underpinning advice please

[SLTA]

I am working on a home renovation where the addition will undercut the existing foundation of bricks resting on soil. (Before anyone freaks out, this is the common type of construction around here in early 1900s homes, and they're usually rock solid.) I have checked the existing foundation and there's no signs of movement, cracking, etc. The foundation is 24' long and has two stories and a basement wall above it.

To achieve the required headroom, the finished floor elevation of the lower level of the new addition has to be about 16" lower than the existing, and is directly in line with the bottom course of brick foundation. I plan to underpin in 3' long sections, with at least 6' left between the sections being worked. So, a few questions:

1. Do I encase the lower portions of the brick, or just pour up to the bottom?
2. For such a small section being worked, do I need to mess with needle beams?
3. Do I put rebar in the concrete or just mass concrete?
4. How do I connect the sections together - drill and epoxy rebar?

I've googled around and haven't found a good reference for underpinning methods, so I'd be grateful if anyone knows of one.

thanks,
Linnea

 

[msquared48]

To answer your questions better, a section through the area would be helpful.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[SLTA]

ok, here goes:

 

[SLTA]

The architect just emailed me that he'd like to shift to a load-bearing wall running parallel with the existing brick wall, to carry the loads from the new structure, so that should be imagined in the sketch as resting on the thickened slab in the new portion.

 

[msquared48]

Three things at first look here:

1. I would be concerned with water penetration from the outside through the brick. Need some waterproofing measure outside of the wall.

2. I would also be concerned with the brick wall "walking" to the inside in a seismic event. Hence raising the slab level 4" or so to engage the first course at least would be good.

3. I would add rebar transverse to the wall at the bottom of the unnderpinning. I would add the longitudinal steel meant for the underpinning in the new slab (parallel to the wall).

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[SLTA]

Thanks, Mike. If you're ever in Asheville, I'll buy you a beer, for all the help you've given me on here.

 

[csd72]

slta,

I assume asheville is far enough from memphis to be out of the high seismic region. If this is the case I wouldnt be too worried about the wall sliding off though you do need to to take into account the fact that it is now retaining.

I agree that groundwater could cause a damp issue, though you can get eggcarton type drywall material that will act as a drain to drain this away internally.

I disagree with the need for rebar in the underpinning as the size of the concrete sections is small and will be under a very low compressive stress (afterall it is just replacing the soil!)

Not sure why you have made the underpinning as L-shaped as this just adds complications in pouring it. I would just use a rectangular section down to the level required and the same with as the exuisting then bring the downturn in the slab to the same level. If you are worried about overturning then dowelling into the slab downturn will help take out the moment.

What I would recommend though is continuous underpinning in a traditional hit and miss method. Unless you can justify a higher bearing stress than used then you need to keep the area of underpinning the same as the existing. Once again just check the bearing stress.

You need to leave 50mm under the existing foundation to be filled with non-shrink grout after the underpinning has had time to get its initial shrink. Otherwise the underpinning will shrink away from the underside of the wall.

One of the structural magazines had a real good article 2 years ago about underpinning, I will see if I can find it for you.

Personally I think that there really should be no need for a secon structural wall here if the existing is in good condition - sounds like a good opportunity to save the client some money.

 

[SLTA]

Thanks, cds72. I appreciate all your thoughts. The second wall is to carry the load of the new structure, which is significant, and I really don't want to get into messing with the existing construction any more than req'd. This plan actually saves a bunch of money!

 

[hokie66]

I would agree with the architect in providing another wall for the new addition. Still need to underpin the existing as you intend.

One other thing to consider is how the basement acts globally. Not sure what it looks like in plan, but the opposite existing wall now will not have the same support at the top unless that is reinstated by the new construction.

 

[SLTA]

hokie, I'm not sure what you mean about the opposite wall and support at the top - can you elaborate, please?

 

[hokie66]

What I assumed, since you said it is an existing basement wall, is that the brick wall in the existing structure retains earth on the right side of your sketch, and that on the opposite side of the building, another wall also retains earth. Therefore, each wall depends on the soil on the far side to resist the force which comes through the floor system. When you remove the earth on one side, that resisting force is no longer there.