Foundation Underpinning

[jjeng2]

Have used this fix once before in a situation where contractor screwed up and had to come up with something. This time I think it lends itself to the application. Wanted to know if anyone had any opinions on my methodology.

Have an old brownstone that has brick walls in pretty bad shape. They are lowering the existing on grade floor level about 18" to below the bottom of the existing wall/ftg. My only role is to create a detail for the lowering of the foundation. Im trying to avoid the typical fix of shoring the wall and adding a new footing and wall below the existing walls in sections because I want to minimize disturbance to the existing walls, save the contractor the hassle of doing this, and also the walls are shared with the adjacent building. I proposed cutting the soil flush with the face of the wall down to the proper elevation and putting a 12" concrete retaining wall with the footing reversed(on inside of building) to essentially act as permanent sheet piling and confine the soil. The back of the new wall will be in contact with the face of the old wall. I would worry about wall rotation/movement causing a loss of confinement and analyze the whole thing as a retaining wall with a surcharge from the exisitng footing loads but im not overly concerned with a 12" concrete wall only going 18" deep.

 

[PSlem]

Dirt next to a footing also serves the function of containing the dirt under the footing. You are going to have to pin it some prior to excavation.

 

[PEinc]

jjeng2,

I would not do what you describe. As PSlem indicated, the dirt you are digging away retains the dirt under the foundation wall and its footing. You need to underpin the foundation in sections to below the new, lower, bottom of slab grade. I would not make a continuous 18" deep, vertical excavation (or any continuous excavation for that matter) immediately adjacent to and below an existing foundation that could easily be bearing at 2000 psf to 3000 psf or more. The dirt under the foundation SHOULD collapse. Even if it only MAY collapse, that is too risky.

 

[jjeng2]

PEInc,
I intended to address this by doing the work in sections of a few feet. Any advice on a reasonable length to do it would help. Also, we have problems with shoring the wall, as it is shared with an adjacent building and we will not be able to put any sort of horizontal beams thru the wall as one end would be in the neighbors house. I would like to include some more contingencies for safety but am having a hard time coming up with them. I could potentially have them brace the wall with posts at 45 degrees. I also thought about temp or permanent sheet piling against the vertical cut but think its cost is way too high for the size of the project. Also, the contractors are not experienced in this type of stuff. I am going to include a note to fill any soil voids with grout or the concrete pour. If the finished product seems like a reasonable way to do this, any advice on making the construction procedure safer/better would be appreciated.

 

[PEinc]

If the wall were to be underpinned, it would be done in approximately 4 feet wide sections. If you deepen the basement your way (which I do not recommend), you would probably have to build the new wall in similarly short sections. However, I would be surprised if you could design a very low, thin wall that could resist the adjacent foundation pressures.

You do not need to needle through the wall into the neighbor's basement. You could use pit underpinning. Refer to Winterkorn & Fang's Foundation Engineering Handbook. It has a good discussion on underpinning methods.

 

[jike]

I suggest that you consider using an underpinning system similar to Atlas resistance piles.

I believe Chance also makes a similar system. There are many contractors thru the US that do this type of work and it is fairly economical.

Good luck!

 

[PEinc]

I do not agree with the Atlas or Chance approach. If the brick wall is thin enough, you may be able to pick up the vertical wall load but you still have to build a foundation wall around the helical piers to hold the dirt under the wall and to make a barrier between buildings. If the brick wall is fairly wide, you may not be able to easily pick up the load from only one side. And, you still need to build a concrete foundation wall.

If you install some simple pit underpinning, you won't need the helical piers and the underpinning is the foundation wall. And, you never undermine too much of the existing wall at any one time.

 

[jike]

If need be, the anchors can be installed inside and out. Once the anchors are installed, they can very easily build a small wall to retain the soil, as originally proposed.

Both methods could be priced out to determine which way to go.

 

[jjeng2]

I had thought about helical piers but there is really not much of a footing under the wall. I would essentially have to build a bond beam or a grade beam to distribute the wall loads to the piers. These are all good ideas but there are a lot of conditions here which is what led me to my initial conclusion. Any way to temporarily contain the soil? What about building the back form of the wall as a temporary sheet pile? Problem is how do you remove it, and then you have a gap....

 

[PEinc]

jike,
jjeng2's original post said the wall is a party wall shared by occupants on both sides of the wall. I don't think it would be possible to work on both sides of the wall.


jjeng2,
You don't seem to want any free advice. You seem to want someone to say it's OK to do it your way. I can't.

 

[jjeng2]

PEinc....
Not at all. I appreciate all the advice. There are difficulties with every scenario we come up with. I spent two hours at the site with the contractor trying to figure out what to do. The least difficult in several ways was what I had originally proposed and basically I am just trying to figure out if there is some way to make it work. I wouldnt have asked for advice if I knew there werent problems. I had thought about putting in sheet piling and using it as the back form of the wall but I am trying to minimize disturbance/vibration as the wall is not in good shape. It is out of my scope but I strongly advised them to repair and repoint the walls before doing this. I could alwalys tell them to shore the walls and just build a new foundation under it in sections but we dont know what we are going to find with the neighboring wall and I felt that might disturb the wall more than a vertical flush cut. Ill just keep thinking about it. If anyone comes up with something let me know. Thanks.

 

[PEinc]

You may want to speak to an engineer and/or contractor who is experienced (specializes) in underpinning. Where is the project?

 

[jjeng2]

Project in Jersey City, NJ. Problem is it can not become a huge job from an Engineering or Construction Standpoint and if it got that involved I would tell them to scrap lowering the floor or tell them if they are not willing to pay for what needs to be done I cant help them. Im now leaning towards telling them to shore the floor joists at each level down to grade as close to the walls as possible so the only load on the wall is the self weight. Its also possible we could shore near the top of the lower level wall so all that would be on the soil would be 7' of brick. If we can do this, I think I would be comfortable making the vertical cut.

 

[PEinc]

I would not do any of that, especially a continuous excavation along and below the wall footing. The major load in a situation like this is usually the dead load of the wall itself.

Call Moretrench Geotec at 973-627-2100. They do underpinning, sheeting, tieback anchors, micropiles, dewatering, etc. Ask for Tom Tuozzolo (ext. 217) or Ken Chadwick (ext. 213). Moretrench is in Rockaway, NJ. I've done many designs for them.

 

[jjeng2]

Ok...thanks.....Instead of racking my brain anymore I think Im just going to tell the client that this is a much more significant undertaking than they think it is and it cant be done as simply or cheaply as they would like. Then we can do it right if they are willing to pay for it.

 

[DRW75]

jjeng,
is it possible to not deepen the FULL size of the basement? Say deepen only within a distance of existing foundation. You may not get the full floor area on the basement, but you will get some, and save the costs of fancy shoring & earthwork and the like.

or perhaps I have misunderstood what you are trying to do.

DRW

 

[jjeng2]

DRW75,
Great idea as most have been and I did consider it. But it is not an option as this is a small space and they already were complaining about losing the 2 feet because of the new wall. Obviously if we could do that it would solve all the problems. This is why this has been so frustrating. Every possible solution that I have come up with or seen presents a problem.

 

[hamlet104]

jjeng2:

A variation on this problem led to this solution:
Make multiple "potholes" under the wall to the depth desired for the new footing. Make them, say, 2' wide, and space them at 6' centers.
Fill the space under the wall with extra-lean concrete (aka "controlled density fill" or "CDF"). This is a very flowable low-cement content mixture that when cured, can still be worked with hand tools.
When this is cured, excavate adjacent similar-sized holes, and repeat; then repeat for the remaining holes.
When all are cured, you can then trim the face of the CDF flush with the face of the wall; sometimes this can be done even with a flat-bladed shovel. The CDF will have an f'c of perhaps 200-300 psi; take the allowable strength in compression fairly low, say, 50 psi. 50 psi x 144 =7.2 ksf, which should be plenty for support of the brick wall.

Good luck.

 

[jjeng2]

Hamlet104,
Thanks. I think that is a start of a way to potentially do this. Does anyone have any other advice/experience/comments of this method?

 

[PEinc]

What hamlet104 is suggesting is really pit underpinning where you are substituting weak concrete or flowable fill for regular strength concrete. If you are going to go to the effort to dig pits, why not use real concrete that will not deteriorate over time? 50 psi lean mix concrete or flowable fill can be excavated with a shovel. I would not want my building, and my neighbor's building to be sitting on diggable, low strenght concrete or flowable fill. In addition, you should also drypack a 3 inch space between the top of the concrete and the bottom of the footing - especiallly if you use high slump lean concrete or flowable fill which will experience much shrinkage during curing. I think you need to talk with someone who is very experienced with underpinning. Design-build contractors who specialize in underpinning would not do what hamlet104 suggested.