Foundation over existing foundation

[warlapen]

Is it possible to build a new foundation over existing foundation ?

While working on a building site located next to a sloped land, we first decided that shear walls or retaining walls are not necessary, as there already were stone masonry structure to support the soil. We completed the foundation of the project and completed further works, and completed plinth tie beam casting.
But now, the client is asking us to build a shear wall.
The problem is, our existing foundation slab extends until the boundary line of the plot, and in order to build a shear wall, we have no choice but to provide its foundation on top of the building foundation.

Is this possible, and if so, how can it be done safely?

I have attached a rough sketch of the situation.

Any feedback is much appreciated.

 

[HTURKAK]

It is not reasonable to construct retaining wall found. on existing bldg . foundation right??

I will suggest you ; demolish the masonry wall and construct the L shaped retaining wall. You will need soil weight to resist OT moment. Make sure that heel side will be at soil side.

..




He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48

 

[warlapen]

Thanks for the suggestion.
The problem is , we can not demolish the masonry wall, and thus can not provide heel for the new retaining wall at soil side.

So the case being that the existing masonry wall can not be changed, could a new footing be constructed on top of the existing one ?

 

[JoshPlumSE]

If I interpret the question correctly, the existing masonry "retaining wall" is going to remain and isn't going to be demolished. But, the new wall will be only used as a shear wall. Is that correct?

If so, I'd say that you'll want to dowel into the existing foundation, make the new and old behave together as a single combined footing. Then analyze them for the combined loads. Ideally, you'd have a small gap between this building and whatever that masonry wall is supposed to be.

 

[warlapen]

Well, we call it a shear wall around here, but technically it would be retaining wall.
Existing masonry wall is going to remain intact, but we plan on building new retaining wall 5-6 feet taller than existing one and backfill.

p.s. i did not know about double posting and have removed the other thread.
thankyou.

 

[warlapen]

How can we effectively dowel new to old ? Could you please help.
Thanks.

 

[EireChch]

Drill and epoxy some bars in, not much to it. Just need to make sure your that when you tie the retaining wall into the house foundation, that it has enough structural capacity. There is no way in hell that it will move laterally so its just structural capacity in my eyes.

 

[SlideRuleEra]

warlapen - As others have explained, constructing & tying a new foundation on top of an old one is practical.

Check effect of vertical loading from the new wall on the existing building foundation.
The dead weight of the new retaining wall is eccentric to the load from the building.
The horizontal overturning load from retained soil will cause eccentric (vertical) uplift on part of the building foundation.
The various combinations of vertical loading from existing building new wall dead weight, and overturning uplift is important.

 

[warlapen]

Thankyou so much for all the responses.

 

[JoshPlumSE]

Drill and epoxy some bars in, not much to it

Yeah, that's what I was thinking. We might have two L shaped bars that are epoxied into the existing slap and lapped with new reinforcement for the new portion of the foundation.

We also, would probably "roughen" the top surface of the existing foundation so that the newly poured concrete gets a bit more friction. But, that's not always done.