Clarification on trench foundation depth - NHBC UK 1

[liam1369]

Good Evening all,

I am a relatively new engineer and would like some clarification and guidance on a query I have in relation to NHBC guidance for trench foundation in cohesive clay soils.

Upon review of documentation and also discussing with other engineers. The terms 'minimum 1.0m deep trench foundations' often comes up.

However,

Is this stating the actual depth of the concrete trenches themselves need to be at least 1.0m deep, or does it simply mean 1.0m deep below existing ground level to underside of foundation? The top of new foundations are generally 450-600mm below ground level, so minimum 1000mm deep trenches will mean the foundation depth is already minimum 1450-1600mm below ground level.

To confuse matters anymore, I have read some references and heard from other engineers, that it needs to be at least 1000mm penetrating into the clay/cohesive soil. So if we have 0.50m top soil and 0.50m of sand/terrace deposits and then hit clay, it needs to go at least 1.0m into that. Whereas, in some references I have seen at least 200mm penetration mentioned.

If possible, is anyone able to provide clear guidance on the typical requirements in a clay soil and in layman terms, describe the actual methodology.

I have attached a sketch aiding with the explanation.

I appreciate any guidance and insight into this subject.

Many thanks,

Liam

 

[GeorgeTheCivilEngineer]

Formation depth (underside of the concrete) is a minimum of 0.75m for low plasticity clay, 0.9m for medium and and 1.0m for high. This is measured from existing ground level.

This is suitable only for flat greenfield type conditions... overlying made ground and varying strata will affect that. Trees also require foundations get made deeper due to their water demand affecting clay shrinkage.

I've never seen that depth be defined as starting in the subsoil strata. A specification of a minimum embedment of 150-300mm into the underlying subsoil is typical.

A trench depth of 2.5m or deeper becomes difficult to construct and piles are likely more economic.

Trench fill over 1.5m deep require clay board or other compressible fill measures to protect the footing.

I'd advise having a look at BS 8103 which much of the Building Regulations / NHBC guidance is based on. The BRE DG 298 is the summary of the research which this is all based on.


The topic of foundation depth is something of a black art, which is influenced as much by insurance company claims in extreme hot weather as by sound engineering. The reason for this minimum depths is because some clays becomes dessicated or heaves depending on water content. Below a certain depth the water content won't alter even in extreme seasonal conditions. What the research appears to have found is that it is the overall depth which matters, since overlying soil will prevent seasonal changes in the clay just as well. Roots tend to only exist in the top 500mm of soil, but their influence stretches somewhat deeper. (I'm somewhat skeptical of the tree influence depth diagrams but I generally follow the outcome of them.)

Working with old buildings which may only have 150mm of foundation - which are perfectly serviceable - teaches you the real role of a foundation. Which is, to transfer load to somewhere competent with an acceptable amount of movement. Modern buildings with brittle cement masonry require much deeper foundations than a lime mortar house because they cannot cope with barely any movement in the subsoil. They may require a stronger strata due to larger spans.

 

[liam1369]

Hi George,

Thank you for the response. That has definitely cleared up a lot of thoughts I had. Especially clarification it is from existing ground level presuming green field conditions.

Agree, if made ground / previous industrial nature. Will change this.

Thanks again,

Liam

 

[BigH]

You also need to consider frost penetration - 1.2 m in the Toronto area; up to 3 to 4 m in more northern Ontario's areas. Actual depth will be based on the "normal" depths for an area (see town/city requirements) - but also can take into account the soil conditions - had a job in northern Ontario that would have required greater than 3 m depth for frost protection but we were able to found at about 1.2 m depth because the clean sand foundation was not frost susceptible and the water table was low.

 

[liam1369]

Hi BigH, isn't frost protection more for the start of the top of the foundation? - In the UK it is general 450mm below to the top/start of the trench. My query was more in relation to the depth of the underside of foundation and the depth of the actual concrete trench itself.

 

[BigH]

The depth of the footing to be below the frost line - it is measured from the ground surface to the underside of the footing.