The laying of strip foundations

[trevorbayliss]

Can strip foundations be laid in stages or do they all have to go in one fill ?

I ask because i have a small issue with a foundation i will be doing on my single story extension.

It is the foundation next to my neighbours 1.8m brick wall.

I will be building about 200mm away from this wall, so the foundation will be touching this walls foundation i imagine (if i go to the same depth which is currently 700mm as done a trial pit).

I then have the foundation at the front of the extension to do and the return on the other side, plus a strip for a block wall dividing up the extension into the two rooms.

If the foundations can be cast separately then all is good as i can do the bulk of the foundation one day and then the following few days i can get the foundation in for the wall next to the neighbour wall.

ALSO is it ok to take out the foundation next to this wall in one go ? I have been told it might be best to do it in stages. So i leave the third in the middle, but excavate the side 1/3rds. Cast these and then a few days later dig out the middle and cast this.

Any help with this appreciated.

 

[John D v2.0]

Your SE should really advise, but in essence you need to connect the different pours with rebar if you can't do in one go.
If you're excavating lower then next doors foundations, you should have already served them with notice of how you plan to protect their wall

 

[trevorbayliss]

IF it is not lower, and i don't think it will be, does this mean i can just dig out this trench in one go and then pour the concrete ?

 

[John D v2.0]

Again your SE should advise, but as long as you don't leave it open for any length of time, it is a lot less risky then going below the foundations.

 

[^woody^]

You can excavate that distance next to the wall in one, as long as you dont go below the neighbours foundations.

You can join two foundations strips using re-bar or suchlike, or a stepped arrangement. For deeper strips this is more involved. You don't really want to have a joint right on a corner, but return the strip around a corner say about 500mm or so minimum from the internal side.

 

[^woody^]

Again your SE should advise

Who has a SE for strip foundations?

 

[John D v2.0]

Who has a SE for strip foundations?

Dunno, we just had a general SE who just overlaid all the details on the architect's drawings, then we printed the lot and served it on the neighbours.
I assumed that was standard but maybe not. The builders did seem irritated that he asked for such deep foundations so maybe they normally work it out themselves?

 

[trevorbayliss]

Not using a SE, will work with BC when he says what depth i need to go to.

I have a pic of the trial pit which shows the footings for the wall. Only a shallow concrete strip for the wall.

So i need to dig out all the foundations and leave this section till last. Try and time it so i dig it out the day before the concrete comes.

 

[wessex101]

Didn't we deal with this a few months ago?

I seem to recall at the time Woody was dead set against joins in the foundation. Don't say I have managed to convince him!

 

[^woody^]

I change my mind as much as my pants. At least once a month.

 

[trevorbayliss]

Yes it was, but i think i have my answers now. Don't need to do it in stages. Digging out will be ok. Prob looking like i will be digging a little further away from the wall as well. So maybe 200mm between the foundations now.

 

[wessex101]

700mm is rather shallow for an extension foundation. What is the ground like? From the photos it still looks like topsoil.

 

[trevorbayliss]

We are on a sandy soil according to BC. The wall has this 150mm concrete foundation and then it is built up in the brickwork to ground height. From the bottom of the foundation to ground it is 800mm. Concrete footing maybe 200mm deep i suppose.

I just hope i don't have to go lower than the foundation.

 

[wessex101]

Surely that was the whole point of digging the trial hole. To discover the depth of the neighbour's foundation and to see what the ground was like to see how deep your foundation will have to go? Is it a firm sandy sub-soil? It looks very black and top soily to me.

Just hoping that your foundation will be no more than 800mm is rather short sighted. As the american military say....Fail to plan.....Plan to fail.

 

[trevorbayliss]

Well i plan on meeting the BC guy before anything starts to run lots of stuff by him. I can also dig out the other foundations not affecting this wall and that will give a better idea on the depth to go to.

If i does go deeper than the walls foundation i am a bit lost on what to do in that case. Maybe it will be as the BC guy initially stated and do it in thirds.

Another option i have read about is to do a wider foundation strip, but less deep. Anyone heard about this. I think you can also put in rebar lengths to strengthen it all up. I am doing it all so costs will be low. A bit of rebar is not a lot really, especially if i can use a shallower foundation.

The soil is good quality in this area. Not clay, but more on the sandy side of things. The land used to be a horticultural nursery in 1900.