Overlapping Stressing due to Building Addition

[geomane]

Is there a rule-of-thumb for distance between footings when it comes to constructing a new footing adjacent to an existing footing (i.e. critical zone).

My understanding is that I can calculate the increase in stress between the two footings caused by the addition of a new footing and its accompanying load by using the Boussinesq's solution (or similar method). What method do you guy's use?

If the existing footings were designed using an allowable bearing pressure of 2,000 psf and the base of footing was 2'4" below existing grades and projects 3" from the face of the building, what route would you go in recommending the depth of the new footings and the minimum clearance between the base of the existing footing and new footing, without over-stressing the soils supporting the existing footings?

Site Specific Data:
-Blow counts in upper 15 feet are 12 bpf.
-The soils are poorly graded fine sands with silt [SP-SM] down to the boring termination depth of 20 feet below existing grades.
-The groundwater table was encountered at 20 feet below existing grades.
-The new footings will be designed based on a bearing pressure of 2,000 psf.
-Column loads are less than 50 kips.

Thanks,

James

 

[BigH]

As a quick approximation (and assuming that you are not going to be "undercutting" the existing footing by putting it deeper) - draw a line at 2V:1H from the inside face of the existing footing (towards you new footing) - put on your new footing and do the same? Are they strip footings? or square/rectangular footings (L/B < 4)?

I would see where they intersect - what I did years ago was then draw the Schmertmann triangle for settlement calculations and then see where the intersection point is on the "triangle" - this will tell you what "order of magnitude" of influence the new footing would have on the old with respect to settlement. - very quick and dirty for green light, red light or amber . . . schematic concept is attached; if the intersection is way down in the tail area, very little influence and footings would be okay - if higher as shown, you may need to space the footings further apart. (See text books for how to do the Schmertmann technique).

Anyway - this is one way to look at it - quicker than doing Bousinesq contours, etc (which I have done in the past).

 

[geomane]

BigH,

The footings will be square.

Thanks for the response!

-James