House addition settlement

[Jameschen6]

This is a imaginary question. In a discussion forum one guy talked about differential settlement for a house addition against original portion. I am a geotechnical person. My feeling is that what he spoke are not in point by enlarging the problem.

His main concept is that all house will need a period of the time for settlement. When an addition is built to the original part which the settlement has finished long time ago. A differential settlement will occur inevitable and result in problems. I think this is right in a certain degree but has been over emphasized by him. If a geotechnical report, structural design, footing inspection and construction are all correct. There should be no concern for the differential settlement. Most of such problem will relate to moisture change, water leaking, weather change, insufficient backfill, tree cutting and etc.

How will a structural engineer see his points? Is there a different design procedure for a addition foundation to overcome differential settlement?

Thanks

 

[oldestguy]

You probably ae right about "no problems". However, for the typical house additon there is no stuctural engineer, but maybe an architect at best. I'd not expect whoever is involved with the additon design and construction will not have a geotech involved, but will just go ahead and build according to any local codes, if there even are any.

 

[Ron]

Agree with OG. Many locations do not require engineering involvement for single family residences.

Getting to your question, the original structure has likely gone through any settlement that it will have. It is also a good indicator of settlement that might occur in the future of any structure founded on the same or similar soil conditions, with similar loading. That brings us to the addition. The addition likely has similar soil loading as the original structure and unless there are oddly changing soil conditions or the addition is built over buried debris or a "muck" pocket, I would anticipate that the addition would behave in the same manner as the original structure. Will there be some minor differential settlement? Yes, but if properly designed, the addition should be isolated from the original such that minor differential settlement will not affect either.

 

[Jameschen6]

Thank you oldstguy and Ron. A couple of times I were in such cases as a footing inspector and did not pay any attention for the structural design. Ontario Building Code defines soil bearing capacity of 75kpa for house. When you said proper design and addition islation, does that mean that the structural engineer will do this attentionaly from founation wall to roof and have the settlement acommandated? How much a settlement will be expected

 

[Ron]

Where are you in Ontario?

 

[Jameschen6]

Toronto

 

[JoshPlumSE]

Your imaginary question is my reality. My house has an addition (built before I owned it) where there is significant differential settlement (2 inches at the worst spot) in the new wall relative the existing house.

Some thoughts on this:
1) My belief is that most of the settlement occurred during the early part of construction phase. This is because you can't really tell when you look at the drywall. There are a couple of minor drywall cracks. But, they're tough to notice unless you're looking for them. And, next time I paint the kitchen, I'll seal them up and no one will ever notice them again.

2) The settlement was very noticeable at the slab. Large amount of cracking in the original tile floor. And, the differential deflection occurred over a relatively short distance (approximately 6 ft). So, there was a noticeable slope to the floor too.

3) This is the main reason why the house was a bit under-market value when I bought it. I'm one of the few buyers that wasn't scared away.

4) I imagine I'll have to disclose this when we sell. But, I don't think it will scare off any buyers anymore. That's because of a number of simple things.
a) No additional settlement in the last 15 years since I bought the house
b) I replaced the tile with a more flexible wood floor. Therefore no cracking in the floor. Much less noticeable.
c) When we removed the tile, we put down a coupe of layers of self leveling underlayment (is that the right word?). That took out an inch or so of the differential settlement. What's left is maybe half an inch in most places. When you replace the floor you will notice the self leveling repair, but other than that it's pretty hard to tell what was going on.

5) Originally this happened, I'm guessing, because of different levels of compaction for the fill under the house vs the area adjacent to the house. Our house is the last one on the block and there was probably no need to take the soil compaction all the way to the edge of the property.

When they did the addition they never had a geotech check the soil 6 or 8 ft out from the house to see if it was engineered fill or anything. That probably caused some real headaches for the original owners and the contractors and engineers who worked on the project. But, some relatively cheap remedial measures have made it mostly a non-issue for us.

 

[Ron]

Josh...you're assessment of your house is likely spot on. It is likely that the addition was placed over uncompacted fill, with the initial foundation/slab load causing essentially immediate "settlement". This is different from consolidation settlement as you know.

Jameschen6....Depending on where you are in Toronto, your soil conditions are generally good for lightly loaded structures such as single family residences. There are, of course, various areas where soil conditions can be bad, immediately adjacent to good soil conditions. Usually post-hole diggers will tell you a lot. You can get down about 4 feet, which is probably just below your frost line. If you have access to a bucket auger, you can get down to 8 to 10 feet, which is usually enough for a lightly loaded residential strip footing evaluation.

 

[XR250]

I rarely see this as a problem in the field unless the area contains plastic clay soils - then I see it alot.

 

[oldestguy]

One common fault I have found, not only with additions, but with the original building itself is related to filling. Sometimes the landscaping is delayed until building is finished off in interior . Then some landscaper adds fill alongside the foundation. That adds load to the soil below the wall and they both go down some. My first case of original building settled was at a corner, where, from the builder's comments of "We removed a boulder from there" it was pretty clear as to cause.

When raising the grade at a building site any more than a foot or so, plan on doing so for all fill that may add load to the supporting original ground both under and nearby the building. Then dig down to planned footing grade for the foundations. Yes this is extra work, but if any settlement would result from fill affecting the building, it had better be caused to occur before any building is affected by it. I know of one case where surcharging had done its work, but the contractor then moved it just to the side of the building to avoid having to pl,ace fill there first. Result was catastrophic.

Of course don't build on an old landfill either.

Another rather infrequent cause of settlement is building on coal ash cinders. For some unknown reason those cinders seem to deteriorate FOREVER.

 

[MrHershey]

Kind of tagging along to oldestguy's post. Few years back helped fix something like this for my parents. But it wasn't a fully enclosed addition, just a built on covered porch. Being just a built on porch it didn't really have much for foundations, was a total afterthought of a design if it was even designed at all. Looked like guys who built it just dug a post hole, threw some cement and water in and backfilled it around the 4x4 post column. 15-20 years later it had settled more than the main house that sat on foundations that actually had some thought put into them. Actually settled a decent amount more than another nearby post built the same way, so wasn't even just the lack of a footing that did it. Jacked it back up, compacted soil as best you can when you're working through an existing porch, poured a small spread footing around the post and has been fine for several years now.

In the end it was technically a settlement issue. But not really because of being built at different times or having different loadings. With residential stuff like this it's entirely possible that it was never designed in the first place and the settlement is just the symptom. If any sort of foundation beyond what you'd do for a basketball hoop had been installed it probably wouldn't have been an issue.