Footing on top of existing

[civeng80]

Ive been working on a project with little nasty surprises along the way.
A retaining wall on a boundary with its footing.
I would welcome comments on the detail attached.
Soil properties stiff to very stiff qallow = 300kPa at least.
Footing of retaining wall competant (not sure of reinforcement).

 

[hokie66]

Should be fine structurally, but you would have to consider what would happen in the future. The other footing encroaches, probably without any easement allowing it to do so. What would happen if the other owner wanted to remove the boundary wall and fence and construct a building on the property line? I would like to see an agreement in place which would bind both owners, the agreement to pass in perpetuity to subsequent owners. If obtaining that agreement is not possible, you may need to resort to placing footings further inside your property, with rectifying beams to carry the new wall loading.

 

[civeng80]

Hokie
At the moment one owner has both titles. So no problems getting an agreement.
Maybe need an easement on the title if one of the blocks are sold.
Thanks.

 

[WARose]

I'd definitely connect the new (footing) to the old (via Hiltis)......at 300 mm thickness (I assume that number is in mm), I'd go with 2 layers of steel in the new.

 

[hokie66]

What would be your logic in connecting the footings? If I don't have a reason, I don't do it.

 

[WARose]

What would be your logic in connecting the footings? If I don't have a reason, I don't do it.

The lean mix concrete footing (being new) will settle at a different rate than the existing retaining wall footing. Such a situation is obviously undesirable as it could (among other things) cause some tilt in the panel (no pun intended) and uneven bearing. Another thing about this detail that bothers me is: no connection between the “new footing” that the tilt up panel sits on and the retaining wall footing nor the new lean mix footing. I would expect some type of re-bar to tie that together. The detail shows some stirrups in it…..but no connecting re-bar. The code mandates a minimum for vertical transfer. (Not to mention some for the horizontal loads that are likely present.)

 

[hokie66]

That's too many concerns for me. It is judgment, but I think what civeng80 has shown (in concept) is fine, with "stiff to very stiff " bearing material. As to "code", civeng80 is in Australia, so not working to the same code as you are.

 

[WARose]

I don’t know what the definition of “stiff’ would be here…..but even if it was sitting on solid rock: I can’t imagine having no re-bar (between elements) tying all that together. If that’s the plan (and I thought it wasn’t shown because it’s a detail in progress) that’s just nuts. Somebody asked me to seal that…..they wouldn’t like my reply.

 

[civeng80]

I wasn't going to tie into old footing. Why would the lean mix settle a different amount to the old footing? They both sit on the same clay foundation. Estimate about 5mm for 100kPa bearing pressure.

The tilt panel is dowelled into the footing its just not showed on this detail, also tied to the floor slab (not shown here)

 

[msquared48]

I have a problem with bearing on top of this short footing, in that with the new high load from the tilt panel, a load the fence footing has never seen, and considering the lack of transverse steel, not only in the existing footing, but also into the new lean mix you propose, I can see the new/existing footing joint below the tilt footing failing and the fence rotating inward toward the tilt panel, allowing the tilt footing to subside.

I think I would negotiate with the owner of the fence to replace it after the tilt panel footing is in place.

Mike McCann
MMC Engineering

 

[civeng80]

msquare48

A concern of mine was the footing sitting on top of the joint, thats why I posted.

 

[WARose]

Why would the lean mix settle a different amount to the old footing? They both sit on the same clay foundation.

They sit on the same clay, but the soil under the wall footing has gotten more compacted over time than the new lean mix footing likely has (unless the contractor did an out of this world job compacting). This thing looks like a potential problem without it all being tied together with reinforcement. (IMHO.)

 

[msquared48]

civeng80... And that's why I answered!

Mike McCann
MMC Engineering

 

[hokie66]

If the new footing is of adequate width bearing on the soil below, I can't see how the presence of concrete between the footing and the soil is a problem. Replace that concrete with virgin soil or compacted gravel, and you have no transverse bending capacity, so why would it help to tie the concrete elements together? To quote WARose, "that's just nuts". Differential settlement is also a nonissue...IMHO.

 

[civeng80]

The settlement of a square footing on the clay with a bearing pressure of 200kPA is estimated at about 5mm conservatively.
Infact it would probably be about 3mm.
The pressure on the old footing from the retaining wall which is only about 1000mm high is about 10kPa (includes the footing load). So the settlement of the old footing would be about very roughly 5/20mm. 0.25mm.

Insignificant, Again to quote WARose "thats just nut" Differential settlement really is insignificant..... IMHO.

 

[a2mfk]

Any soil testing done right next to the existing retaining wall? I would like to see some CPT data performed along the heel of the retaining wall.

How tall of a building, ie, load on the footing?

Though I see Hokie's points in theory, and it may indeed be fine, but there seems to be a lot of unknowns for me to stick my neck out if I am the EOR. You can bet if you have any settlement issues down the road and someone sees this detail, you will get questioned on it big time. I fall on the side of the fence with Mike/WArose on this one. I would be concerned if you have only a cold joint between the retaining wall footing and the new concrete and no tie in that you could get a hinge effect there, and then you would have some unintended high edge bearing pressure on the retaining wall footing and the lean concrete.

Mike- I am not sure if civeng's sketch was implying rebar or no rebar in the transverse direction. Are there drawings of this wall? I would anticipate them to be there, but.... I would feel good if there were transverse bars and you tied the two sections of concrete all together into one foundation.

Also, if you have good soil (you indicated stiff clay), micropiles (steel augers) may be a good solution here, then design a grade beam to span between them. If there are contractors in your area that do this they can be surprisingly inexpensive. Then you are working with knowns and not touching the retaining wall.

 

[WARose]

You can bet if you have any settlement issues down the road and someone sees this detail, you will get questioned on it big time.

As Gomer Pyle use to say: Shazam. And I also question that settlement number.....unless the Geotech is [red]guaranteeing[/red] that number (i.e. 5mm) in writing, I would take it just as what it is: an approximate guess. Every time I've tried to get those guys to give an absolute limit on settlement (i.e. foundations (for equipment) where settlement would be a big issue).....they've been hesitant to do so and told me to go to piles instead. Settlement isn't an exact science.

 

[hokie66]

Fellows, just look at the picture. Forget the retaining wall and the fence. Replace the old retaining wall footing and the new lean mix concrete with virgin stiff clay. If the bearing of civeng80's footing would have been OK on that, it will be OK on what he has shown. How does "hinge effect" come into play when bearing on soil?

 

[jayrod12]

I see the hinge effect because the existing footing will try to redistribute the load from the overlapping portion to it's whole existing base. The portion of the new footing that is not overlapping will apply a slightly higher stress to the soil then the load going through the existing footing. This will cause the soil below the unlapped portion to theoretically consolidate more and if there is no fixity between new and old footing it will cause the hinge point everyone is talking about.

You cannot pretend the existing footing doesn't exist. It is there already with slightly consolidated soil beneath its base. The soil beside the existing footing will not be consolidated the same amount and in fact may have been disturbed during the installation of said footing and therefore will not be as strong as the soil below the existing footing. I see issues all over the place with this proposal and would never try to partially rest a new building footing overtop of an existing retaining wall/fence footing.

 

[hokie66]

Gee, I thought I was conservative.