Floating Slab on Deep Foundations with High Water Table

[craigory28]

Hello:

I am wondering if anyone has come across this condition before. We are looking at a building with a basement which is located in an area with a very high water table. Therefore we need to use a mat foundation which we can waterproof around it and use its weight to resist the buoyancy forces.

The problem is that the soil engineer has indicated that the soil under the basement is not suitable for bearing. He is suggesting that we use piles. The problem is, how do we waterproof the locations that we will have to penetrate the mat for the piles to go down?

Has anyone encountered a similar situation? How have you resolved it?

Thanks,
Craig

 

[jdonville]

craigory28,

Typically, the piles are driven before the mat is constructed. The piles should provide bearing for the whole structure and the primary resistance to buoyancy. Thus, you may not need the mat for buoyancy resistance. However, you may still need a mat for settlement reasons, but it will likely not need to be as thick.

Talk with your geotech and a senior structural engineer for more information.

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

 

[craigory28]

Jeff:

I understand the engineering princpals. However, I am more concerned about the waterproofing. The basement not only will act but has to act like a boat so we don't get water into it. The piles seems to be a conduit for water to infiltrate the structure.

If we were to use just a mat we could waterproof around that and up the walls.

Craig

 

[JAE]

What if you used a grid of geopiers under the mat - (drilled piers that use compacted rock instead of concrete) and then capped the geopiers off with a mat of gravel, then the waterproofing, then the mat. Essentially, there would be no connection between the piers and the mat other than vertically down...since uplift doesn't seem to be an issue here.

 

[JAE]

...and - for that matter, why not use concrete piers with a cap slab to span between piers, then the waterproofing, then the mat. With the conditions you mention, there is probably water so concrete piers would need casing and possibly tremie work, but the gravel piers might do the job. You'd best clear this with the geotech (the geopiers or piers).

 

[SlideRuleEra]

craigory28 - Where I live and work, the water table is assumed to be at the ground surface. Constructing an underground, pile supported, watertight structure is not that unusual on industrial sites. No matter how light the load, 20 inches is often considered the minimum thickness for a pile supported mat. Piles are traditionally detailed as projecting 4" to 6" into the mat - this leaves at least 14" of monolithic, reinforced concrete above the pile tops. With proper care, concrete itself can be made virtually watertight (several full size concrete ships were constructed during World War I). I have an out-of-print Portland Cement Association document titled "Watertight Concrete" for download on my website - good for general information.

In my opinion, the best waterproofing for walls and construction joints, is bentonite. One trade name is Volclay at this link

http://www.cetco.com/bmg/Products/Waterproofing/Volclay_Panels.htm

For waterstop, there are also bentonite based products such as this

http://www.cetco.com/bmg/Products/Waterstop/Waterstop-RX.htm

Best Wishes

http://www.SlideRuleEra.net

 

[pba]

craigory28,

Hydrophillics, as recommended by SlideRuleEra are very good and might help prevent much water ingress. Generally, where work is below water table, it is sensible to provide a method of positive drainage within the basement. I've previously specified GRACE cavity drain products for this but there are many similar systems.

Outside the drained cavity it is usually acceptible to rely on the concrete with water bars or hydrophillics at the joints as there is no need to totally eliminate any possible ingress.

 

[SperlingPE]

Underfloor drains?