Recommended foundation method for Clay soil nr trees

[JD44027]

Hello,

Looking into building a house, this would be located on what is currently an old stretch of road (still tarmacked). The soil around is solid clay from about 3-600mm down, plus there are several mature oak trees within 4m of what will be the foundation. The site is level and sheltered, little-no risk of flooding/ground movement as such.

 

The house itself would be pretty simple rectangle shape, chalet bungalow. No particular stand out structural or design elements.

 

Any advice as to a recommended type of foundation to deal with this, anyone had similar situations? Guessing with the existing road surface the ground beneath would have been worked on at some stage so may be more stable?

 

I believe the next nearest house was built on a raft type foundation in the 90s although that is further away from trees.

 

This would be the first project of this type i've worked on so a bit clueless...

 

 

Edited June 10, 2021 by JD44027

[Thedreamer]

If it was me I would get a trial hole dug and a structural engineer to review the clay and provide a foundation solution.

 

Oaks are very thirsty trees. How high are they? A photo might be useful.

 

 

[Thorfun]

I can only advise you to speak to a structural engineer. our building control needed to see structural engineer calculations for our build (a lot more complicated though) but I presume that'll be the case for even a simple build.

 

 

[Temp]

+1

 

Not all clay is a problem. We"re on clay and have trees close on two sides. SE looked at ground condition report and decided we only needed trench foundations lined with compressible board for the house but the garage needed small piles due to old pond. House few doors down needed very deep piles for some reason so the SE probably saved us a lot.

[tonyshouse]

That is cheapest way out even with 4m deep trenches, piling is easier.

 

absolutely must get a design done, step one is get samples and soil report

[Hilldes]

Not all clay is equal- ours has high volume change potential which lead us to pile (we also have mature oak trees within 4m of the foundations). 
 

The next step would be a Soil Investigation which will entail drilling bore holes and taking samples at various depths for lab analysis. Our SI cost £2300. Although I’m not sure if simply drilling trial holes with a digger would suffice - our Structural Engineer wanted the full soil investigation.

 

Choosing the structural engineer is key - eg if you want something like an insulated raft foundation for passive house choose an SE with experience of this.

[Dave Jones]

screw piles are also an option, less hassle than piling but around same cost.

 

http://www.geologicfoundations.co.uk/home.html/index.htm

 

 

[joe90]

We are on clay and the BC just wanted slightly wider footings (thankfully).

[Tony K]

I'm on clay with lots of trees and went for steel screw piles, though partly that was due to other logistical factors too. Not cheap, you'll need an engineer to design it and there are only so many firms who do that work. 

 

The trees near me are much closer than yours though. Presuming the trees are to be retained, get a soil survey done and an arb survey of the roots, then you can get an engineer to tell you your options regarding foundations. 

 

Also consider drainage at the earliest stage. The soil test will tell you if a soakaway is viable or not, and you'll need to know that ASAP. 

[JD44027]

Thanks all, v helpful.

 

Will definitely get a soil investigation/structural engineer review done, 

 

@Tony Kwhat size footprint was yours and what sort of costing did this come to, if you're happy to share?

 

Probably best if I budget for the worst case.

 

Thanks

[Tony K]

@JD44027 It was about 40k for the piles, that's 39 piles at about 6-7 metres deep. Then add the cost of the slab.

 

My costings are a bit unique due to my odd site. There's another thread nearby where costs are discussed, so take a look. 

 

Good luck! 

[dangti6]

14 hours ago, Thedreamer said:

Oaks are very thirsty trees. How high are they? A photo might be useful.

 

 

 

BC work on the basis of the species mature height don't they?

 

Annoyingly enough I had a single unruly Laurel near one corner of my extension. Because it was about 5m in height and very obtrusive it was spotted and I had to go down 1.7m deep that end. 

 

At the other corner was a fence with a run of several laurels just poking over the top that are kept pruned to just above the fence line. They didn't take much notice of those and I didn't point them out despite being positioned at the same distance.

[Temp]

Your SE will most likely ask for a soil condition report with lab tests. These should show if the clay shrinks or not. That and the presence of trees will determine the foundation design. If you need trenches over 3m deep then piled foundations may work out cheaper and easier.

 

I recommend getting a percolation test done at the same time as the soil report. If you already have access to an excavator and driver offer that as it will reduce cost.

[George]

As an aside, 4m is quite close so there may be push back, since you'd be chopping through a large number of roots with traditional foundations. This is reduced with piles, but you are still blocking water from getting to the roots.

 

Allied with that, during construction there may be a requirement to protect the roots/soil from compaction. This can result in building a temporary works platform over the roots area.

 

I think to deal with any volume changes, and to protect the tree, I'd be looking at a screw piled foundation solution. Anything else is going to risk killing off the trees.

[JD44027]

15 minutes ago, George said:

As an aside, 4m is quite close so there may be push back, since you'd be chopping through a large number of roots with traditional foundations. This is reduced with piles, but you are still blocking water from getting to the roots.

 

Allied with that, during construction there may be a requirement to protect the roots/soil from compaction. This can result in building a temporary works platform over the roots area.

 

I think to deal with any volume changes, and to protect the tree, I'd be looking at a screw piled foundation solution. Anything else is going to risk killing off the trees.

Ok thanks.

 

Would the fact that the foundation will pretty much be on the same location as an old B road (so presumably compacted/levelled etc at some point) make any difference?

 

Thanks

[George]

Potentially but I was thinking more about compaction due to plant movement and material storage around the footprint of the structure. 

 

There's too many variables to give much advice - but some of the ideas in this thread will be good so you can challenge any consultants you use to make sure they're suitably experienced.

[Adsibob]

We're on London clay. We built a full width extension that is about 8m wide and 4.5m deep on ground floor and has a partial first floor above that as well. The SE does a lot of work in London so he should have known about the clay and was familiar with our garden which is overlooked by several very tall leylandii (about 15m tall and around 8m from the extension) and a much shorter (3m) holly tree which is within three feet of the extension.

 

Originally the foundations were to be concrete trenches about 1.25 to 1.5m deep, depending on the relative proximity to the trees. BCO didn't like this and insisted on adding 75cm of depth to that, which added about £2k in cost. So we now have 2m to 2.25m of foundations. Engineer thought it was slightly overkill, but then he would say that wouldn't he.

 

Anyway, the extension has been up for a couple of months and looks to be faring well. 

[George]

It's a complicated area. Building control have a very blunt tool when it comes to determining foundation depths - some of which is based on research/recommendations from BRE and NHBC who had more of an eye on insurance claims rather than engineering accuracy.

 

Personally, I cannot see the potential for volume changing once you're well away from organic matter/root zone. What's the point going another half metre deeper into identical clay strata. So on the face of it I agree with your SE but they weren't in a position to contest it, unfortunately.

Edited June 11, 2021 by George

[Adsibob]

I guess the holly tree could keep growing and so its roots could keep growing too. But yes, it is pretty silly that the BCO who signs off on this is not an engineer. Maybe they should have specialist BCOs with some engineering training to only do the foundation check.

[Faz]

As mentioned above you will need the engineer to design the footings.  I imagine they will go a bit deeper and spec Claymaster poly board (use Claylite - same stuff but cheaper or, if you are close to Cambridgeshire come and see me as you can have a load I have left over!)

[Thorfun]

6 minutes ago, Adsibob said:

I guess the holly tree could keep growing and so its roots could keep growing too. But yes, it is pretty silly that the BCO who signs off on this is not an engineer. Maybe they should have specialist BCOs with some engineering training to only do the foundation check.

our BC said that she would send off the SE calculations to the BC engineer to look over. 

[JD44027]

4 minutes ago, Faz said:

As mentioned above you will need the engineer to design the footings.  I imagine they will go a bit deeper and spec Claymaster poly board (use Claylite - same stuff but cheaper or, if you are close to Cambridgeshire come and see me as you can have a load I have left over!)

I'm based in devon nr chudleigh/a few China clay mines, might be a bit of a trek ?

 

Thanks all for the help, appreciated.

[Faz]

Yeah - that would be a bit of a mission for £100 worth of board! Best of luck with it mate.

[patp]

We had to to put in a 1.2m trench and heave boards down one side of our foundations that were near a hawthorn hedge (we confessed to removing it and he said that can make matters worse  ) 

[George]

15 hours ago, patp said:

We had to to put in a 1.2m trench and heave boards down one side of our foundations that were near a hawthorn hedge (we confessed to removing it and he said that can make matters worse  ) 

 

Strange, they're usually not mandated until foundations are 1.5m deep.

 

Heave can cause issues as the ground reuptakes moisture.Ideally you'd have a good idea f the full soil strata and site history. But as that would mean a huge expense, only a very blunt 'worst case' tool is available. 

 

Because I'm an SE my BCO just left me to it on the structural things. He pointed towards a nearby tree but due to the local geography I assured him it wouldn't cause issues.