Another subsidence question thread!

[daltontrees]

I thought there was still some debate on the water demand category? In that non-arbs require a pigeon hole for each genera whilst arbs recognise that there are simply far too many factors to take into account. Does a kilmarnock willow still fit into a high demand category the same as a white willow:lol:

 

Not to say that very broadly speaking, that they are not to be disregarded.

 

Wewll, the Kelly/O'Callaghan article (at p.11) clarifies that the 'water demand' term is a misnomer and that vertical distribution of roots is what matters. Ove rhe years some species have been moved category, suggesting that the debate is not over.

[Gary Prentice]

This thread just goes to show the breadth of factors that must be considered with trees and structures. Starting to read 'Trees roots in the built environment' again has reminded me how many factors are involved.

 

I looked at a Norway Spruce last week, initially for a quote to fell. The owner was obviously attached to it, indeed had purchased the house because of it, but was now becoming concerned about the proximity to the house.

 

What can you say? Yes, the property is within the trees zone of influence. But I'm not a geologist nor a structural engineer, so can't predict or even make a wild guess at the probability of subsidence. It's difficult when the client wants a yes/no answer:confused1:

 

Oddly, in this instance, the owner was never required to get a report when he applied for a mortgage fifteen years ago. The tree is only about 6-7m from the house.

[daltontrees]

What can you say? Yes, the property is within the trees zone of influence. But I'm not a geologist nor a structural engineer, so can't predict or even make a wild guess at the probability of subsidence. It's difficult when the client wants a yes/no answer.

 

It has always looked to me like the NHBC guidelines are set up for engineers rather than arbs. After all, if the engineer can identify the tree species, he can look up everything else on the tree side of the equation. Can the arb do the same? Probably not, because the Plasticity Index of the soil needs to be known and unless you are going to dig trial pits and do cone penetrometer tests and be able to interpret the results, you can't complete the equation.

 

So what can you say? There are nearly no shrinkable clays in Scotland but I am asked about it regularly, it is good to be able to cover the basics but anyone who wants a yes/no answer in the course of a (free) tree quote is dreaming. Firstly there is no such thing as yes/no, only balance of probabilities after all factors have been investigated. Secondly, why would anyone stick their neck out on such a potentially litigous and expensive minefield for nothing? They wouldn't. I think you're quite right to start with 'I'm no expert on the engineering side, and the soil type hasn't been confirmed, but for that species and distance the general industry standard suggests that ....'

 

If I had a Norway Spruce that close to a house on stiff clay soils I'd be more worried about a shallow rootplate and the whole thing blowing over.

[Gary Prentice]

If I had a Norway Spruce that close to a house on stiff clay soils I'd be more worried about a shallow rootplate and the whole thing blowing over.:lol:

 

The thought had crossed my mind. Anyway it's staying put (hopefully) for a while yet.