Soil testing for foundations near trees

[Alex Phillips]

We’re planning an extension and have trees nearby. The species have been identified and distances calculated, so we just need to know the VCP/PI of the ground (medium would put us at around 1.7m, high would be 2.5m+/engineered). I’ve looked into getting the professionals in, and have a few quotes to carry out the work, but really don’t want to spend that kind of money (2-3k), especially when it may all be for nothing. I’m wondering if there’s a way to get the answers I need without going to such extremes, so have been looking at taking samples and sending them away myself. I’ve already done a bit of digging and it’s clay with gravel at around a meter deep. I’m yet to check existing foundations but the house is over 50 years old so I’m expecting shallow strip?

I’ve spoken with BC who have advised a depth of 1.7m for the closest/most problematic tree (an Ash) but won’t confirm how they came to that figure. I can only presume that the calculation assumed medium VCP as that tallies with LABC and NHBC. The SE obviously wants confirmation from BC before basing their calcs on the same, but BC obviously don’t want to be held to anything so want the SE to make the final call. Without confirmation from BC, the SE will have to work to worst case/high VCP. In that case, the SE has suggested a raft foundation at a depth of 1.25m, which seems like a reasonable way to go if needed as access isn’t really good enough for a huge machine to get to the required depth for trench fill, and I can take on the laborious aspects of the work needed to keep costs down. The two figures that jump out there are 1.25 and 1.7m so perhaps I already I have my answer…

P.S. Any thoughts on raft foundations also welcome. I’ve read/heard mixed feelings on them, particularly when used around trees, although on the whole ok.

 

[^woody^]

It is up to your engineer to design and calculate things and not rely on the building inspector to tell him. An inspector can't argue or dispute the word of a qualified engineer.

You need to be discussing the pros and cons of the three foundation type options, including costs and longer term movement potential of a raft.

 

[Alex Phillips]

It is up to your engineer to design and calculate things and not rely on the building inspector to tell him. An inspector can't argue or dispute the word of a qualified engineer.

You need to be discussing the pros and cons of the three foundation type options, including costs and longer term movement potential of a raft.

Thanks. The SE wanted confirmation from BC as he assumed they know something about the ground conditions that we don't... if they do they aren't saying. I've read on here that BC may/will know these things. Anyway, my question was really about testing the soil myself and how exactly that should be done. BC said some SEs will collect samples from a pre-dug hole and have it tested, perhaps that's an option.

 

[stuart45]

LABC have got their own calculator now, which is better than the old NHBC one. It means that if the clay just falls in the high shrinkable group the foundations depth will allow for that. Last time I got a soil test 10 years ago it was £40 from a place in Dorchester.

 

[^woody^]

Thanks. The SE wanted confirmation from BC as he assumed they know something about the ground conditions that we don't... if they do they aren't saying. I've read on here that BC may/will know these things. Anyway, my question was really about testing the soil myself and how exactly that should be done. BC said some SEs will collect samples from a pre-dug hole and have it tested, perhaps that's an option.

Your engineer should be arranging the ground test and interpreting the results, and then reporting to you on the options and not relying on or waiting for any information that the council may have, but can't disclose to you, because they have most likely not actually tested the bit of ground in your back garden.

 

[RonnyRaygun]

Agree with the above, but if the cost of the extra excavation and concrete based on high volume change potential is significantly less than the cost of a soils investigation, then just dig down to the required depth for high VCP.

Building control do usually have knowledge of local ground conditions and will usually share that knowledge with your engineer, but at the very least your engineer should be looking at the BGS Geology of Britain Viewer
https://mapapps.bgs.ac.uk/geologyofbritain/home.html
This should at least give a good idea of whether or not the local soils are even shrinkable - if not trees are not an issue.

A 2 or 3m borehole and soils test indicating VCP shouldn't be that expensive. Would be worth getting a desiccation test done too.
https://k4soils.com/soils-testing for an indication of current prices.