Residential addition foundation options

[TroyD]

I'm assisting an architect and home remodeler with a residential addition. Typical 1960s construction. I have not yet verified the basement wall height, block thickness, etc. The attached sketch shows 2 options. I proposed the crawl space foundation option. The GC proposed a slab on grade foundation, supported along the existing foundation wall with 4" cmu block, and a frost-depth footing at the perimeter. I'm not totally opposed to this method, but my RetainPro model shows the existing basement wall is overwhelmed by the lateral loads from the increased backfill height. If there is any reinforcing in the existing block wall, I'm sure it is minimal. The new 4" cmu block provides bearing, but really no resistance to lateral loads. There is currently a 4-season porch there that will be removed. It is slab on grade construction, and appears to be performing well. Required frost depth is 42". Has anyone done a slab on grade addition like this? I know the existing 2x rim board will need to be protected with metal flashing against the face of the concrete. Any input is appreciated.

 

[oldestguy]

I don't like the slab with thickened edge on a wall. Unless you well reinforce the slab it may sag. Certainly you can't compact earth against the block wall to prevent this. Wall likely will bow in.

 

[azcats]

If you can't be sure that wall will work for compacting backfill against it, I don't see how it's even an option.

 

[r13]

Wasn't that existing wall has earth pressure against it all the time been? I like your option better, as it does not bear on the existing footing, which may not be able to take the additional load without strengthening. You still have to check the load from the addition though.

 

[XR250]

I would not add anymore backfill to an existing basement wall unless you reinforce the tension face and top connection. You could add some carbon fiber strips to the interior face and then tie the wall back to the new slab. I have done this in the past and knocked the face of the top block (or brick) off and let the slab bear on the wall. The slab has to be reinforced to span over the backfill and the whole system has to hold until the concrete hardens.