Residential Basement Feasibility

[epr]

This is more of a survey than a question. . . .
Assume that you are the engineer who has been asked the feasibility of constructing a basement under a dwelling that is yet to be built. A soil boring has been performed by a soil scientist which estimates the seasonal high water table to be 6 feet below existing grade (based upon soil mottling).

Question: at what depth should the basement floor be set relative to the soil boring data?

 

[jheidt2543]

IMHO, the basement finished floor elevation should be a minimum of 10" above the high ground water mark (6" for drainage fill under a 4" slab), that would be at - 5'-2" below grade in your case.

In our area most basement slabs are at 8'-6" below finished floor elevation. This is arrived at as:

Wall form or block coursing height 8'-0"
Subtract basement slab thickness -0'-4"
Add 2x10 joist on basement wall +0'-10"
---------
That leaves a clear basement ht. 7'-6"

That means the house finish floor would have to be raised 2'-4" higher than "normal". This shouldn't be any big deal and should actually promote better drainage around the house.

If, for some reason, you can't raise the basement floor slab, then you have to use a thick (40-60 mil) membrane waterproofing material under the basement slab, around the footing and up the exterior of the basement wall; completely enclosing the basement. You would also add perimeter drainage pipe, a sump and duplex ejector pumps.

 

[JAE]

jheidt2543 is right on the waterproofing description - but note that water tables are not static - they change constantly through seasons so setting a basement level based on a one-time measurement of a water table is not a good idea. With a water table that high, you would want to waterproof the entire basment system.

 

[Focht3]

[blue]JAE[/blue] has it right - remember you're building fo 30-50 years, not 12 months. Lots of unexpected things can happen in that time frame.

Did the "soil scientist" produce any boring logs? Mottling alone is not a good indicator of prior groundwater levels. It just means that oxidation has occurred. Any indications of calcareous or ferrous nodules or pockets? Ferrous (black to reddish brown) specks or streaks? These accretions can be an indication that the water table was/is near the level of the accreted materials -

For me, I design with "belt and suspenders" wheI'm near the water table. Guess I've done too many forensic investigations on basements/below grade walls that weren't designed that way.



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[epr]

Thanks jheidt2534,JAE and Focht3.
You are confirming my point of view.
This question stems from a lawsuit in which I was one of the experts. The opposing expert stated that there was no need to worry about the estimated seasonal high (and this was after a basement had been flooded to a depth of two feet for most of a winter, and even two 1/4HP sump pumps did little to help the homeowner.

I generally use a lowest floor elevation of two feet above the estimated seasonal high groundwater table in southern New Jersey, where it's quite sandy. And this has worked over the past 25 years for me.

However, I was unable to find a published source with any guidelines for placing the basement floor slab (which would have helped me immensely in that particular case). Most of the published sources simply indicate that a good drainage system should be installed when a problem is foreseen.

The major problem that I'm seeing now is that real estate values are causing more people to build homes with basements, almost no groundwater investigation is performed, and the owners fully expect a dry, well ventilated space that they can finish with drywall, carpet, etc.

Anyone else care to comment?