partial underpinning of existing footing 2

[jabarch]

First time posting on this site. My apologies if this is the wrong place for this post. I am a residential home designer and I currently have a client who wants to have their 5' deep crawlspace lowered to 8' deep. A specialized foundation contractor has proposed underpinning the foundation. Their proposal however is varying from what I understand to be the typical practice. The contractor is proposing to only excavate and underpin to the exterior face of the 8" foundation wall (not the full depth to the exterior face of the footing (as seems to be typical practice). He is proposing that the underpinning have a monolithic footing at the bottom of the new wall. He will vibrate the whole form, no dry pack at the top. He is proposing that the new drainage mat, rather than being placed on the inside of the new underpinning, be placed on the exterior of the new underpinning, before forming and down under the new proposed monolithic footing, this also seems to vary from standard practice. And lastly he has proposed that approach pits be made in 6' widths as opposed to 4'widths (2 stage sequence) as would be typical. Any problems with this foreseeable construction, does anyone here recognize this a a standard or precedented way to underpin a residence. Is it reasonable for residential underpinning? I have attached a sketch to help clarify, the footing in the sketch is shown as being only to the interior, however it may be the intention of the contractor to dig out a footing projection on the exterior side too. Existing foundation dates from 1970 is cast in place concrete, with a presumably 20" to 24" wide footing.

Thanks in advance

 

[oldestguy]

Usually something like this is done in sections, maybe each 5' long at the most. If possible some re-bars should be used to help tie the individual new sections together. One technique that would be good is to leave a small space between the old footing and the new wall. Then, shortly after the new work hardens, dry pack in that space a stiff concrete mix to transfer some of the old load to the new footing.

 

[PEinc]

IMO, you could underpin just to the rear face of the existing wall but that is not usually done. Having the existing wall sitting toward the rear of the underpinning helps increase overturning resistance of the underpinning piers. However, this does reduce the usable floor area in the basement, which most people do not want.
The projected portion (toe) of the underpinning piers toward the drain pipe is very thin and probably would need to be reinforced. Instead, I recommend making this projected concrete at least as thick as the length of projection. Then, you could say that this "footing toe" is in compression.
The pier concrete projecting up in front of the existing wall footing does nothing. So why have it? The only thing it might do is allow you to not dry pack the existing footing. I recommend dry packing and eliminating the new concrete in front of the existing footing's edge.
I do not like the underpinning bearing on top of the drainage board for a couple of reasons. I recommend that the underpinning extend deeper than shown, first, to increase the toe thickness and, second, to allow more depth to run a small PCV outlet pipe, from the vertical drain board, through the underpinning, and over to the collector pipe in front of the underpinning. You should not need too many of these outlet pipes. There can't be very much water to drain.
Underpinning can be wider than 4' as long as the existing foundation wall can span the width of the underpinning piers. The underpinning pier excavation should be shored to prevent sloughing or collapse of the pit sides and prevent additional undermining of the existing footing.
Do not trim off all of the existing footing's toe or you could cause settlement by such a great reduction in bearing area. Rather, just trim off the footing toe for each of the underpinning piers as they are being sequentially being constructed.
Make sure the underpinning provides equal or greater bearing area than the original footing.
Underpinning very rarely needs reinforcing steel. You don't need it.

http://www.PeirceEngineering.com

 

[jabarch]

6' does seem long on a 20' long wall (4 of them, 1 wall only an internal retaining wall against the other half of the non raised basement slab) Especially so because there is only 2 forming sequences.

As to some of the points about the raised portion of the pin that extends inwards, I have only sketched it this way as I know they will "overpour" the pin as opposed to dry packing the gap. This projection is only schematically represented in the sketch to indicate what they would require to give the required head vertically as well as the space to fill the form horizontally. No structural intention. I think dry packing would be more reliable as well, though obviously much more arduous.

The toe is to thin (not to scale) I assume it will be the typical footing projection dimensions.

I do not like the idea of the drainage board behind the pin either. Seems complicated to place properly. Useless under the weight of the footing. Could possibly act like a giant slip sheet not allowing the pin to key into the adjacent soil. Shouldn't the drainage board simply be installed inside the pin, from grade down to the weeping tile?

Unlikely that much shoring will be employed in such a residential project. The contractor did mention that there will be full height steel columns fastened to the bottom of the underpin in some fashion and fastened to the floor framing above. This is to negate the hinge point where the new and old foundations interface.

Agree about the point regarding the footing being removed sequentially.

One of my original concerns, The new footing is unlikely to have the same bearing as the old, as it is not poured to the full depth (back) of the existing footing. Unless they intend to key a small footing into the soil at the back and bottom of the excavation?

Seems to be some varying opinion on whether underpinning requires reinforcing. No steel is needed horizontally to take up differential settlement of the pins? No steel is required vertically to tie old foundation and new foundation together and thus negate the hinge effect?

 

[PEinc]

Underpinning VERY RARELY has reinforcing steel. The concrete piers are large and have minimal compression and tension stresses. Rubble stone foundations are common and they have no reinforcing steel. Block foundations are thinner and have little to no reinforcing or grout fill. Which do you thick is a better foundation - a rubble stone wall with lousy mortar, a thin block wall, or an unreinforced concrete foundation of equal or larger size? If horizontal reinforcing were needed to prevent differential settlement, then the existing wall would already have significant cracking. The concrete surfaces between adjacent piers bond together. If you are still worried, you could add shear keys between adjacent piers.

Why would you expect the underpinning have less bearing capacity than the existing foundation if they are founded on the same soil and the bearing width is similar or greater?

Does the existing crawl space have a drainage problem now? Does the existing foundation wall already have drainage board? If not, why does the slightly deeper, underpinning need drainage board? Most 8' deep basements that I see and work on don't have perimeter drainage. If you have that much water now, you will have problems installing the underpinning.

http://www.PeirceEngineering.com

 

[Ron247]

Are you temporarily shoring the floor system above? If not, when you cut off the interior projection of the original footing, you will get a fairly significant change in soil bearing from both the reduced bearing area and new eccentricity.

If this is a crawl space now, why is there a concrete slab in it?

If you install steel columns periodically to provide a "backbone" for the 2 wall sections, are you going to "preload" those columns so they are pushing back on the walls now or are you going to let the walls shift a little and then laterally load the columns.

 

[jabarch]

Ron, I think this would definitely be a concern if the entire crawlspace was excavated down 3' all at once as opposed to doing sequential pinning. I suppose there is some misunderstanding on my part as to whether the standard approach to underpinning a small residential area (approx. 400 sq. ft.) is typically done by way of complete excavation of the interior area combined with shoring of the existing floor system vs. only removing enough of the supporting soil at a time in a sequence, so as to leave enough soil to support the existing walls.

I was under the impression that typically, enough soil would be left around the perimeter of the exterior walls so as to support the angle of repose of the existing footing. (in this case maybe a 4' width.) Then, approach pits would be dug perpendicular to and under the existing footing. These approach pits would only be excavated every other, or perhaps every third section at a single time, with each section being perhaps 4' or 5' long at max. I have perhaps been ignorant in assuming that this is the typical practice.

As regards the crawlspace, This is a 1970's build. Located in Alberta Canada. Split level house. The crawlspace is the portion of the basement located under the lower half of the "split level"

 

[PEinc]

This is a typical underpinning job. The only real difference is that you are trying to underpin from inside a low-headroom, crawl space. Removing spoils will be slow. Make sure that, if you dig out the middle of the crawl space to give you more headroom when underpinning, you leave sufficient berm in front of the walls to be underpinned. The excavation of the approach pit and the underpinning piers should be shored to prevent collapse of the sides which could undermine much more than your theoretical pier width of 4 or 5 feet. For the short runs of walls that you have (about 20' long), 6' wide piers are probably too much. I would keep it to 4 or 5 feet maximum.
Check out my web site for my the presentation titled Philadelphia’s Recent Building Collapses – Causes and Prevention. It has information on underpinning.

http://www.PeirceEngineering.com

 

[jabarch]

Thank you everyone for all the information. Very helpful. I will be sure to check out the presentation. Thank you.