Minimum distance between two isolated footings 2

There's no minimum distance between isolated footings. Mat footings exist, which are continuous. The only reason to separate isolated footings is if they're at different elevations, since one can surcharge another or create soil slope instability. It doesn't seem like the case here.

Since they're so close, you can probably do a combined footing.

Might give an idea.

milkshakelake, I too had the same concept that if they are in the same leve/ elevation at the underside of footing then the minimum distance shouldn't really matter, however I went through the above-posted forum and find something about overlapping stress of the soil when they are closely placed. So, how do you consider that and your opinion on it please.

Thank you

The soil pressure under a footing is a "bulb":



Although pressure deceases with depth, soil pressure bulbs can overlap, overloading soil.

A simplified, traditional way to estimate if overlapping pressure creates a problem is shown below:



Here is a basic example for the OP's 3.90m x 3.45m footing.

1) Assume a depth of 2m.

2) At that depth, soil pressure is estimated to be:


3) Do the same for the second footing. If the pressure areas overlap, add the two pressures.

4) Where the pressure areas overlap, the total of the two values should be < allowable.

@palk7 EIT I didn't consider that. Sorry, I don't know much about it. It's always good to have assumptions challenged. Based on that post though, a combined footing would help the problem because OP won't have two footings that can tilt towards each other.

In my experience:

1) There is no hard minimum for the spacing between footings. Hence the raft foundation logic.

2) Depending on the soil properties, footings placed close together may need to be designed to a lower bearing stress in order to limit settlement equivalently. That, for the reasons well elucidated by SRE above.

3) This is information that is often included in geotechnical reports. Use XXX PSF for footings up to this big and YYY PSF for footings bigger than that... Usually YYY < XXX.

The bulb interaction stuff tends to be less influential in locations like Manhattan where you're generally bearing on bedrock.