Elevator pit next to existing footing

[JoeBaseplate]

Please see the attached sketch for a visual explanation of the situation. And as the title suggest, one of the edges of the proposed elevator pit are right underneath an existing footing. The footing is 4’ square. I think this preliminary detail should be fine, except I am not sure about the underpinning and its consequences on the existing footing. Since they will have to over excavate the area under the footing to place the wall, how would the contractor be able to place a compacted fill under the footing. Would a lean concrete mix be a better option?

 

[msquared48]

You are creating a hard line of bearing for the footing edge that concerns me, and it will be transmitted to the elevator footing regardless what you do.

May sound like an overkill here, but I would strongly consider shoring the existing structure that the column is supporting, removing the existing plinth and fooring, repouring the footing at a depth to match the top of the elevator footing, repouring the plinth to the existing top elevation, and re-attaching the column to the plinth.

Structurally, I just feel better about it here.

Mike McCann
MMC Engineering

 

[Teguci]

Did something similar except it was a bearing wall and footing. We braced the wall to either side of the excavation and placed the new concrete under the bearing wall footing.

Lot of work. In addition, we had notes recommending the contractor use an excavation box and how to use it.

 

[JoeBaseplate]

Considering what you guys mentioned here, I am wondering if it would be cheaper to support the existing column/footing with push/helical piers.

The elevator shaft may also move in a way that the pit wall is right underneath the column, in which case I think bearing the column/plinth combo on the new wall would be the only option.

 

[JoeBaseplate]

msquared48, actually the first idea that came to mind was to excavate under the existing footing and fill it with a concrete, in essence a very thick elevator pit wall under the existing footing.

 

[csd72]

What i generally do in this type of situation is draw a 45 degree line from the base of the excavation up to the footing and ignore the support of any soil above that line.

If the footing still works during permanent and temporary conditions then it is fine.

In your case I would think your detail may be okay in the permanent condition but would think that you may need screw piles to ensure that it can be safely constructed.

I dont think standard underpinning would be suitable for this case.

Dont forget to check the uplift case as this is often the critical case for this type of footing.

 

[JoeBaseplate]

csd72, I think its for this reason that I am considering going with helical piers for a temporary and permanent solution. I think the piles would also minimize any new settlement. I guess the sequencing will work in a way where they would install the piles first and then they can work on the elevator shaft. Though I am not sure how the excavation for the shaft will effect the piles, may be this is a question for the helical pier supplier.

But I got a question based on your comment. You mentioned that the screw pile can be used for temporary loading. I wasn't aware that the contractors provided these on temporary basis, I though they were always a permanent fixture.

 

[EBSEngineering]

We have used helical piers to support this type of application before. May have to use a helical pulldown micropile from Chance. If the shaft of the helical pier can't take the bending forces when the soil is removed, then an HSS needs to go around the pier with grout in it to support the shaft of the pier. The helical piers would permanently pick up the load once the soil beneath the existing footing is disturbed.

 

[bgatto]

I would try to do something like the sketched in the attachment. Would pour additional concrete around and under the free edges of the footing so that the part that's over the elevator pit could be neglected (demolished). The change of the shape in plan is a try to avoid or minimize the change of the position of the gravity centre of the footing. All this is accompanied by addition checks of the footing.

 

[PEinc]

Without a plan view of the area, including adjacent footings, one can only suggest potential methods of underpinning the column footing.

The existing footing could be conventionally underpinned using 3 underpinning piers as long as you can have access from the side opposite the elevator pit. The first underpinning pier would be about 3' wide by 4' long and would extend under the existing pier for 1' wide by about 3' long. This pier would be poured to 3' x 4' = 12 sf. The third pier would do the same. Then, the second pier could be 2 feet wide by 3' long, under the existing footing, between the first and third piers. The total bearing area of the three piers would be greater than 16 sf. If you do this, you need to HAND-DIG the piers and install pit shoring as the underpinning pits are being excavated. You need to know what you are doing or you need to hire a contractor that does know how to install underpinning. This is not a job for a backhoe or for digging an open hole and then standing up a concrete form.

OR

You could also do as msquared48 suggested. I've done this both ways. I would not use helical piers for this particulat situation.

Whichever way this is done, you need to do a pre-construction survey of the building and then monitor the column as the work progresses.

http://www.PeirceEngineering.com

 

[JoeBaseplate]

PEInc,

Thanks for the detailed response. Here is the plan view of the existing. The architect is still playing around with the location so at this point it appears that not only two column footings are going to be effected but at least the existing South wall will need to be addressed as well, and possibly the west wall to a lesser extent.

I am surprised to hear that you would not consider helicals. Actually thats what I have set my mind on. I think with a lit bit of ingenuity I might avoid underpinning of the column footings all together. The helicals would be installed first, followed by use of excavation box to avoid undermining the newly installed piers.

I am not sure about the south wall yet, I think I might go with helicals on that too.

The reason I am considering helicals is because this is an existing 3 story structure (1950s) with concrete deck on bar joists and wood trusses at roof. The column spacing is very small too, 11'-4" so I want to keep any potential settlement to a minimum and I think helicals would be much better then my original detail.

 

[PEinc]

It looks to me like you should underpin the two walls and pick up the two column footings like msquared48 suggested.

http://www.PeirceEngineering.com