Hawthorne tree removed for extension – heave?

[MartB]

Hello,

 

I had to remove a hawthorne tree (7m tall) when building our extension. The (now removed) tree was 0.5-1m away from the new foundations of the extension. We have clay soil, and I am worried that heave may occur. I am thinking about laying a patio over the area where the tree used to be and direct any rainwater away with an ACO drain. Would this help against possible heave?

 

The details: I cut the hawthorne tree down to 1m above ground level in Sep 2020. I also cut down two smaller trees (3m tall) growing right next to the fence. The soil is clay, and the effect was that during heavy rainfall, the water was standing on the lawn in this area (which did not happen before removal of the trees). Construction works began in early April 2021. The position of the hawthorne tree is 0.5-1 m away from the foundations of the extension. Foundations are 1.5m deep (trenchfill). The area between the new extension and the garden fence, where the trees used to be, is about 2.5-2m wide. My thinking is to lay a patio there and direct any rainwater away from this area with an ACO drain. The thinking is that if the clay soil cannot get wet, it cannot swell (?). Even if this thinking is correct, I do wonder if this could potentially cause subsidence issues due to the clay soil drying out.

 

Any advice would be much appreciated.

Many thanks.

Martin

[daltontrees]

There's a big difference between clay soils and shrinkable clay soils. Where are you?

[Paul Cleaver]

2 hours ago, daltontrees said:

There's a big difference between clay soils and shrinkable clay soils. Where are you?

Hi Jules ,

I thought all clay soils were shrinkable to some extent, although the the plasticity index varies considerably. 

Your thoughts please 

[MartB]

I live in Milton Keynes (Loughton).

 

Also, I just had a look at my messages with the builder and this was his assessment: "there is no desiccation, no roots visible below 500mm from the removed tree; the moist clay that we have uncovered is low to medium shrinkage". Builder's assessment was that under these conditions, heave will not be an issue.

 

I have also attached a picture of the trench showing the clay soil (if that helps).

Edited May 9, 2021 by MartB

[daltontrees]

Parts of MK are on Oxfprd Clays, parts on Lias Group. These are both capable of being shrinkable. Other parts of MK are Great Oolite, zero shrinkage. There are also superficial deposits of glacial clays.

 

If your builder has assessed for you, keep a note of it and maybe you don't need to do anything else.

 

Heave needs shrinkabel clays, a history of dessication by trees, then removal of trrees but fuindamentaly the foundations need to be of inadequate depth. If the new foundations are suitable there is no issue. You maybe don't knwo the situation with the existing foundations. But a skittery wee hawthorn? Seems unlikely to cause heave to the existing building whcih will be separated fromrehydration by hte new building.

 

A patio and Acodrain won't stop rehydration. It usually happens over quite a period of time.  If it's going to happen it's going to happen. Doesn't look like an issue based on what you have said. If there are no signs of tree roots at 0.5m and the founds are to be at 1.5m, I can't see the issue.

[daltontrees]

1 hour ago, Paul Cleaver said:

Hi Jules ,

I thought all clay soils were shrinkable to some extent, although the the plasticity index varies considerably. 

Your thoughts please 

Yes all clay shrinks on dessication. But if it has bigger particle sizes mixed in, the volume changes and loss of bearing capacity are less significant ('modified plasticiy index'). Throw in the usual consideration of persistent soil moisture deficit. I am in Scotland, most of the country is smeared with glacial tills (clay) but in all my years I have never seen a subsidence or heave case. Maybe that's because it rains a lot of the time.

[Paul Cleaver]

43 minutes ago, daltontrees said:

Yes all clay shrinks on dessication. But if it has bigger particle sizes mixed in, the volume changes and loss of bearing capacity are less significant ('modified plasticiy index'). Throw in the usual consideration of persistent soil moisture deficit. I am in Scotland, most of the country is smeared with glacial tills (clay) but in all my years I have never seen a subsidence or heave case. Maybe that's because it rains a lot of the time.

We have glacial  boulder clay around here ( South Cheshire) The worst shrinkable clays are in the south. I know the London clays are particularly bad. 

[daltontrees]

15 hours ago, Paul Cleaver said:

We have glacial  boulder clay around here ( South Cheshire) The worst shrinkable clays are in the south. I know the London clays are particularly bad. 

As the name suggests, boulder clays have a wide range of particle and clast sizes. The clays shrink in the voids between the matrix of sand and silt particles without changing the overall volume. The worst clays are pure clays where shrinkage must result in volume loss.

[MartB]

Thanks Jules. Your advice is much appreciated!

Martin