foundation's: excavation/ concrete depth

[ashworth108]

I had a brief chat with a building inspector who advised me that the area where I live the normal foundation depth is 1 Meter. Is this the actual depth of concrete of just excavation depth? Thanks[/list]

 

[jeds]

Excavation depth. Technically concrete could be as little as 150mm thick for a 600 wide foundation but I'd go 200 or 225mm.

 

[theprinceofdarkness]

Do a search on "foundation blocks", great big blocks to quickly bring the top of your concrete up just to below ground level when you can start laying the bricks. So from your finished floor level or brick courses, work out the courses of blocks and bricks plus some concrete, to get the concrete base at one metre depth.
Frank

 

[ashworth108]

Thanks for that Frank. I had a chat with the building inspector- he now wants me to got 2.1 meters deep as I have a blossom tree near me. Seems too much? I take it the foundation blocks are the thickness of 2 blocks + cavity. ?

 

[theprinceofdarkness]

2.1m !!!, you will need shuttering and props for that depth. Yes, the blocks are about 300mm square, can't remember and I am not digging them up.
Frank

 

[stuart45]

Are you on shrinkable clay?
Trench fill is probably the best option.

 

[angelboy]

I had to go 2.5m deep along one 8m trench for a garage!!! Might as well dig out a basement!

 

[ashworth108]

Hi I am not sure if it's shrinkable clay? It is on a slight slope though! I am very worried about going that deep. I take it ply wood with mini acros will be ok to support the excavation ?

 

[RonnyRaygun]

If it isn't shrinkable clay there would be no reason to go to even 1m deep, unless sewerage pipes or similar are nearby so assuming the BCO knows the soil type we can assume it is shrinkable clay.

These depths can be checked by downloading the NHBC Technical Guidance, section 4.2, Building Near Trees.

Today's constructions are not designed to move as buildings did in the past, so foundations have to be deep enough to prevent local tree roots going deeper than the base of the foundations. This means the clay below the foundation's base will not suffer from heave or dessication and should not move.

Be aware that any foundation greater than 1.5m in depth will require Claymaster to the side of the foundation - the thickness is based on whether the soil is low, medium, or high shrinkability.

 

[ashworth108]

Thanks for that information Ronny. What do you mean by clay master?

 

[jeds]

Hi I am not sure if it's shrinkable clay? It is on a slight slope though! I am very worried about going that deep. I take it ply wood with mini acros will be ok to support the excavation ?

You need to do a proper investigation. It's a monumental waste of time and money digging a 2m+ trench unless ground conditions require. My advice is do a trial dig and take a sample of clay from about a metre down. Have it tested for 'volume change potential'. The difference between low and high change can add 1.5m+ to your trench depth. Then you need to identify the tree more accurately. The difference between species and sub-species can also make 1.0 to 1.5m difference in trench depth. Then measure the distance from tree to foundation.

When you have all that you can determine an appropriate trench depth.

PS. If you do end up with a 2.5m deep trench don't whatever you do climb into it to try and prop it or anything. Even good looking trenches can go without a seconds notice and if it goes while you are in there nobody will be able to get you out in time. Just have concrete ordered for the same day get it in as soon as possible.

PPS. Claymaster is a compressible board material that you lay against the inner foundation wall before you pour. It allows the clay to expand against the concrete later without pushing it outwards.