Foundation Shallow Underpinning - Overpour v Drypack relative to Shrinkage

[EngineerEtcetera]

Smart People,

I’m working on renovation of 3-story c1927 house (unfinished walkout basement, street and upper level finished, 30ft x 30ft footprint). Basement is to be finished, including lowering by 18in (underpinning) to increase ceiling height from 6ft 8in to 8ft 0in (incl flooring).

Footings appears to rest on 2ft to 4ft of ML / sandy clay loam native soil over deep SP / loamy sand native soil (40ft + likely - based on familiarity with soils local to site). The seasonal high water table is probably 20ft + below footing.

I’ve read various magazine/journal articles, engineering texts, and posts at this site regarding opinions on proper foundation underpinning process. Most seem to feel the drypack method is generally best, with greater assurance of required bearing area contact and reduced shrinkage effects apparently the primary drivers for this opinion.

I’m dealing with an underpinning depth of 18in and maximum thickness of 29.75in (36in maximum pit width), which is very minor compared to projects related by those who appear to have the most extensive experience in this process. (Image of preliminary design is attached.)

References I have found regarding shrinkage of normal concrete indicate expected values of 0.00040ft/ft to 0.00310ft/ft (with 0.0005ft/ft to 0.0008ft/ft being often noted). At 1.5ft footing depth, this yields 0.0072in to 0.0558in vertical shrinkage, with the conservative estimate being less than 1/16th inch.

Given the shallow depth of pour, small magnitude of operation, and lack of available concrete tradesmen experienced with underpinning/drypacking, would overpour (utilizing form with 2in wide x 4in deep lip for overpour pressure and pencil vibrator access) be a better option than drypack? I’m expecting most will say drypack, but this situation appears somewhat different than most I’ve seen which discuss this.

Any and all input appreciated - thanks!

 

[oldestguy]

If you keep the water content of the concrete just enough to place it, shrinkage is likely no probe's. Notice I didn't say Pour the concrete. Use a vibrator to assist in placement. Then either way for getting that support will work. If in doubt go with dry pack well after concrete sets.

 

[PEinc]

Drypacking is the way to go. it is the only way you can be sure of having full contact between the footing and the underpinning. I would not worry about any shrinkage unless you used a very wet, high slump mix - which you should not do. With a typical 3000 to 4000 psi concrete mix, shrinkage will be negligible. Place the concrete and drypack the next day. No special waiting periods are necessary.

Now, why the odd shaped underpinning? Why extra width? What are the embedded anchor bolts for? Why are you digging below the underpinning for the drain? Don't chance undermining the underpinning for installing a drain pipe. Raise the pipe or lower the underpinning.

http://www.PeirceEngineering.com

 

[EngineerEtcetera]

oldestguy ...

Much appreciated!

PEinc ...

Current plan is for placement of “typical 3000 to 4000 psi concrete mix” (as you note), typical to slightly dry slump, machine mixed bags along exterior footings in 1, 3, 5, 2, 4 sequence (2 pits per 30 ft wall - 8 pits per day - one 6-day week completion).

Thanks for the questions! My attempt at responses ...

Q: Now, why the odd shaped underpinning? R: The loading magnitude/ distribution will be somewhat different through building with the renovation process. This is largely due to two factors: 1) Conversion of gambrel roof to a gable roof (increasing upper level floor space from 570sf to 852sf); and 2) All joists and various other structural members in the house being undersized (even with much higher quality ~100yo wood) and being replaced. The loads and spans are such that reconfiguration is required to do that as well (no engineered lumber or steel beams are being used - though there are six built-up flitch beams), which includes rotating attic/roof loading by 90deg.

Q: Why extra width? R: See above. Also, the soil receiving building load will now be a combination of ~100yr loaded and previously non-loaded (excepting cohesion/friction), so this was - frankly - a rough compromise configuration to address the loads.

Q: What are the embedded anchor bolts for? R: The footing j-bolts are for anchoring of vertical wood beams. The basement walls are 3-wythe unreinforced brick. Though this is likely adequate for the lateral loading, I’d rather incorporate restraints to be sure (especially with odd existing header sequence). These are coincident with the joists and all will be exposed along with the brickwork - a combination functional/aesthetic component (higher grade wood for these).

Q: Why are you digging below the underpinning for the drain? Don't chance undermining the underpinning for installing a drain pipe. Raise the pipe or lower the underpinning. R: The building drainage is not adequate at all - No gutters, ground partially sloping towards foundation, no existing subdrainage, etc. Even with relatively high infiltration soils and deep seasonal high water table, there is water intrusion during heavier rainfall events. Gutters/downspouts to direct water downgradient are to be installed as are several subdrains. Raising the subdrain would interfere with basement floor elevation. Increasing proposed underpinning footing depth would reduce effectiveness, but I could do that (and some exterior subdrains are to be installed). If the basement exterior could be excavated or interior wall sealed (exposed brick), water management would be easier.

Thanks again!

 

[oldestguy]

For that drain, don't forget to encase any gravel backfill with filter fabric OR use concrete fine aggregate for the drain backfill to keep from losing soil into the drain and beyond. Single size gravel is not a filter.

 

[EngineerEtcetera]

oldestguy - Understood/agreed. Subdrain gravel will be installed on filter fabric wrap, followed by slotted PE pipe, then gravel to height, with filter fabric wrapping / overlapping to complete subdrain assembly. Additional subdrains outside basement walls will be bored in - moled or water-jetted, though those will likely be PVC with well screen slitted sections at intervals / leading edge. There's a couple - at least - things to dodge (such as oil UST), so I'm mulling approach options with flexibility needed dependent upon what comes up during install.