New wall over existing foundation

[Buleeek]

Hi everyone,

I am to design a new 4-story building. It is going to be a row house in NYC and it will be adjacent to the existing row house. It seems the previous building has been demolished together with foundation and the hole filled.
1. The new house wall will be on the property line each side.
2. We do not know much about the existing foundation - no size, material or elevation. I would assume it is concrete and it encroached the property line.
3. The existing building is about 100 years old with clay brick walls
4. My concern is, the part of the new wall will rest on the existing foundation.
5. In the attachment I presented two options: dowel new mat foundation to exisitng footing, or cantilever the mat over the existing footing without touching anything what is existing.
6. We have a soil report:
6.1 The soil capacity is as follows:
The first 4-6 feet is fill - probably filled after demolition. Anyway, no use
below 6 ft -> 1.5 tons
below 10 ft -> 2 tons
below 15 ft -> 3 tons
6.2 no water was found

My idea is to pursue with the option B (cantilever mat foundation)

Can you look at my sketch and help with the following:
7. Which option seems more suitable for this situation
8. Suggestions on the cantilever mat foundation: shape, reinforcing, etc.
9. What should I know about row houses built over 100 years ago

Thank you in advance.
Buleeek

 

[SlideRuleEra]

IMHO, use cantilever mat foundation, no question about it.

My approach to these type situations is to avoid doing anything (like installing dowels) to the existing structure, because:

1) Minimize below grade work. Goal is to excavate, perform subgrade construction & backfill... as quickly as practical.

2) Keep subgrade work as simple as possible (KISS Principle). Working conditions are far from optimal. Extra work (like installing dowels) increases the possibility of unexpected problems (drill hits rebar in existing foundation or existing foundation "crumbles" when drilled) which can lead to delay (while the excavation remains open).

3) Tying new and existing foundations together is not going to matter unless differential settlement occurs.

4) If differential settlement occurs with foundations tied together, outcome is likely "bad":

a) Existing foundation is damaged, with unintended consequences.

b) New foundation is damaged, with unintended consequences.

c) Dowels "work"... now, one side of the new foundation (side tied to existing) does not settle. Other side of the foundation does settle... now, the new foundation has it's own differential settlement, with unintended consequences.

d) Best outcome is the dowels shear cleanly & easily when new foundation settles... in which case the dowels did no good and there was no reason to install them.

 

[mfrad]

Buleeek
Before you design your foundation system first do a test pit along the existing building foundation to determine its depth and footing encroachment if any. It may have been a party wall only four feet of depth and not an independent foundation wall. You may have to underpin existing building. Check landmaps and previous surveys regarding party wall construction. Your foundation system appears too complex unless for a seismic design requiring 10’ depth.

 

[KevinChez]

Careful the contractor doesn't use the adjacent wall as a form.

 

[Buleeek]

Thank you for your responses.
1. Good point about contractor using the existing wall as a form. To prevent it, I am thinking about switching to 12" cmu wall below grade and attach it to a cantilevered foundation.
2. Any suggestions on how to make sure the cantilevered mat slab will work as cantilever? Is placing 5" EPS insulation between the existing and new foundation enough?
3. I am looking for some examples and suggestions on proper reinforcement of the cantilevered mat slab. Can someone share their experience on this matter ? I am looking at about 4 kip/ft ASD load.
4. From your experience, are there any general notes that definitely should be placed on the drawing in regards to existing foundation and making sure nobody touches it?

Thank you once again for you input,
Buleeek

 

[SlideRuleEra]

Is placing 5" EPS insulation between the existing and new foundation enough?

Be sure the material has adequate compression strength to withstand hydrostatic pressure of vibrated fluid concrete without too much distortion.