Trees and subsidence

[Marc Lewis]

Thanks all.

 

Not sure if house or tree is older.

 

What is heave? Don't think I have come accross this term.

[Gary Prentice]

Yes, but the concern was about subsidence due to rotting roots. That wont happen.

 

Why will it suffer from heave from rotting roots?

 

The houseowners concern is unfounded with regard that the property might subside, due to underground voids that may appear as the roots decay. There is unlikely to be that large a diameter roots 10ft from the tree.

 

The public, at large, have a limited understanding on the affects of vegetation and soils - shrinkable clays with a high plasticity index.

 

The main concern, forgetting rotting roots, is the possibility of a change in the soils volume due to moisture content/removing the tree.

 

With shrinkable soils, water uptake during the summer months leads to a water deficit in the soil, its below its field capacity, and its load bearing capacities are reduced. Re-wetting in the winter months allows for expansion of the soil, hence the seasonal variations and symptoms of cracks in walls opening and closing, windows sticking in the summer etc.

 

When trees are removed, the soil is able to reach its full 'wet' volume-maximum expansion and heave can occur. This may be particurlarly significant when trees are located near the corners of structures, as the volumetric differential is more apparent.

 

Dalgea O'Callahan's written a couple of good articles on this, informations available from the BRE (Buildings Research Establishment) and the TSO's 'Tree roots in the built environment' are all worth reading.

 

Oaks, in particular, as a species, have been included in the high category for water uptake. This is in studies going back to the thirties and more recently by Biddle et al The opening poster would be well advised not to comment on the possibility of soil movement.

 

Arboricultural Consultantents, with their PL insurance, have to work with geologists and structural surveyors to assess these things. Pockets of clay exist even in sandy soils. The geologist calculates the PI of the soil- how much it is able to shrink and expand in relationship to water content. The structural fellow looks at the foundation structure. If the property is on a raft, piled or has sufficient depth below the level of the roots influence, all may well.

 

As I said, its outside our field of knowledge to answer the clients question. At the distance the tree is, it's well within its zone of influence to affect the soil volume. I think O'Callahan reported a case, an estate was built in accordance with NHBC guidelines, where no trees were present. What was taken into account, was that new owners were going to plant trees and hedges. The soil was shrinkable, add a dry summer (seasonal re-wetting didn't occur) = subsidence.

 

Simple answer is not to stick your neck out.

Edited March 22, 2013 by Gary Prentice