About heave

[Sylvia]

Last Easter we had to remove a large Beech growing between retaining walls in our garden - no sign of any give in the walls so far after some good downpours but does heave occur within days/weeks of rain if it's going to happen - or is heave more to do with freeze/thaw - ie won't know if the walls will hold until after a decent freeze?

[Alinicoll]

Heave occurs when a desiccated shrinkable clay soil re-hydrates, its nothing to do with freeze thaw. I believe it can take years for a very dessicated soil to do this but you would think you might of noticed something after a year.

[Pommie]

Heave is very difficult to predict. Did the felled tree pre date the house ?

[Sylvia]

Thanks -

the Beech was there a good few years before the house -

I didn't realise heave was a problem of shrinkable clay and not just any soil - we're on limestone - no sign of clay - so hope that means there's not a prob.

Can't dig down far because the old tree roots are so dense so I hope they'll keep the soil held together.

[Gary Prentice]

Having just been to a seminar with Giles Biddle........

 

 

Seasonal contraction and expansion doesn't cause heave after removal of the tree. This only occurs where a persistent soil deficit occurs. Say, for example, the soil has a 40% moisture content when fully hydrated. By the end of September the content may be 20%. During the winter - if the soil fully hydrates, then tree removal isn't likely to cause heave.

 

But, some soils aren't that permeable and don't fully rehydrate, so maybe two metres below ground m.c only reaches 35% in the spring, next spring its a little less -say 30%.

 

This downward spiral continues, the house is subsiding and the tree is removed. The soil at deeper levels which have never fully hydrated over winter may have MC's at 5-10%. With the tree gone they slowly return to full hydration - max expansion> This is heave. It may take several years, depending on the permeability and climate (Rainfall) but its a continuing expansion. I think Giles mentioned a property that's still rising after 30 years.

 

Anti -heave engineering methods, along with underpinning, are available - at a cost. Normal shrink-swell can be quite dramatic, we were shown level monitoring data showing 27mm falls (subsidence) measured over a six week period.

[Gary Prentice]

Thanks -

the Beech was there a good few years before the house -

I didn't realise heave was a problem of shrinkable clay and not just any soil - we're on limestone - no sign of clay - so hope that means there's not a prob.

Can't dig down far because the old tree roots are so dense so I hope they'll keep the soil held together.

 

Nothing to do with holding the soil together. If you go to the British Geological Survey site; Please enter the address of your report location you can enter your address and get a ground stability report.

 

If its the same as the app I have, its a fiver and gives you information on the risk of subsidence. You probably don't need it tbh, but you can get small pockets of clay amongst other soils, even gravels.