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When to tie-in to existing foundation

When to tie-in to existing foundation

When to tie-in to existing foundation

Hi all,

I am a part of a project that involves adding a new mat foundation for a new skid (the foundation will only be about 12" deep). The location of the foundation is currently being decided, but we know it will be near an existing foundation.

At what point does an engineer decide to tie-in to the neighboring foundation? I would rather not have to tie-in since that requires extra engineering, but I don't know if there's a minimum distance between foundations that should be observed. For example, what if the layout determines that the new foundation is only 6" away from the existing? Is it ok to install it like that? The new foundation will be shallower than the existing one so there is (I think) no risk of digging under the existing foundation.

Hopefully my question makes sense. Thanks for any help.

RE: When to tie-in to existing foundation

sentrifice - IMHO, there are few, if any situations where tying into an existing foundation is a good idea. I suppose is the soil is really "good", ties would be ok... but "good" soil is not the case where I worked.

If the foundations are close, but not joined, they can be evaluated... may be something for a geotech.

If the foundations are joined, an entire new set of issues appear:

1) How is the load distributed between the two... not necessarily what the Engineer predicts.

2) If there is differential settlement, what happens to the connectors...
... do they eccentrically load both the old and new footings?
... do they damage the existing footing?
... do they damage the new footing?
... perhaps, what I consider the best outcome, they cleanly and easily shear without causing problems, in which case there was no point in putting them in.

3) Then there is the problem of installing the connectors... drilling into existing footing rebar, wasting time working below grade (where a change in the weather can flood the workers out, or worse).
In your case how would that be done if the new footing is at a higher elevation than the existing footing?

Putting two footing close together but at different elevations is risky, some of the upper footings load will be picked up by the lower footing.

RE: When to tie-in to existing foundation

In my opinion I agree with SlideRule... When I have to have foundations close, I still generally don't tie them, But if bearing on granular soil I will inject the bearing soils below both the edge of the new and existing (if mat) to a common depth/elevation with chemical polyurethane grout/soil binder to increase bearing and control differential. Have had great success.

If elastic silt or clay, bunch of other factors come into play.

RE: When to tie-in to existing foundation

My past rule ;
Provide separate foundations for each separate equipment , skid etc.

I preferred to use polystyrene board when the distance is not allow the use of side form.

But be careful for the bottom levels . If the existing foundation bottom level deeper than 12'' no problem..

Don't underestimate a nail. A nail saves a horseshoe, a horseshoe saves a horse, a horse saves a commander, a commander saves an army, an army saves a whole country.. GENGHIS KHAN

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