ensure that it's base is always under water, even during "droughts".

Likewise for floatation (and water-proofing & water-stop), assuming a signicantly higher water table than the "maximum" may be wise.

We don't even attempt to put in exterior perimeter drainage; the ground is flat - no where to drain to, and don't want to depend on pumps. Of course your case may be different.

Best Wishes

http://www.SlideRuleEra.net

 

[smwpe]

Birchy

Where are you with this problem?

It makes a difference.

 

[Birchy]

Borough of Belmar, Monmouth County, New Jersey.

Two & 1/2 blocks (maybe 1,000 - 1,500 feet) inland of mean high tide.

 

[cap4000]

Birchy

When I mentioned the "Hudson River", I did not think that was your problem off Sandy Hook. Rain & Flooding in March was a nightmare in NJ . Good Luck.

 

[Prepakt1]

Its possible to use bentonite matting as a sealer it comes in 10 mm or 20mm thickness and is self sealing