# expansive clay



## cleveman (Dec 28, 2007)

I have a friend who had a home built about 10 years ago. He was acting as the general contractor himself and didn't keep a close eye on what was going on.

Now, the house is moving around a bit. It is a two-storey home with a walk-out basement. The perimeter appears to be stable. The field in the basement is moving up and down, including the columns supporting the floors above.

He doesnt' know how thick the concrete in the basement is, the size of the footings under the support columns, or the amount of rock under the slab.

He called in an engineer, who told him the home is sitting on expansive clay and the solution can be either of two methods:

He can take out the slab in the basement and excavate out 4' of soil, re-fill with crushed rock, and pour a new slab.

Option 2 is to drill holes through the concrete. Diameter and depth and spacing is unknown, and fill these holes with concrete. Then remove existing concrete, wait two years, and re-pour the concrete. 

My suggestion is to remove the slab and put some footings under the support columns which won't move. Next, excavate enough dirt to allow step three to take place. Step three is to put in a footing near the center of the building or a set of columns to set a beam on, then frame up a basement floor resting on the footings and center footing or beam. I suppose a pt2x4 could be set on the footing edge to hang the floor joists on. A vapor seal would be installed on the basement floor. The floor joists would rest on the PT 2x4 and be notched as necessary to arrive at the same floor height.

My initial concerns about this method are that the floor joists should be tapered going up to the sill plate on the footing. There shouldn't be much dirt excavated close to the exterior wall footing. On the parallell wall, the plate on the footing could simply be topped with a 2x4 joist, which would give you the same height as the old concrete floor.

Any thoughts or comments would be appreciated.


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## rino1494 (Jan 31, 2006)

This could be a hydraulic issue. Does your friend know if there is a footer drain, is there a frost footer where the walk out is ??

He might want to cut a small section of concrete out and take sample of the soil to get tested. It will be cheaper to test the soil to know if you have a problem instead of going out and trying this and that. 

It is possible he may need a subdrainage under the slab.


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## cleveman (Dec 28, 2007)

I would have to hope that there is a frost footing under the walk-out portion. The exterior is tiled and daylites. The interior goes to a sump and the sump has never run. I believe it is connected to the exterior as per commom practices here.

The residence is on quite a slope. These hilly, rocky, timbered areas are problematic.

I also have to assume that the engineer tested the soil before he pronounced it expansive clay. 

I think the logic is that the spongy stuff down there is always getting wet and heaving, then drying out and shrinking. The solution is to put a cushion there for the pushing. Or to stabilize it with piers.

I can't quite understand why the interior column footings are heaving, but not the perimeter walls. Thought is that the perimeter walls (poured concrete with two more floors above) have enough weight on them to keep them from moving. However, the columns must a bit of weight on them as well (2 floor systems).

The solutions are pretty scary in my opinion. If you drill holes and fill with concrete every 8' on center and this doesn't work, are you supposed to come back and drill holes every 6' on center? Or should you just drill 12" holes 6' deep every 4' on center the first time, and will this even work?

Likewise, if you excavate out 4 more feet of soil and fill this area with rock, what if that doesn't work? Do you then come back and excavate out the rock again, stockpile it, and take out another 2 or 4 feet of dirt and put the rock back in and add more rock and re-pour the concrete for a third time?

Hope I live long enough to find out what happens.


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## Sar-Con (Jun 23, 2010)

Is the basement finished or unfinished?

Is the slab isolated at the columns? That would be my first trial and error. Cut the slab around all columns and install some expansion joint material. Give it a year, then go from there. That is assuming that this "moving around a bit" isn't creating an unsafe living condition....


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## thom (Nov 3, 2006)

There are a couple issues here, responsibility and fix. 

If I remember correctly, clay is about 20% expansive, with moisture. Put another way, wet clay takes up 20% more space than dry clay. If there is clay below that has a changing moisture content, expect the house to move. 

Fault? Probably your friend, the GC. Did he have an engineering study done on the soils? If so, did he incorporate the results into foundation design? Of course it was his (the GC's) responsibility to do this, not the excavator or concrete sub. 

Fix, put it on something stable, that might mean excavating to a point where the moisture level is constant. 

Look up foundation designs for Colorado's front range. They have some extreme conditions that require dealing with this. 

Tell your friend good-luck, he's going to need it on this one.


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## rino1494 (Jan 31, 2006)

cleveman said:


> I would have to hope that there is a frost footing under the walk-out portion. The exterior is tiled and daylites. The interior goes to a sump and the sump has never run. I believe it is connected to the exterior as per commom practices here.


You say there is any interior drain that is connected to the sump. The problem with that is, it only collects water is that on top of the clay. The possible underlying issue is that you have percolation deeper than that. 




cleveman said:


> I also have to assume that the engineer tested the soil before he pronounced it expansive clay.


If so, then ask him for the boring logs. He may also just giving an educated guess.


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## Santa's Helpers (Jun 12, 2009)

We don't have to deal with cold temps in florida however we do have pipe clay which can create the problems you have. One solution for us is to keep the ground saturated so there are no fluctuations. This probably isn't an option for you. Bentonite may be a solution. It is a clay that can be pumped in a slurry form. When it is wet it expands and is used as a waterproofer. It can be pumped around foundations to prevent water from getting through to the foundation.
I'm not sure that it can be used for your application but, it might be worth looking into.


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## cleveman (Dec 28, 2007)

The basement is unfinished. I think the entire slab is broken up, so the coumns are pretty much independent of the slab now. But you have a good point there to isolate them completely. Owner/GC told me that the plans called for something like a 4x4x2' thick footing under each column and he ended up with a "puddle pad" which is fairly common.

Noone is concerned about responsibility.

We clearly do have some percolation down below. Something is making the clay wet from below (water table rising) or from above. The unknown is how deep the vein of clay is. I suppose it could extend down 30 feet below the basement floor and it is continuously swelling and shrinking. I'm not sure the house went a year before the slab started to bust up.

I'll keep everyone posted as this progresses. It is surely one to learn from.


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