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The Best Remedy for Settling
Tim K. Garrison — the Builder’s Engineer
Dear Tim,
I’m remodeling an older home that has significant settling over the living room area. I could probably jack and shim it back up, but I’m concerned that might not be the best remedy. What do you suggest?
— Robert P.; Fort Bragg, CA
I’ve been involved in many settling projects, and the best permanent solution I’ve found is pin piling, also known as underpinning.
Permanent is a key word in the above sentence. Jacking and shimming rarely provide a permanent fix because those operations don’t stop the footing from future settlement.
Another issue with shimming: in doing so, you disconnect the footing from the structure. A diligent contractor may try to reconnect, but many don’t bother. Even when a contractor tries, it is difficult to get the connection as good as it was originally. The problem arises during wind or earthquake events, when houses can be knocked completely off their foundations. Proper connection between shear walls and foundations avoids this. In fact, this very issue has been the catalyst for many stringent code revisions in recent years.
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What causes settlement? Here are a few common culprits:
- Building over an old, abandoned slough or depression. Old-timers typically filled such depressions with logs, stumps, trash or anything else they could grab. In time these rot and settle.
- Building over poor native soils, such as perpetually saturated clays, peat bogs, loose sands or soils with a lot of organic material that will eventually rot.
- Building on improperly compacted fill. An example could be the last lot in the subdivision, where all the spoils from road construction were dumped.
Pin piles are small-diameter (usually 2-inch to 6-inch) schedule-80 galvanized pipe, driven with hand-held or excavator-mounted jackhammers. Capacity ranges from 4,000 to 10,000 lbs. each. Short pipe lengths, usually 6-feet, are used, with simple pin couplers joining the lengths. They are driven at a slight angle from vertical adjacent to the footing. In the old days, the top of the pile was pushed sideways under the footing using a jack. Now, heavy steel brackets are commonly used to connect the top of the pile to the footing.
An engineer is the best person to determine how many pin piles are required because it depends on the weight of the structure and the strength of the footing/stem wall. Common pin pile spacing along continuous footings for residential applications is in the 3- to 6-foot range.
Pin piling and jacking are highly specialized disciplines that should be attempted only by experienced professionals. Many expensive and dangerous things can go wrong during these operations, such as: sheetrock and window cracking; triggering settling of adjacent buildings; footings slipping off jacks/piles; etc.
Tim K. Garrison is a licensed professional engineer from Mount Vernon, WA, the author of “Basic Structural Concepts for the Non-Engineer," and the president of ConstructionCalc.com. He can be reached by e-mail.
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.
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