Underpinning contractor did not leave room for drypack 3

[Ben29]

The underpinning contractor installed 14 pits so far. The inspector just now told me that the contractor is not leaving room for drypack. The contractor is trying to do avoid this step. Should I make him sawcut the top 2 inches of the underpinned pier in order to drypack it?

 

[PEinc]

Ben29, in your sketch's plan view, I would not expect a void like you show to cause a problem. There should be plenty of bearing area between the footing and the underpinning. I would not expect the bearing of the existing footing to cause more than about 20 to 40 psi (about 3000 to 6000 psf) bearing - very low for concrete compression stress. Even if you discount half of the bearing area due to a possible void, you may be talking 40 to 80 psi (6000 to 12000 psf) bearing on concrete. A small void probably will not matter. If a big void is present, the odds are the drill holes will find it.
EDIT: The entire, top surface of the underpinning pier should be leveled off using a flat 2x4 so that you know there is at least 1.5 inches available to ram in the drypack. Ideally, there should be a 2.5 to 3 inch, or more, gap between the top of the underpinning pier concrete and the entire bottom of the existing foundation being underpinned. Pouring the underpinning concrete up against the bottom of the footing, even with some extra head, MIGHT provide full and proper bearing but also MIGHT NOT and is also hard to confirm.

bookowski, I have designed underpinning in NYC where everyone, especially DOB, expects to see shims, wedges, permanent jacks, etc. It is my experience that shims, wedges, jacks, etc. are rarely, if ever, needed. Proper dry packing will evenly transfer and distribute the building load to the underpinning. Wedges, shims, and jacking can cause high, possibly damaging, load concentrations on the existing footing, especially if it is a rubble stone foundation wall. NYC DOB has previously questioned me about my non-specification of wedges and shims and has accepted my explanations. (Of course, I probably just jinxed myself for my next underpinning project.) My clients and I have designed and built underpinning projects in many different northeast and mid-Atlantic states and have never used wedges, shims, or jacks unless needed to temporarily support some loose stones in a foundation wall until the pier is ready to be dry packed.

http://www.PeirceEngineering.com

 

[Ben29]

PEinc: So I can essentially issue the details that I sketched above and be done with it? You wouldn't suggest that I tweak anything about these details?

Thank you so much!!

 

[GC_Hopi]

Tell the contractor that they are digging themselves a hole and you want the work verified (drill a few holes and use a borescope to check the bearing) As PEinc stated the bearing pressure should be low but you can check the numbers. For peace of mind you could have the contractor drill alternative holes if the grout does not flow through on the first set of holes and inject all four holes.

 

[bookowski]

@PEInc - Could you elaborate on your explanations to them? Have you attempted to quantify this, i.e. if the existing footings experience 1tsf bearing how do you show that drypack alone preloads to X% of that 1tsf? Or does the argument boil down precedent. I'm not against it, genuinely curious. I will say that I have not seen any foundations damaged by wedging or jacking. In the case of rubble foundations the jacking or wedging is done against a concrete "lintel" of sorts, not directly against the rubble.

 

[PEinc]

Unless 2 or more drill holes hit the same void or intersect each other, you will not get grout flowing into one hole to come out of another hole. I would also drill the probe holes tangent to the bottom of the existing footing, not 1 inch down.

http://www.PeirceEngineering.com

 

[PEinc]

bookowski, properly prepared and installed drypack will be much stronger than the soil that the footing originally sat on. Shrinkage of a thin depth of drypack will be insignificant. You can conservatively calculate the shrinkage drypack as the shrinkage of normal concrete. On page 75 of Underpinning by Prentis and White, 1950, the authors describe steel plates and wedges but then say, "In lieu of wedging and grouting, if time be available, a good method of putting the load on the pier is to "dry-pack" it and leave the wedging out. This is done by bringing the concrete to within three or four inches of the bottom of the foundation and, after permitting the concrete to set so as to take up all of the shrinkage, to dry-pack the space left with a stiff mortar. This is thrown by handfuls into the thin space between the footing and the top of the concrete pit and rammed home, solid, by some suitable means, for instance by a short 2" x 4" piece of wood and an 8-pound hammer..... This method is much more reliable than grouting, because there are many difficulties in getting a good grouting job."

http://www.PeirceEngineering.com

 

[bookowski]

drypack will be much stronger than the soil that the footing originally sat on

The issue isn't strength - it wouldn't take much to match the strength of most soils. The issue is preloading such that movement is minimized.

Without preloading the underpinning, and therefore building, have to move until the load is transferred. I've also seen calcs that look at the progress of load transfer with/without. For simplicity say you are doing A to D pins (4 sequences) and the existing building is experiencing 1tsf. When you dig your A pins you've now ony got 3/4 the footings left, so they should be at 1.33tsf. If you don't preload the A pins then they are at near 0. So when you dig B's you've got 50% of originals + the unloaded A's which will now load up but be less than C and D. etc etc etc. There are calcs that run through this in more detail. If the loads/bearing are small enough I can see how the movement to reload the pins is small and acceptable, but I don't see how that is true for many cases. And I don't see how much preload you can really get by drypacking, I'd think it's quite small.

I don't doubt this is being done successfully, I was just wondering if you had anything more quantifiable when you said that you've presented the rationale to the reviewers.

 

[PEinc]

bookowski, whoever said or proved that using steel shims and wedging on a large, deep underpinning pier is any more effective at preloading than is drypacking? Real experience in proper underpinning construction proves that properly mixed and placed drypacking is effective.

http://www.PeirceEngineering.com

 

[SwinnyGG]

The issue is preloading such that movement is minimized.

I'd argue that as long as a well-designed sequence is followed for getting an entire footing pinned, the amount of displacement the footing will actually see while partially unsupported areas are being grouted in is extremely small.

I see the argument that the unsupported section of the footing sinks by some tiny amount, is then grouted and held at the new elevation, and then the rest of the footing comes down to match it as each area is pinned.. but in the vast majority of cases, if a footing settles .005', do we really care?

 

[PEinc]

bookowski, please read the attached excerpt from Ratay's Temporary Structures in Construction, Chapter 11, Underpinning.

www.PeirceEngineering.com

 

[bookowski]

@PEInc - Yep, I've got that book and i'm not doubting that plenty of jobs with drypack only are working. I only went down this hole because of your comment that "DOB has previously questioned me about my non-specification of wedges and shims and has accepted my explanations" and assumed that you had some data/calcs as part of that explanation. We won't design underpinning, I just see it on a good number of our projects so was curious if there was engineering to this since it deviates from what we see around here (and would presumably be much cheaper).

Since we don't design underpinning I've only given limited thought to this. But to your question about proving which provides better preloading - I would say that at a minimum it's a wash in the worst case. Both jacking and wedging are finished with drypacking, so it can't be any less preload. And jacking is done with load increments and monitoring to determine when movement under a certain load has stopped, so in jacking you can define the preload. For wedging you know the displacement - if the inspector reports fully wedged you know the pin moved down say 1/2", or if he reports refusal at half length then you know 1/4" etc. We recently had an 8 story x 20' wide unreinforced masonry building that had to be underpinned and the jacking was fairly substantial, and movement of the pins non-negligible until it stabilized. It's hard for to imagine having the stomach to just drypack such a condition and let it find it's own place. If there is no calc associated with preload for drypack only is there no check or limit? If that's the case then underpinning would seem to boil down to copy/paste a typical detail with no engineering behind it.

If we're playing the quote old book game I'll throw in "when the pit has been sunk to the required depth it is concreted to within two feet, or other convenient working distance, from the bottom of the footing and allowed to set. Then it is wedged up. This important operation, which conveys the load of column to pier, is sometimes done by means of a pier and wedging stones or better by wrought iron wedges" - from Modern Underpinning by Lazarus White, 1917

but in the vast majority of cases, if a footing settles .005', do we really care

I'd agree that 0.005' probably isn't an issue - but buildings being underpinned are always monitored (at least around here) and you'd be thrilled if you only had 0.005'. All of the underpinning jobs i've been involved with move, usually it is in the 1/8 to 1/2" range and we've seen jobs with a lot of movement (3" in extreme case). On top of that it's not uniform, due to the sequential nature of pit underpinning you shift that building around piece by piece so even 1/2" on an old unreinforced building can wreak havoc. You often impact adjacent buildings as well.

Long story short it sounds like there's no engineering based assertion that drypack alone provides any quantifiable level of preload, but that it's been done a lot and usually works.

 

[PEinc]

It's only time and money. Use wedges, plates, and jacking if you want. I have successfully designed and/or built a couple hundred underpinning jobs and have never wedged, plated, or jacked the underpinning.

http://www.PeirceEngineering.com

 

[PEinc]

bookowski, try plating, wedging, and/or jacking this foundation without damaging it.

http://www.PeirceEngineering.com

 

[bookowski]

You seem to think I'm questioning that you've successfully underpinned projects with this method. That wasn't my intention. I thought that maybe you had some info that could be useful. But when someone starts reaching for specific extreme examples it's usually a sign that they don't have much to back up the general. If I say that men are usually better basketball players and you come back with oh yeah do you think you could take sue bird in a game of 1 on 1 then that probably means i'm right. We agree that you've underpinned buildings, you can sleep soundly on that. But if you are saying that you explained to code officials how this preloads the same as jacking or wedging and it turns out the backup is here's a picture of a scary foundation then i'm not very impressed any more.

Yes, I agree that's an ugly foundation (and pretty sketchy shoring to boot). Jacking should not be adding any load to the building, it should be attempting to creep up to what was already there. For that particular picture (without knowing much) I would've expected that the rubble wall be repaired and stabilized first, that corner piece stitched into the wall such that it can be safely assumed to cantilever (which it appears to be already) doing, and then the rubble wall underpinned and jacked/wedged if determined necessary. No new load would be added, if anything the intent is to more gently transfer loads. This looks like a semi diy level operation with a guy jamming timbers in various places under a very precarious foundation. If anything I'd think that such a situation requires a higher level of care not less.

 

[PEinc]

bookowski, I do not think that you are questioning my experience. Sorry if you got that impression.

With respect to the photo I posted, when my crew got on site and saw this foundation, they quickly set the approach pit ring boards, blocked up the loose stones and, shortly thereafter, mortared up the loose stones before proceeding with the pit excavation. My point for showing YOU the photo is not that it is a reference for not shimming, wedging, or jacking' but, rather, that not all walls are in good enough condition or are not uniform enough to "survive" shimming, wedging and, jacking. Should my crew have mortared that particular wall before trying to install a couple approach pit ring boards? Of course, but that didn't happen as soon as it should have and everything did work out all right. The building was not damaged, did not settle, and nobody got hurt.

As I said before, if you want to shim, wedge, and/or pre-jack the foundation wall before installing grout or drypack, go for it. I'm just saying that this isn't always needed or required, is definitely more expensive, could possibly damage a bad foundation wall and, while mentioned in various underpinning references, it is neither the only published method not the most commonly used method for transferring a building load to the underpinning.

You wrote above, "In my area the code has specific provisions for underpinning 'Pit-piers shall be preloaded by wedging, use of permanent jacks, or by other means designed by the engineer'." Can you please show me or link me to the code in your area that has that specific provision? Thank you.

http://www.PeirceEngineering.com

 

[bookowski]

I didn't realize that you're actually an underpinning contractor. I agree that not all situations require wedging or jacking, but I would not say that no conditions require it (or even that most don't require it).

The provisions I was referring to are in the nyc bc chapter 18, excerpt below (sect 1817) and also for free on nyc dob website. These provisions in general have been stepped up due to the number of failures and issues associated with underpinning jobs in the area.

The code does require preloading but there is a "by other means" out after jacking and wedging, so I guess you are saying that you believe dry pack alone serves as preloading (not just bearing) - but again I don't see how you quantify that and vary it for different building.


1817.7 Additional requirements for pit-pier underpinning. When the method of support selected is pit-pier underpinning, the design shall meet the following minimum criteria:
1.After installation of each pit-pier, the approach pit shall be back filled.
2.In no case shall the general site excavation expose more than one third of the total height of a pit-pier, unless:
2.1.A pit-pier bracing system designed by the engineer is installed or;
2.2.The calculated capacity of the individual pit-pier to resist lateral loading at a greater depth is identified on the drawings.
3.Pit-piers shall be preloaded by wedging, use of permanent jacks, or by other means designed by the engineer.
4.Any voids between the bottom of the foundation and the top of the pit-pier shall be filled with dry-pack or equivalent. Dry-pack shall be composed of nonshrink material and be no more than 3 inches in thickness.
5.The need for jacking and all associated jacking requirements shall be determined by the engineer responsible for the underpinning design.
6.The width of pit-piers shall not exceed 4 feet unless the calculations determine that a wider pit-pier is acceptable.7.Shear transfer shall be designed and installed between adjacent pit-piers.
8.The bottom of pit-pier elevation shall be a minimum of one foot below the bottom of the future adjacent excavation.

 

[PEinc]

1817.7, No. 3 leaves the door wide open. If you present yourself well to the reviewer (i.e., appear knowledgeable, experienced, and confident but not cocky), you may get (not guaranteed!) approved what you want.
Attached is another good reference verifying the use of drypacking vs. wedging and jacking.

I used to actually design and install underpinning when I worked for Schnabel Foundation Company and managed their Philadelphia office. Now I just design it and keep my clients out of trouble.

http://www.PeirceEngineering.com