Technical Question - High Water at Basement


I am a licensed architect with a completed project near Orlando, FL which includes a 9'-0" deep basement/pit for a lube center.  The floor and walls of the pit are all CIP concrete.  The pit is dry, water-proofed inside and out, has gravel around the walls and under the slab, filter barriers and a continuous french drain around the perimeter with a separate, detached sump pit that houses two pumps (primary and back up).  Grade around the building is mostly paved and sloped away from the building.  

Since the project's completion about a year ago, the pumps have run continuously, pulling water from around the pit and dumping it into a retention pond which is at a lower elevation and about 100'-0" away.

 While there is no evidence of a problem with the project, I am concerned that failure of a pump or some other unpredictable condition might lead to a major problem.  

I'm looking here for some solid technical advice on recommendations I might make to the owner as protection against this type of thing.  I welcome your advice and/or questions.

Nov 21, 12 3:51 pm

first: how thick is the concrete (floor as well as walls?) that's a critical point. As a rule of thumb (depending on the quality of concrete) about a foot of concrete should be enough to withstand the pressure of the water, and avoid any leakage. 

If that isn't the case, or if the execution of the concrete pours wasn't quite perfect (and thus the basement being porous), you could inject the weak spots (as is done in the construction of swimming pools), or attach a watersealant to the concrete. Preferably the outside of the walls, of course - even though that might be a bit of a problem in a built project. If that's the case, you could opt for a double walled (pit in pit) construction. But first things first: how thick is the concrete?

Nov 21, 12 4:20 pm
won and done williams

usernametaken's questions are more helpful than mine, but did you have a civil engineer on the project? Seems like the water table issues should have been discovered in design/engineering or at latest during construction.

Nov 21, 12 4:30 pm

won and done is asking the right question.  I assume the rest of the building is a simple slab-on-grade.  The gravel fill around the grease pit creates little air pockets.  If there is ground water, it may be migrating toward the voids in the gravel, since water takes the path of least resistance and is under slight pressure from simple gravity.  Is there a perimeter drain around the outside of the building, too?  If so, how is it dispersed?

I would try to determine where the water could be coming from and whether there is a way to disperse it without relying on a mechanical pump.  Can it be piped to daylight, or is the site too flat? 

Nov 21, 12 6:43 pm

Sounds like you might have wanted to use geo tex benonite waterproofing in the pit. I have used it and it is a pain, and expensive.  It does however keep water from coming into the building.  benonite is white mineral clay which swells when wet and creates a barrier. You cast it into the top of the footing and foundation wall   to get a great seal. It has to be certified by a testing lab so there is extra cost involved.....but as the saying goes a flood can be a lot more expensive.  I think this application is not  realistic at this time but for future reference.

Nov 21, 12 6:48 pm

You also apply it to the outside of the foundation wall.

Nov 21, 12 6:49 pm

It sounds to me like you have a drainage issue on site.  As mentioned above, consult the civil or geotechnical engineer on the project.

Nov 21, 12 9:56 pm

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