Foundations - when does "original ground" become original?

[kandgmitchell]

Our plot is basically a bit of unused agricultural field. It's triangular in shape and probably got left over when the main rectangular body of the field was sold off. Over the years it became overgrown with a lot of self seeded trees and scrub. It slopes down to a drainage ditch.

 

When the landowner applied for outline planning they cleared many of the small self seeded trees central to the plot leaving the more mature trees around the edge. It then took them a good 5 years to gain outline approval and we now have full planning. As the site is near the medieval village church we had to have an archaeological excavation done. Whilst they found no artifacts they did see a "buried top soil layer which may be remains of a medieval ridge and furrow surface". This lay about 900mm down. The strata below this level looked the same as that above (it's a moderate plasticity silty clay)

 

So, there has been change in ground level - probably some attempt to terrace the ground and reduce the slope but it certainly isn't recent, given the tree cover that was cleared and from talking to locals.

 

Now, in one corner the foundations will be influenced by a retained tree to the extent of making the foundations 1.4m deep but NHBC Chapter 4.2 says for existing trees that is measured from original ground level. Well, if that is the extant ground then it's 1.4m but if that's the medieval layer then it's 2.3m - a bit of a difference!

 

Hence my query, at 1.4m I'd be some 300mm into the original sub-strata below the medieval soil but not technically 1.4m below it. Can a ground layer ever achieve the status of "original" for the purposes of Chapter 4.2 ?

[ToughButterCup]

I don't know the answer to your question. I think, while interesting, your question could be reformulated.

 

Can I suggest you have the ground surveyed - so you can get a good indication of what will be needed for the foundation design? Your ground sounds much like ours - possibly glacial till.   8 cores (vertical soil profiles) didn't cost all that much. The results were used by the piling company and the groundworkers.

 

A ground condition survey of some sort almost always reduces risk, and may be a requirement of Building Control and others.

[Alan Ambrose]

>>> 8 cores (vertical soil profiles) didn't cost all that much

 

Possible to give us a ballpark? TIA, Alan

[kandgmitchell]

Ah but those cores will not tell me much more than the 25m x 1.8 and 5m x 1.8 "trial holes" illustrated when they were looking for King Alfred.....

[ToughButterCup]

£1200 ish I think.

The key value is that other professionals in the build workflow (in our case at least) required the Groundsure report. It helped with the foundation design and the groundworks design.