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Proximity of Adjacent House to New Retaining Wall at Property Line

Proximity of Adjacent House to New Retaining Wall at Property Line

Proximity of Adjacent House to New Retaining Wall at Property Line

(OP)
Hello everyone.
I am designing a retaining wall against a property line. The wall will be a precast inverted T-shape with the heel nearly against the property line. The adjacent property has a single story house and either a basement or crawlspace underneath that is 1-ft off the property line. The wall height varies from 12-ft to 0-ft along the length of the house. A couple questions I'm hoping to get insight on:

1. Is there a good rule of thumb for estimating the loads from a residential building? Like X psf for each floor plus X psf for the roof equally distributed to each exterior wall? I'm trying not to recreate the house design just to get the bearing pressure, but still have a reasonable conservative value. Anything else I should also consider with this loading?
2. The proximity of the house to the wall and the property line obviously creates some stability issues for the existing house. The contractor will be responsible for creating a shoring/underpinning plan that will be reviewed by the EOR for the project. Shoring/underpinning for the adjacent house is outside of my scope, but I want to ensure I qualify the minimum requirements for the contractor within my wall design drawings to protect myself if there. Any thoughts on this?
3. The contractor's tentative plan is to excavate and install each precast wall piece individually within a 6' wide trench box. I am not going to pretend I know enough about residential construction to know if the house can tolerate a 6-ft wide excavation less than 3-ft from the foundation. The site soils are predominately sandy. Any thoughts on if this plan is reasonable?

The house is essentially condemned, and my client has repeated tried to contact the owner to buy the property with no replies.

I am a specialty engineer responsible for just the wall stability.
Replies continue below

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RE: Proximity of Adjacent House to New Retaining Wall at Property Line

The house location sounds awful close. You should get the homeowners permission to take a lot of project record photographs. Is the depth of the retaining wall excavation below that of the house?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

RE: Proximity of Adjacent House to New Retaining Wall at Property Line

This could have trouble written all over it. The key is, will the wall and the house's foundation depth be the same? I guess you don't know if you're not sure if it's a crawl space of full basement. You should get clarification so you can try to match footing depths and avoid having to underpin the house.

Edit: I'm surprised a 12' tall wall will be allowed on the property line. There isn't any setback requirements in the local code?

RE: Proximity of Adjacent House to New Retaining Wall at Property Line

A lot more information is needed: elevations of existing adjacent basement, bearing pressures and footing sizes for the adjacent building's foundations, soil conditions, and ground water level. Your inverted T wall being built in segments from inside a 6' wide trench box seems questionable in terms of OSHA, worker safety, and protecting the adjacent building. Underpinning would require permission to work on the neighbor's property and, at 12' deep, would need lateral support, at least temporary, maybe permanent. While I don't normally recommend doing this to support an adjacent building, a conservatively designed, drilled-in, permanent, soldier beam wall with a finished concrete facing, just inside the property line, might be more appropriate than a T-shaped wall with its heel up against the property line and its front face about 2' inside the property line. There is a lot more to this project than just designing an inverted T-shaped retaining wall.

www.PeirceEngineering.com

RE: Proximity of Adjacent House to New Retaining Wall at Property Line

(OP)
dik - Attempts to contact the homeowner have been unsuccessful thus far (it has been uninhabited for many years and the owner owes quite a bit in back taxes). Because of that, we have been unable to verify the house foundation depth.
MTNClimber - No setback requirements for this zoning. It is in an urban downtown area.
PEinc - Unfortunately, my client has been unable to make contact with the owner to gain access to get information on the existing structure. The water table is well below the bottom of wall elevation. I proposed a soldier pile wall with concrete facing to the contractor, but they weren't interested in it at the time. It may be something that gets revisited. The EOR specified a segmental retaining wall for this location, so they obviously did not think about the constraints. I totally agree there is more to this project than just the wall design.

I am in the odd predicament that this is all well beyond my scope, but I want to make sure the right thing is done.

RE: Proximity of Adjacent House to New Retaining Wall at Property Line

Proceed at your own peril...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

RE: Proximity of Adjacent House to New Retaining Wall at Property Line

I would talk to the EOR and tell them you need more information to properly design your wall.

RE: Proximity of Adjacent House to New Retaining Wall at Property Line

Property lines are one thing, but in my neck of the woods there is an easement adjacent to most property lines. So even if you are theoretically on your side of the property line, building anything within the easement is still off limits. What is the need for a retaining wall that close to the property line, are you building something right on the property line? From a technical perspective, a retaining structure in this situation is totally doable, I think your issue is more logistical than technical. What information are you hoping to get from the adjacent property owner?

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