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. Over 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.
What are pin piles? Pin piles are small diameter (two-inch to six-inch) schedule 80 galvanized pipe, driven with hand-held or excavator-mounted jack hammers. Capacities generally range from 4,000-10,000 pounds each. Short pipe lengths, usually six-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, after the pile was driven, its top was pushed sideways under the footing using a horizontal jack. Now, heavy steel brackets are commonly used to connect top of pile to footing.
The number of pin piles required is best determined by an engineer 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 generally in the three- to six-foot range.
I would be remiss if I didn’t also mention helical screw anchors. These are similar to pin piles, consisting of short lengths of pipe or square tube, but at the end of the first length is a screw helix (plate). Rather than being driven, they are screwed into the earth using an excavator-mounted rotary motor. Discussion of these is another column altogether.
Re-leveling the structure. Once pin piles have been installed, the structure can be re-leveled via jacking. Usually, 20-ton hydraulic jacks are placed along the settled footings, between the pin piles. Jacking must be done slowly and evenly and be carefully monitored to avoid introducing undue stresses in the structure. As the structure is raised, permanent adjustable pile cap brackets are screwed upwards. Once level, pile caps are snugged into their final positions and the jacks are removed. Concrete can be poured around the piles and caps and under the now-raised footings to give the structure lateral stability against future earthquake movement.
Cost. The cost of these installations varies wildly, depending on soil conditions, existing footing type and condition, extent of settlement, proximity of adjacent structures, etc. I’ve been involved in projects costing less than $10,000 and others exceeding $100,000. An experienced contractor or engineer ought to be able to assess your specific case and come up with a ballpark range fairly readily, however.
Don’t try this at home. Pin piling and jacking are highly specialized disciplines that should be attempted only by experienced professionals. Many expensive and dangerous things can go wrong: sheetrock and window cracking; triggering settling of adjacent buildings; footings slipping off jacks/piles; etc. Also keep in mind that to install these systems, you have to undermine and remove a big portion of the existing supporting soil — a dangerous operation. Properly done, however, otherwise ruined structures can be resurrected; literally raised from the dead.
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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