Potential heave/subsidence from oak tree

tipsychick

I know it's a long shot but I just wondered if anyone here is a structural engineer or at least a good gardener!

I live opposite a 70 foot-ish mature oak tree, owned by the local council and right on the edge of my opposite neighbour's garden. I'm probably about fifteen metres away from it, with a road in between. The closest house would be the opposite neighbour's and the tree is no more than five metres from his property. The soil is London clay.

Apparently my opposite neighbour complained to the council a few months back about the tree "smelling". (No other neighbours have smelt anything though). The council have now done some kind of expensive ultrasound test of the trunk, deemed it to be infected with a fungus making it unsafe and the tree is marked for imminent felling. The tree looks absolutely beautiful, is probably a hundred years plus old (pre-dating the houses which are 1930s) and has no outward signs of infection - leaves, branches, etc. all healthy.

This is purely supposition on my part but the oak tree is bang slap in the middle of the opposite neighbour's driveway, which is making parking their three cars pretty difficult (particularly as his eldest has just learnt to drive) and I imagine he is keen to remedy this by paving over the whole garden.

None of the surrounding neighbours are too thrilled at the thought of subsidence/heave, etc. and we have a meeting with the council's head tree honcho tomorrow. (He has said that with the road in between, our side of the road is unlikely to be affected but obviously we don't know for sure).

I'm keen to find out if there are any specific tests the council could do to map the spread of the roots and at least ensure they're not under our place or the adjoining neighbours. Anyone got any ideas?

(I've made sure we've definitely got subsidence/heave cover in place on our insurance!)

jimbo_the_jetset

N.B. - the advice of a geotechnical engineer is what you require.

http://www.thebgs.co.uk/science/landUseAndDevelopment/shallow_geohazards/shrinking_and_swelling_clays.html

"The London Clay Formation contains significant amounts of high plasticity clay that can result in shrink/swell clay subsidence due to volume changes associated with drying out at times of drought or trees with a high moisture demand. Where there is a sufficient covering of superficial deposits that do not contain clay, the potential for shrink/swell subsidence is much reduced."

So the problem that you are faced with is that the tree has to come down by the sounds of it. The loss of this tree will mean the loss of its roots mopping up moisture in the clay (if the roots are present in the clay). If the clay becomes too moist this can lead to problems. Details are all very site specific (i.e. hard to generalise). It is possible to take samples of the clay to measure the moisture content and make an assessment of the shrink/swell potential.

http://www.ukgeohazards.info/pages/eng_geol/subsidence_geohazard/eng_geol_subsidence_swellshrink_refs.htm
For detailed documentation - you can at least refer to this to scare the bejesus out of them

tipsychick

Thanks so much for that. OMG, those pictures of subsidence/heave are just nightmarish!

I'm assuming that this ultrasound test is incontrovertible proof that the tree is diseased (although it definitely doesn't look it) and will have to be felled. The council weren't budging on that at all.

I'll suggest to the council tomorrow that we'd like them to obtain the opinion of a geotechnical engineer to take soil/clay samples, etc. At least that way we will know what we're in for.

Would you know if there's anything they can do to minimize the chances of problems? I read that bringing the tree down in stages would help but the council were quite dismissive of that and said it wouldn't make any difference.

A friend of ours is a farmer and has some woodland (albeit in a different part of the country) but he thought that with such a large oak, we may well have already seen signs of heave/subsidence in the driveway and garden walls if not the houses themselves and felt that the fact we haven't was hopefully a good indicator for when the tree comes down.

The council said that they've take down half a dozen similar sized oaks locally in the last year without any problems (yet!), so although I know it is very localized and soil specific, we're hopeful that is a good indicator too.

If the tree comes down and we are going to have problems, would you know how long it would be before we see them? I imagine it could be at least a year, as presumably we'll have to experience all the seasons at least once.

Thanks so much for any help. I'm trying not to panic but I can see the potential nightmare unfolding ahead!

fiddiwebb

Would be interesting to know what fungus the oak is infected with...It might be honey fungus in which case you do get a smell from that and also it should show signs of growing fungi at the base of the tree.

Are there any problems with the bark of the tree?

Are you in a area where there are preservation orders on trees?

tipsychick

I'll find out tomorrow what the fungus is. They is no obvious signs of fungi growing at the base of the tree. There is no smell at all according to the other opposite neighbours adjacent to it and certainly none that I've ever caught whiff of.

The bark is pristine, as is the canopy. Really it is a crying shame to take it down if it isn't absolutely necessary.

I'll ask him tomorrow about tree preservation orders but I'm not aware of any.

spectre

interesting, there has recently been a very mature oak felled near to our house which had a fungal type problem.

Talking to a few of the neigbours there was subsidence problems approx 15yrs or so ago due to roots searching long and far for water.

All very similar circumstances 1930s style houses clay soil etc.