New foundations next to existing ones 2

[nivoo_boss]

Hey everyone!

So I'm designing this new building with quite heavily loaded RC columns (~800 kN design load) and the foundations are going to be placed close to the pre-existing ones that are also RC columns from the Soviet era from the 70s. I work with Eurocodes. Are there any methods that can help me determining the effect of the pre-existing columns' loads on the new foundations bearing capacity?

EDIT: The foundations are pads and not piles.

 

[BigH]

Outside of using a computer programme of which I am sure that there are, you could use elastic methods to provide the existing stress levels in the existing foundations and then see what added stresses will occur when you add the new foundation. You can find this in Poulos and Davis' "Elastic Solutions for Soil and Rock Mechanics" - google the book name and you will find a URL that permits a free download of the book.

Another method I tried once was to use Schmertmann's method and determine the overlap and can then compute the settlements - and this can be done at several Sections between the foundations. I checked this against a very old paper (this was years ago too) that determined the effects of two foundations adjacent to each other - and things with Schmertman worked out well. Sorry but I can't remember the authors of the old paper.

 

[KootK]

This question might be best moved to either the Geotechnical forum or, at the least, to the foundation forum. There, your odds of receiving input form dirt wizards such as BigH will be improved. My thoughts on the matter, assuming that your allowable bearing pressures are settlement governed, as I expect:

1) The presence of the existing footings may actually improve matters for the new footings. I'd expect the long standing stresses applied to the area from the existing footings to sort of consolidate the soils in that area.

2) Your larger concern is probably the effect that the new footing will have on the settlement of the existing building. To that end, common foundation approaches include:

a) Using generously sized strap footings for the new construction.

b) Tying the new and existing foundations together to help with differential settlement.

 

[MSUK90]

KootK, how do we achieve option 2b suggested by you at site?
And wouldn't tying the new footing to the old one will overload the old footings?

 

[nivoo_boss]

"KootK, how do we achieve option 2b suggested by you at site?"

I would anchor rebars to the existing one with injection mortar for example.

 

[kostast88]

The answer is: it depends on the ground conditions.

Since you said no piles, I imagine you have granular soil, with little or no clay.
I also imagine you know where the groundwater is and you know it's effect on the bearing of the footings.

You can use the methods BigH proposed, calculating the settlements for the overload, say 2xB below your footing. Either method is acceptable and commonly used.

As Kootk proposed, generous footing area would be the safest bet. You will have an easier time avoiding the problem, rather than number crunching it. We're talking about soil at the end of the day.

I would personally stay away from tying the two footings together. It sounds like you don't know enough about the existing building -until the moment you excavate the footings. Too risky in my opinion. Could work for small loads, but yours sound big.

 

[KootK]

KootK, how do we achieve option 2b suggested by you at site?

It depends on a bunch of details of the new and existing construction that I'm not privy to but, similar to nivo_boss's suggestion, often this takes the form of rebar dowelling.

If your root concern is that it may well be impossible to tie the new and existing foundations together to transfer the full, factored, differential load that may occur between them then I quite agree. My detailed response to that would be:

1) Franky, such convincing interconnection is rarely even possible with lightly loaded foundations if one is honest with themselves about the realities of load eccentricity and anchorage design, often involving shear directed towards a free edge.

2) I have been assuming that the two buildings in question here will be part of a common facility with passageways connecting the two. Where that is not true, I would rarely connect the new and existing foundations. Where that is true, one of the more dire, practical concerns with differential settlement is differential floor movement at the interfaces between the two buildings. It is for this reason that I, and many before me, choose to create some manner of interconnection between new and existing structures even when transferring the full differential load from one side to the other is impossible. Often, the interconnection will help to prevent serviceability issues from arising during the service life of the structure. And, also often, the consequences of the interconnection failing are not not, themselves, any more dire than the development of the differential movement issues at the interfaces.

3) One of the primary purposes for installing grade beams in piled foundation systems is to help iron out differential settlement between the piles. For that purpose, we usually provide grade beams designed to transfer a very small share of the maximum pile load between piles. That, in acknowledgment that one doesn't necessarily need to transfer a huge proportion of the total load between piles in order to positively impact the differential settlement performance. I see the pile and grade beam business as being analogous -- albeit imperfectly so -- to the situation of new footings installed against adjacent footings.

And wouldn't tying the new footing to the old one will overload the old footings?

Possibly, however:

4) With capable enough interconnection, the new and existing buildings will share their combined loads among their combined footings which, hopefully, yields a satisfactory result for both.

5) Out of necessity, foundation design usually winds up being an immensely pragmatic exercise. I find that, if I restrict myself to avoiding every possible, on paper issue, nothing gets done. So I choose to focus my efforts primarily on the things that I really and truly believe may cause real world future problems.

6) In a sense, the new footings may overload the existing footings even if the two are not interconnected. That, because the new and existing footings are likely to share a common stress field below their combined area. But, sure, some thought certainly ought to be given to the foundation construction above the soil interface and whether or not new loads imposed there may cause problems. That's the "engineering" part of the exercise after all.

7) For loads of this scale, I would insist on having the assistance of a geotechnical engineer on the project and would tailor my solutions to suit their recommendations.

 

[MSUK90]

niboo_boss, can you draw a section between the old and proposed foundation as per your design proposal?
I want to have a look..
KootK, thanks for this detailed explanation. Personally, I always try to look for some solution which doesn't have any connection to the existing structure; don't know why but doesn't feel GOOD.

 

[KootK]

KootK, thanks for this detailed explanation. Personally, I always try to look for some solution which doesn't have any connection to the existing structure; don't know why but doesn't feel GOOD.

Yeah, one's preferences in that regard seem to have a good deal to do with their personal philosophy of things and how they were "raised up" as engineers. I try to tie things together as much as I can unless I can think of a good reason not to (seismic, acoustic, blah...). I'm usually trying to avoid complex building envelope details, differential movement problems and, straight up, the cost of functional movement joints. Clearly, you are at the other end of the spectrum.

 

[nivoo_boss]

"niboo_boss, can you draw a section between the old and proposed foundation as per your design proposal?
I want to have a look.."

I haven't designed anything yet. But the situation is something like in the link below. So a new building is built right next to the old one and they remain separated by the pre-existing brick wall. I do not know the depth of the existing foundations. Right now the new foundation is just below the floor (the existing ones are probably about 1,5 m below that). One idea I had was to excavate until the base of the existing pads and cast the new foundation at the same level. Since the columns are prefab and are already cast at that point in time, the new pads would have like an in-situ column part in them that would be as high as needed.

https://imgur.com/dBdvo7g