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How shallow can foundation be? Chad 12-22-2006
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Posted by Chad on December 22, 2006, 8:36 am
Hi

I started to build a dormer recently, its 235 sq m.
I observed the foundations being poured and about 36-40 metres of
concrete went into it.
The trenches are very wide in places because the foundations were dug
more than once (don't ask!).

I meaured the depth in a few places and which varied from over 3000 mm
in places to 2500 mm in others but worringly as low as 1750 mm on an
external wall corner. Steel mesh has gone in also. The ground is
without doubt solid and excellent and has been inspected by an
engineer. The foundations contain a number of steps.

Should I be concerned? It is obvious that this is well below the
official spec so I was a bit worried on seeing it. Have people seen
houses built to this spec which survived long term? Can mesh and width
compensate for shallowness? BTW there will be concrete slab floors
upstairs.

Thanks.


Posted by dpb on December 22, 2006, 10:18 am
Chad wrote:
...
> I started to build a dormer recently, its 235 sq m.
> I observed the foundations being poured and about 36-40 metres of
> concrete went into it.
> The trenches are very wide in places because the foundations were dug
> more than once (don't ask!).
>
> I meaured the depth in a few places and which varied from over 3000 mm
> in places to 2500 mm in others but worringly as low as 1750 mm on an
> external wall corner. Steel mesh has gone in also. The ground is
> without doubt solid and excellent and has been inspected by an
> engineer. The foundations contain a number of steps.
>
> Should I be concerned? It is obvious that this is well below the
> official spec so I was a bit worried on seeing it. Have people seen
> houses built to this spec which survived long term? Can mesh and width
> compensate for shallowness? BTW there will be concrete slab floors
> upstairs.

What was the "official spec"? This is almost 6-ft depth at the minimum
which would be well below frost-line at most any place. If the ground
was undisturbed and solid and seen and the excavation was inspected and
approved by an engineer it would seem adequate.

But, that's just a guess based only on the depth and your description
and a generality about frost depths; it would require far more to make
anything sort of realistic assessment.

Why not ask the architect/engineer/inspector?


Posted by Chad on December 23, 2006, 4:42 am
Official spec
300-400 mm concrete, no steel.
bottom of concreete 600 mm below finished ground level.

Engineer was happy with trenches but does not inspect concrete during
or after pour.

dpb wrote:

> What was the "official spec"? This is almost 6-ft depth at the minimum
> which would be well below frost-line at most any place. If the ground
> was undisturbed and solid and seen and the excavation was inspected and
> approved by an engineer it would seem adequate.
>
> But, that's just a guess based only on the depth and your description
> and a generality about frost depths; it would require far more to make
> anything sort of realistic assessment.
>
> Why not ask the architect/engineer/inspector?


Posted by dpb on December 26, 2006, 7:39 pm

Chad wrote:
> Official spec
> 300-400 mm concrete, no steel.
> bottom of concreete 600 mm below finished ground level.
>
> Engineer was happy with trenches but does not inspect concrete during
> or after pour.

Well, one would assume he would have observed the depth and compared to
the spec and that the concrete would then be at the bottom of the
trench. :) Consequently, if he approved the trench for pouring, it
would seem as though the depth was considered adequate.

You quoted a "minimum" of 1750 in your original post which is nearly
three times the depth of the minimum "below grade" spec and over 4X the
actual required concrete. W/O seeing it, surely sounds as though it
well exceeds the design.


Posted by Chad on December 28, 2006, 5:27 am
My mistake.
I should have said 175 mm, not 1750 mm.
Trench was deep enough alright but in this trouble spot the concrete is
7 inches short of the top.


dpb wrote:
> Chad wrote:
> > Official spec
> > 300-400 mm concrete, no steel.
> > bottom of concreete 600 mm below finished ground level.
> >
> > Engineer was happy with trenches but does not inspect concrete during
> > or after pour.
>
> Well, one would assume he would have observed the depth and compared to
> the spec and that the concrete would then be at the bottom of the
> trench. :) Consequently, if he approved the trench for pouring, it
> would seem as though the depth was considered adequate.
>
> You quoted a "minimum" of 1750 in your original post which is nearly
> three times the depth of the minimum "below grade" spec and over 4X the
> actual required concrete. W/O seeing it, surely sounds as though it
> well exceeds the design.


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