FOOTINGS 1.8m

[blake floyds]

Can any one tell me if this sounds right

i have doug my footings for a single story
I have doug down 1.2 m it is rock hard clay about 2oomm below the grass
the building inspector came round and told my misses we have to dig down to 1.8 m because there is a connifer tree 6m away

this sounds wrong to me

 

[Shytalkz]

He's right.

 

[wms]

last one we did like this Bco insisted on a barrier to prevent roots coming into contact with the footings 8x4ft sheets cost about £17.00 each trade some bco insist on 2.4 when trees are in close proximity.

conifer trees are the worst as a normal tree will have a root ball protruding just past the width of the widest point to obtain water ,conifers however are the monsters of trees roots go in search of water everywhere.

 

[blake floyds]

thanks


better get digging

 

[^woody^]

And what if this conifer tree was removed?

 

[Bowness72]

Sounds about right to me. The reason the clay is rock hard is probably because the conifer tree has taken moisture out of the soil which causes the clay to shrink. The new foundations need to be taken below the level where the roots will extract moisture otherwise there is a good chance you will get foundation movement due to the clay shrinking during dry weather and also when the clay rehydrates and expands again in wet weather.

 

[Shytalkz]

some bco insist on 2.4 when trees are in close proximity.

It's not an arbitrary figure plucked out of the air on a whim by them, you know: it's based on NHBC Standards Chapter 4.2 "Building near trees". Takes into account tree species, water demand, mature height, distance from foundations and heave potential of clay soil. In the absence of soils test results, most BCOs assume high shrinkage soil.

 

[Bowness72]

If the tree was removed there would still be a potential problem with clay heave, although this could be accomodated by waiting until after the soil had rehydrated if timescales allow

 

[^woody^]

Yes, but it removes the need to go down 1.8m to below the roots.

 

[Bowness72]

Not so sure about that. If he didnt go down below the level of root activity and the clay were to swell it could lift the foundations above

 

[^woody^]

OK then, a staged pollarding?

Ask the BCO what depth would be acceptable if the tree was not there. Then lay footings to this depth, with a commitment to gradually reduce the tree?

I'm just developing the topic

 

[Shytalkz]

Yes, but it removes the need to go down 1.8m to below the roots.

Rubbish. Removal of a tree or trees will result in gradual rehydration of the clay and consequent swelling, or heave.

If you remove a tree, it takes several years for the clay to sort itself out and return to comething approaching its normal moisture content. The amount of rehydration depends on the age of the tree and how near its ull development it was and the heave potential of the soil; clearly, if its a small tree, then the amount of rehydration is likely to be small.

In general terms, if you remove a tree to facilitate construction in clays, you determine foundation depths as if the tree was still there, but take heave precautions in addition: for foundations >1.5m depth, you will need compressible board on the outside of the foundations, thickness dependent on soil classification.

 

[^woody^]

IIRC, conifer roots are shallow and not wide reaching.

The extent for heave would depend on the clay permeability and plasticity, and other local factors.

Also, the tree variety and size/maturity would influence how removing it would effect the ground.

6m is a fair distance for a conifer, and unless this is a massive mature tree or fast growing juvenile/semi-mature, then the potential for heave is at the lower end of the scale. In which case, a normal footing with expansion board may well do.

There are a lot of factors missing from the OP post, so its not possible to determine what will or wont work in his particular situation

 

[GaryinAshby]

When my BCO came round to survey the site, she used a table of tree types, distance from the building etc, etc. She told me that I either dig down 1.8m if I left the conifers in or 1m if I removed them. However, as some of her other comments have left me scratching my head I now doubt or question anything she says......

 

[Shytalkz]

The extent for heave would depend on the clay permeability and plasticity, and other local factors.

Also, the tree variety and size/maturity would influence how removing it would effect the ground.

6m is a fair distance for a conifer, and unless this is a massive mature tree or fast growing juvenile/semi-mature, then the potential for heave is at the lower end of the scale. In which case, a normal footing with expansion board may well do.

Isn't that what I said?! 6m is well within the ZOI for conifers (at mature height); however, if it is just a weenie one, then, yes it is unlikely to have affected the soil. however, the soil he describes is clearly desiccated from a shallow depth, so I'm guessing that it's a bit more than fence height...