Pad footing excavation and existing footing

[NicB5858]

Hi, we’re soon to commence a renovation and I dug one of the two pad footing holes for structural steel (in preparation for Buidler to pour footing etc.), adjacent to an existing strip footing. The footing pour has been delayed (weeks) - is there any risk of undermining the existing strip footings by leaving the hole as is? It was a 550 deep excavation in one place, and didn’t go under the existing footing. Thanks

 

[NicB5858]

Some further context

- the footing holes / works have been fully engineered
- I am performing non structural demolition, and digging the required 2 footing holes
- I have commenced the work I committed to doing before the builder commences - simply, digging a footing hole per the engineering specification
- the builder will validate, building surveyor inspect, prior to concrete pour

My concern is that in a typical scenario, the duration between digging the hole, inspection and concrete would be substantially shorter than what i’ve done - am I now at risk of the existing strip footing being undermined? Only questioning as we’ve dug right next to it, per the proposed location of the new pad, at a depth lower than the existing, again per the engineering.. Should I be filing it back in with dirt, compacting and waiting until the day it’s actually required? Once the concrete is in, it’s not an issue, the concern is the extended period prior.

Thanks

 

[Sheer Force Engineer]

Hi NicB5858, could you just give your friendly Structural Engineer a quick call and ask? It would be a pretty straight forward answer if he/she is already familiar with the design...

In my opinion, which is restricted to knowledge based only on your photo and part-plan, I have the following thought bubbles you can consider:

- Time dependency may not be an issue if you are on sandy soil or rock. If you dug the hole you should know... are you on either? If its clay, there may be an argument that ongoing movement may occur due to the length of time the excavation is open but looking at the arrangement I'm not sure it would be of much concern...
- Are their openings in the brick walls on either side of the excavation? In reality this portion of strip footing is most likely not experiencing much load as your brick walls are adding quite a bit of stiffness and in worse case scenario will be cantilevering in a brick arch kind of way (cantilever arch action because there is a kink in your wall)
- Why is the new pad lower than the existing strip footings? Is this simply because of founding depth? Does the pad need to be structurally that deep? If you are trying to hit a particular founding depth and are concerned about the open excavation you can pour some blinding concrete in the hole to match the founding depth of the existing adjacent strip footings. You will need to confirm this with your Structural Engineer though as he/she knows what that pad is doing better than I!




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