Extending Existing Footings

[SIPslidingaway]

Hi all,

 

Currently have footings and out of ground foundations on site for a traditional block and brick house. (2012)

 

I am looking to build a timber frame home on the site. Due to the restrictions on site (TPO's Sewerage) I am having to build in the same spot. My foot print will be slightly larger but will line up on 2 walls of the previous plot foundations.

 

I be pecking up all but the footings of the previous plot and using that for hard core and gravel.

 

My question is.......to extend these footings and attach what i will need for my larger footprint, what do i need to do to satisfy building control?  

 

Can I drill rebar into the existing footings, secure and then pour my extended footing with that interface between the 2?

 

Please advise.

 

Edit: Insulated raft is not an option. Removing original footings ins not an option.

[George]

Dowels into the existing foundation would certainly do the job, might even be a bit over the top. 

 

I think BC will be looking for matching foundation depths, verifying what is there is suitable (esp if trees are close by - as indicated by the the TPOs) and any special features like steel post pad foundations. 

 

I suspect as it's a bit out of the ordinary and needing to verify existing footings, you'll need a Structural Engineer and/or geotechnical engineer to provide a report or opinion. 

[SIPslidingaway]

Cheers George.......just had a chat with a Geotech and he says its all possible. Many thanks

[saveasteading]

2 hours ago, George said:

Dowels into the existing foundation would certainly do the job, might even be a bit over the top. 

 

When footings are poured, there is often a 'stop end' to make a clean joint at the end of the load.

It isn't normal to put any dowel to link to the next pour. 

Therefore I don't think you need to either.

The loads are not complex, just down and perhaps some upwards for a light building in a hurricane.

These loads will spread across your joint.

Just make sure to clean the existing concrete, so that there are no gaps.

[George]

On 08/10/2021 at 17:25, saveasteading said:

 

When footings are poured, there is often a 'stop end' to make a clean joint at the end of the load.

It isn't normal to put any dowel to link to the next pour. 

Therefore I don't think you need to either.

The loads are not complex, just down and perhaps some upwards for a light building in a hurricane.

These loads will spread across your joint.

Just make sure to clean the existing concrete, so that there are no gaps.

My only concern would be if the existing footing have been previously loaded by masonry and have settled due to soil consolidation. Then casting new footings in virgin soil adjacent with timber frame loading, the future settlement behaviour would be different. Dowels would help minimise any differential settlement (which is the only type to be really concerned about). 

 

But yes, agree its likely ott. But you can pay for a lot of dowels for the price of a geotechnical report... 

 

Certainly wouldn't be done for new foundations, but dowels are quite common when pouring new next to old. 

Edited October 10, 2021 by George

[saveasteading]

28 minutes ago, George said:

settled due to soil consolidation.

Unlikely to be significant unless these are old, shallow footings.

In any case a dowel would be drilled in how far? 200mm? would spread a very small proportion of the new load onto the old footings, and I can't see what benefit that would bring.

In any case a tiny movement of the new structure would be better on the junction than at some random position.

Plus a drill hole horizontally into an old footing is a pain to do, down the bottom of the new trench (perhaps even under water), , and might weaken or break the old concrete.

 

 

35 minutes ago, George said:

dowels are quite common when pouring new next to old.

 

A dowel is fine, though if it makes everyone happier. No harm will be done. I reckon most of them go in a very short distance and the end gets thrown away. (I have seen this when the culprit has forgotten to hide all the ends)