Soil Settlement — The Most Common Structural Failure? - 11/8/2004 - Home Foundation Structure Framing

Soil Settlement — The Most Common Structural Failure?

• Long-span beams breaking in two?
• Shear walls ripping apart during wind or earthquake events?
• Decks failing under heavy live loads?
• Implosion of stick-framed roof systems depending on collar ties for vertical support?
• Connectors (nails, screws, pre-fabricated connectors) failing, allowing members to fly apart?
• Corrosion?

The answer is no, six times. The most common failure, I believe, is soil settlement. I didn’t conduct a survey or do research to come to this conclusion; it’s based on personal experience and observation. Anyone willing to dispute it?

Consider that 99.9999% of residential and light commercial structures use soil for support (the other .00001% is built on rock). And soil, by its very nature, is compressible. So, it is virtually impossible for zero settlement to occur within a structure. Lots of times we don’t notice the settlement because it is minute, or because it happens uniformly over the entire structure. But make no mistake, it is still there.

When does settlement become structural failure? When it is differential settlement, of sufficient magnitude to distress building components. In simple terms, say one corner of a house settles an inch, while the remainder settles only 1/16-inch, the difference, 15/16-inch is differential settlement, and will cause drywall to crack, siding to droop, footings to break, etc. It doesn’t matter what town you live in, you shouldn’t have to look far to see examples: sidewalks tilted, slabs cracked and uneven, posts settled, foundations cracked and broken, roads with potholes, entire houses tilted. It’s everywhere, especially in older neighborhoods.

What causes differential settlement? Here are three common possibilities:

1. Improperly compacted fill (number-1 culprit)
2. The presence of organic material in the soil that over time decomposes
3. Naturally occurring, isolated weak soil zone. For example one corner of a house may be built over a saturated clay lens while the rest is on sandy loam.

Any moron knows how to properly compact fill, right? Wrong. I recently made this moronic mistake myself.

As you can see in the following sketch, my home has a basement and a covered front porch. The main wall footings are on bedrock and will not settle.

The porch post footing is supported by fill. I knew this — I put the fill in and compacted it in lifts myself. Why, then, did the post footing settle three inches? There are two reasons:

First, Mister Cheapskate Tim used a puny 150-lb. vibratory plate compactor — not nearly enough compactive effort for the 8-inch thick lifts (layers) used.

Second, Mister Moron-Cheapskate Tim used native material for fill, which is very clayey. Tim did not pay attention to the dry, powdery nature of the fill at the time he put it in the hole. In other words, the moisture content of this soil was too dry to obtain optimum compaction. Water should have been added.

The settlement showed itself with the first rain. All that dry, poorly compacted clay sucked up water, became more dense and shrank in volume. Down went the post. Great.

Fortunately, the porch slab and steps were not poured yet, so it wasn’t too great a job to hog out all of the fill and replace it with granular, structural fill — what should have been used in the first place.

Of course, in any structural engineering job I do professionally, I use a very stringent structural fill spec. I would never allow what I used on my own home. But, being a cheapskate, I thought I could do it myself, save a few bucks.The shoeless cobbler’s children come to mind.

Lessons learned: If it can happen to me, it can happen to you. Do not cut corners when it comes to compacting fill that will or may be used to support something. Use granular material, at the right moisture content, compacted in shallow lifts with equipment heavy enough to get the job done. If in doubt, call a good geotechnical consultant for help. Fixing settling problems always costs orders of magnitude more than doing it right the first time.

Tim K. Garrison P.E. of ConstructionCalc.com has authored books and short courses and lectures on topics relevant to builders. Got a technical or management issue? E-mail buildersengineer@constructioncalc.com. Tim reads every one.

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The views expressed in this article represent the personal views, statements and opinions of the author and do not necessarily represent the views, statements, opinions or policies of the National Association of Home Builders. NAHB does not necessarily endorse any of the views expressed by the author and NAHB is not responsible for any direct or indirect consequences arising out of the views expressed in this article.