Underpinning
Underpinning
(OP)
Can the top 2" of non shrink grout be omitted and instead vibtrate the conrete to the underside of the existing footing when underpinning?
My hesitation in allowing this is that some of the voids under the existing foundation will not be filled with concrete which may result in some settlement in the future.
Does anyone have any experience in this?
My hesitation in allowing this is that some of the voids under the existing foundation will not be filled with concrete which may result in some settlement in the future.
Does anyone have any experience in this?






RE: Underpinning
I wouldn't trust the top layer of concrete to be vibrated firm enough to carry the load.
RE: Underpinning
RE: Underpinning
If you pour to the underside of the footing, there will be shrinkage of the concrete. The amount depends on the height of the underpinning pier. The bottoms of many footings are not level. Therefore, if you do not dry pack, you can not be certain that you have full bearing between the top of the pier and the bottom of the footing.
Non-shrink, high strength mortars are a waste of money. The load on dry pack is usually less than 50 psi compression. Dry pack is usually a 2 or 3 (sand) to 1 (Portland cement) mixture. If you calculate the shrinkage of 3 inches of concrete, the shrinkage is negligible. Dry pack should shrink even less than concrete.
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I usually see the concrete placed with 18 to 24 inches of head and vibrated in place to provide good consolidation and contact with the underside of the footing (as indicated by jh.
I'm not sure what authority "reference books on underpinning" have other than indicating a common practice.
RE: Underpinning
RE: Underpinning
"ACI Committee 209 suggests that the long-term total shrinkage might vary from l/2 to 1 1/4 inches per 100 feet, and that the factors affecting the rate of shrinkage are so variable that it could take from two weeks to more than six months to experience the first half of the total shrinkage. When it comes to a hands-on prediction of shrinkage, "floormeister" Alan Face's dictum No. 1 is that "every 20 feet of concrete is looking for 1/8 to 1/4 inch of shrinkage." Considering the combination of hydration, chemical shrinkage, plastic shrinkage, and drying shrinkage that occurs in concrete, Face's dictum No. 2 is that "wild things are happening in the concrete in the hours immediately following placement."
While we may agree or not that the amount of shrinkage is small, the part that stands out to me is the "two weeks to six months to experience the FIRST HALF of the total..."
RE: Underpinning
If what you wrote is entirely true, all underpinned buildings would continue to settle noticeably for months after they are underpinned. Field monitoring of underpinned buildings shows that this is not the case unless, for example, there is a soil bearing problem under the underpinning piers. I have monitored and plotted the settlements over time for many underpinned buildings. In almost 30 years, I have never seen concrete shrinkage to be a problem in underpinning.
You quoted, "wild things are happening in the concrete in the hours immediately following placement." Underpinning piers are usually dry packed the day after the concrete is poured, or later. Therefore, I guess that the "wild things" must have already occured before the pier is dry packed.
RE: Underpinning
I'm not sure what Dr. Hover was referring to (my reference was from an article he wrote). The language of your post seemed strikingly similar.
I was only commenting based on my understanding of shrinkage behavior over time and the reference cited. In my experience, the overwhelming majority of underpinning piers (that I have been involved in) are relatively shallow (5 to 8 feet) and therefore, I suspect not much concrete shrinkage is going to occur in any case.
I have no reason to doubt your experience in measuring underpins. It seems that in the cases where much greater depths are involved concrete shrinkage may be more meaningful. Along those lines, I think that if reinforcing steel is included in the pier installation it would have a beneficial effect on shrinkage. Perhaps that is a factor in your experiences also.
--Kenneth C. Hover, Ph.D., P.E. is a structural/materials engineer and professor of structural engineering at Cornell University, Ithaca, N. Y. An active member of numerous ACI committees, he is also a popular speaker at Hanley Wood's World of Concrete.
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Great picture! I've always liked working on underpinning projects, although I've never had to go that deep. There are fewer contractors around that know how to do this type of work, even the "simple" ones of one or two 8' lifts.
The underpinning projects I've worked on it seems there was more concern about getting the concrete poured before the soil fell into the open excavation, than worry about settlement. If the sections are small enough, 4' wide or so, and there is plenty of cure time between sections, then there was very little, if any settlement - none that hurt the building. It seems that the job is all about alternate sequencing of the underpinning sections and timing.
RE: Underpinning
Some "underpinners" dig a sloped excavation downward from the edge of the footing and then dig an open cut into the slope and under the footing in order to form and pour the pier. This is wrong even though many try to do it this way. It is wrong because the unshored sides of the excavation can collapse which can damage the building. Also, in order to form the front face of the pier, someone has to enter the excavation. If your unshored excavation is over 5' deep, you are probably in violation of OSHA regulations and someone will get hurt.
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Good points, but just to clarify, we NEVER let anyone into the underpinning pit under an existing footing. A 2' to 4' wide footing can be hand excavated without getting someone under the footing.
We face-formed the underpinned area and braced it back to the outside. And you are also right about the depth of excavtion and the OSHA requirements. I think the deepest we went in one lift was 8', one form panel.
RE: Underpinning
Just this week, a client of mine updated me about a job where a building facade was being underpinned by your method. One contractor was hired to dig the pits. Another contractor was hired to install the concrete. No one had overall responsibility for the underpinning. The contractor wanted my client to install tiebacks in the underpinning. I told my client to stay away from the job. Luckily, he did as I suggected. The "underpinning contractor" was trying to install 17' deep concrete underpinning piers in stacked lifts of about 5' per lift. Essentially, the contractors were going to underpin the underpinning several times. The tieback anchors were going to be installed in the upper lift of underpinning. I don't know what the contractor thought was going to brace the lower sections of underpinning. Over the weekend, rain caused excavated, unshored, underpinning pits to collapse. This further undermined the foundation wall. The building facade is now significantly damaged.
There are good reasons why underpinning should be done today the same way as it's been done for many, many years. The proper method is low tech but it works. No one has found a better, surer way to do underpinning.