New home basement slap leaking water 4

[jayhawk78]

Two year old home basement slab (in Georgia clay) has several cracks, in different areas of the slab, with moisture showing 6" either side of cracks. French drain at base of foundation w/water piped to buried catch-basins located on downhill side of house. All downspouts are piped into similar catch-basins. Basement is a total dugout, no daylight, on a flat lot. There are no cracks in the foundation wall. What's causing the moisture to appear, is it a serious problem and how to correct?

Thanks for the input!

 

[ashjun]

If the foundation walls are sound and have not settled unevenly, then I suppose the problem may not be dangerous.

I really don’t know about the Georgia clay; would it be possible to know the clay activity. An activity of just over 1 would suggest that one merely needs to fill up the cracks with good mortar or jet grout them. The cracks probably generated due to poor compaction technique adopted before the construction.
However, if the clays are quite active, it would be better idea to stabilise the soils below the basement slab prior to the filling up the cracks.

 

[Focht3]

You almost certainly don't have subfloor drains. They are expensive - and aren't common in residential construction. At least in my area (central Texas.)

Unfortunately, this is a circumstance that cannot be fixed cheaply. You will probably get effluorescence in the wetted areas from chemicals leaching out of the concrete. This will take some time to develop, and will depend on the amount of water passing through the cracks.

It probably isn't cause for concern about the house falling down. But forget about putting down carpet, vinyl flooring, ceramic tiles, etc. - they will only make things worse. And mold may become a problem.

If the basement ceiling is high, other options are available. How much money are you willing to spend? What has the builder said about the problem? Have you hired a lawyer yet?

 

[RDK]

The clays that I am familiar with (Winnipeg Manitoba area), have a high swelling pressure. There are two pressures that are of concern in the clays. The first is the swelling pressure and the second is the pre consolidation pressure.

Swelling pressure is the amount of pressure that will be exerted by the clay due to the removal of the overburden. You took about 2.4 m of soil off the top of the clay and replaced it with 100 mm of concrete. The material under the footings is holding up the entire house and is restrained from swelling due to this loading.

The pre consolidation pressure is the amount of loading that can occur before the pressures cause the water in the clay to migrate out and the soil will consolidate. With the Winnipeg clays they are generally stable under normal residential loadings, except for the swelling under the slab.

The main exception is due to prolonged dry or wet spells or something else (water line leakage, tree growth etc) that fundamentally changes the moisture regime in the soil. Water plus clay equals expansion. Water removed from clay means shrinkage.

Now to answer your question. The cracking is most likely due to swelling under the floor slab. If the cracks are open at the top and closed at the bottom then this supports this scenario.

The moisture is most likely coming out of the clay assuming that the perimeter drains are installed correctly (lower than the floor slab). In time I would think that this will stabilize since once the water comes out of the material as there is no recharge because the perimeter drains will take care of this water. (You could have hit a semi artesian water source in which case the water will continue for a long time.)

The solution is not easy. You cannot do anything about the swelling of the clay. (except fill in your basement). Some high end houses in Winnipeg use a structural floor system, either a structural concrete slab (very expensive) or wooden joists spanning to the walls (less expensive and allow under slab ducting and wiring).

The moisture can only be stopped by constructing under slab drainage. This would have been simple during construction, all that would have been required is a course granular layer (100 to 150 mm sand or gravel) under the slab that connects to the perimeter drainage.

As to if it’s a real problem, if you view basements as dark holes in the ground suitable only for storage of root vegetables, then this is not a major problem. If you view basements as extensions of the living space then this is a major problem.

Check out local residential construction techniques and building code requirements.

Contact a local civil engineer familiar with residential practices have him or her see the situation and then you can get some specific advice.

Call a lawyer about the responsibilities of the builder in this issue.



Rick Kitson MBA P.Eng

Construction Project Management
From conception to completion

http://www.kitsonengineering.com

 

[Focht3]

jayhawk78 didn't mention heave - only cracks and water. Lots of things cause slabs to crack...

The moisture is probably collecting as a result of both a temperature differential and relative humidity differential (both caused by the conditioning of the air inside the home.) The vapor barrier probably has holes in it - most do.

 

[VAD]

Just an idea. Have the basement floor cored with a concrete coring machine (6 inch dia) and use a hand auger to check the soil. At least this would allow you to assess soil etc.

This can be done through your local geotech/ materials service.

 

[jayhawk78]

Thanks to all (VAD, RDK, Focht3 and ashjun) for the input. There has been no heave in the slab, only minor cracking previously mentioned. Since the area has received 10" rain in May alone (definitely not normal for this locale), the idea of the clay being saturated make sense. We parted ways w/builder when home was 80% completed. I tried contacting slab sub, but he's no where to be found. Will contact local civil engineer to discuss issue.

Thanks again for the input. I now have a better idea of the possible problem and what to do.

 

[Focht3]

The GC is still be on the hook for the work he did - unless you signed away all your rights to get rid of him...

 

[NickDemos]

I have found some excellant educational material on this site:

http://www.concretenetwork.com/

If the cracks are hairline width then they are likely only shrinkage. If they are in the floor and wider at the top then the bottom, then it may be "curling" which is when the top of concrete dries faster than the bottom. Neither of these types of cracks are structural defects, just cosmetic.

To keep the water out in these cases, I would use an epoxy grout troweled into the cleaned out cracks and paint the whole area with a waterproof paint (call a concrete product supplier for products).

Surface drainage plays a big role in wet basements. Make sure that the land around the house slopes away from the house. Normal codes are 6" in the first 10 feet.

If the two possibilities mentioned above do not seem to be the cause then call a local engineer to come out and make a determination. Good Luck.