New Addition/Footing next to Existing Rock Foundation Wall

Other options probably better than yours

6- Install helical piles under the new ftg...

7- Ask politely to the neighbor to fill the cellar with sand and gravel..

JungleJoe said:

Should I still be worried about the weight of the SOG influencing the soil behind the URM wall?

Click to expand...

Yes.

IMHO, based on the sketch, the existing unreinforced masonry wall will have to "strengthened" before any of the options can be specified. As suggested, filling the basement will do that; so will constructing a new retaining wall inside & adjacent to the existing URM wall. What type new retaining wall may depend on site conditions. One thing I would not do is "remove the URM wall and build a new retaining wall in its place".

You need a lot more information (dimensions & loads) before you can make any informed decision..."a very rough sketch", "approximately 5' tall" and "footing will not have much load on it" are for limited discussion, not necessary mathematical calcs. If you don't have all dimensions / loads try to obtain them or make reasonable conservative estimates.

Thanks everyone for the insight.

HTURKAK said:

Install helical piles under the new ftg...

Click to expand...

Yeah we discussed that, but I'm worried about influencing the existing setup when the piers are installed.

SlideRuleEra said:

"a very rough sketch", "approximately 5' tall" and "footing will not have much load on it" are for limited discussion, not necessary mathematical calcs.

Click to expand...

Yes, understood. I'm scheduled to go out on site next week. This sketch I drew is based on a sketch provided by the owner so I wanted to have some potential ideas in mind before I go out there.

The owner took some measurements and pictures and sent them to me so I've updated the drawing. See below for another idea proposed by a colleague. I don't see any major issues with this other than ensuring that the existing rock wall isn't undermined during excavation.

JungleJoe said:

See below for another idea proposed by a colleague. I don't see any major issues...

Click to expand...

Your colleague's proposal has significant problems and risks (for the project, not just the Contractor). You may not see them because they occur during the construction phase. Visit the site next week. If you choose, we can discuss the details after that... or not.

SlideRuleEra said:

You may not see them because they occur during the construction phase. Visit the site next week.

Click to expand...

I'd love to get your input on this now that I was able to go out to the yesterday morning.

The owner of the home is a contractor and will be building this addition himself. The detail I posted last Thursday matches the conditions I saw on site. The new retaining wall would need to be 6' tall from top of wall to bottom of footing to match the top of rock wall and bottom of block wall as shown in the detail. The existing back wall of the home is 40' long. This cellar area is directly in the center of that 40' long length, but the cellar area itself takes up just 10' of that 40'.

The owner/contractor is aware of the risks with digging next to the existing structure, but he'd rather do this than any of the other solutions that have been proposed. I'd appreciate hearing more about the problems and risks you see for this method. Thanks!

Glad you were able to visit the site; that is worth far more than the time it takes.

Your colleague's proposal, with vertical excavation, has several technical and safety draw backs... but I skip all of that for now. Unshored vertical excavation of a soil, other than competent rock (granite, sandstone, etc.), is an OSHA violation in the USA, see: OSHA Excavation Sloping Shoring. Since the excavation will be deeper than 4' and open for more than 24 hours there are no "loop holes". This may be considered a "Willful Violation" to which you and your firm contributed with a design specifying vertical excavation:



With that said, take a look at one "technical" problem. The proposed design will require a one-sided concrete form since the other side of the wall directly contacts the (vertical) soil. Assume the wall footing is 1' thick; then the wall is 5' tall. All forces to contain the 5' of freshly placed concrete have to come from outside the wall (no form ties between the two side forms). This is not particularly easy, but can be done on the formed side of the wall.

The soil side of the wall is another story. The soil supporting the existing structure is horizontally "thin". Forces are resisted by only a URM wall of unknown structural properties. Force from the fluid concrete will tend to (and IMHO, will) "blow through" the "thin" soil... taking down the existing structure adjacent to, or on top of workers placing concrete in the wall forms. Sometimes a valid argument can be made that placing concrete at a very slow rate, say 2' to 3' an hour will drastically decrease fluid concrete pressure, and that is true.. but not in this case. The wall is so "short" (5 feet), that ACI 347, used to determine concrete form pressure is of little help. See my sketch below:



Don't take my word for this, check it out yourself and, hopefully, look at other options even if the Owner does not prefer a different approach. Sometimes math and engineering principles override hopes and wishes.

I' be happy to discuss this more. We are preparing for tropical storm winds coming through tonight. If I don't get back to you right away we may have lost power.