Tolerances on foundation trenches

[acurachris]

Hi

I have had the trenches dug (825mm deep, 450mm wide) for my extension and the piling guy is coming tomorrow to lay the piles and ring beam. The trenches are in the right positions although a marking out line shows they are not 100% square.

The ground is really poor so the dig out left parts of the trenches wider than 450mm and some parts 500-550mm.

The depth varies in parts of the trench too, from 810mm to 870mm.

Basically, i'll get the walls square within the trenches although some will be closer to the trench edge than others (so the wall will take a slight diagonal along one trench from end to end; does that make sense?). One trench over by 75mm at one end. My logic would say it's near impossible to get a digger to be mm perfect, so am i being pedantic? Is the height or squareness a problem?

I'm constantly worrying about this foundation!

Cheers, Chris

 

[John D v2.0]

With ours it seemed to be the verticalness and flat bottomness that was the important bit, the position was all over the place. the se said it's fine to build on the edges as domestic the loads are not much. The pad under the column was the most important bit too although won't apply to youy and, he had to do specific calculations to prove it was correct.

 

[Notch7]

I hadnt realised you could dig the ring beam before the pilings.

I realise it cant be dug by machine after, but I just wondered if the ground will support the piling machine -assuming its a driven pile.

I doubt a ring beam needs to be super accurate. I presume its have reinforcing as well.

The trench bottom of a trench foundation needs to reasonably flat and level, and with loose crumb scraped out. Im not sure the bottom matters with piles -because the load will be supported on the piles not the ring beam bearing on the ground.

 

[^woody^]

8.2.5. Lol

Pass the feeler gauge.

 

[KenGMac]

acurachris, good evening.

It is a well known fact that in poor ground trench excavations will go over size by width and depth.

With all due respect, the over dig, horizontally and vertically is very small, relative to the overall dimentions of the excavation, you should try to excavate in dry sand??? or see poor soil after a torrential downpour / thunderstorm???

In general terms, Building tolerance is graded, from very wide in excavations, getting smaller with setting out and building brick to timber finishes such as miter joints in Architraves, or joints in Worktops, it is a sort of a grades scale.

The Foundation specialist you are waiting for will have their own set of tolerances.

Hope this assists?

Ken

 

[KenGMac]

Almost at 23,000 Woodie,

Dare you to go for one more here???

Cheers on the number.

ken

 

[acurachris]

I hadnt realised you could dig the ring beam before the pilings.

The piling guy said that's what he needed. 450mm width and depth as whatever we needed for services etc.

assuming its a driven pile

I don't actually know! I'm pretty sure it's driven piles.

I presume its have reinforcing as well

Yes I believe so.

With ours it seemed to be the verticalness and flat bottomness that was the important bit

What type of foundation is yours?

Everyone just keeps saying, it'll be fine, stop worrying, but i can't help it, this is one part of the build that can't go wrong!

 

[acurachris]

8.2.5. Lol

Pass the feeler gauge.

I was going for 827.5mm but i thought that was being too picky! Your comment made me laugh, i suppose i didn't give much thought to the exactness i was expecting.

With all due respect, the over dig, horizontally and vertically is very small, relative to the overall dimentions of the excavation

If it's to be expected then i'm happy. I don't know if the piling guy will just pile in what's there or whether he'll check everything first, i'd hope the latter but i don't want to sound condescending to him by saying "you're going to check it's all okay first aren't you?".

I've been on edge the last week about these foundations. I think my house is on an old industrial or dumping site. The stuff that's come out of the ground is not good! It stinks too (building control want 2 x 1200 methane barriers!).