Felling mature oak. Ground heave?

Question

We have a dying (decay in trunk) mature TPO oak approx 7m from a conservatory on a raft foundation. We're on clay soil. Both an arborist and tree surgeon have advised to have the tree felled sooner rather than later. An application has been made to the LA.
Given the tree is on clay I understand removal could cause the clay to re hydrate and cause heave on the conservatory foundations.
So there's risk if the tree is not felled and risk if it is!
Thoughts?

Jason · Accepted Answer

It’s always best to follow the advice of the professionals, if the tree was to fail due to weak integral strength it would cause more damage than small ground movement.

Generally when the tree is felled and the stump is either ground or killed off the root system will no longer cause any issues or reach for moisture. Also any moisture would dissipate into the mold of where the tree was leaving the risk on the very low end of the scale. It’s highly unlikely to cause risk once removed

Daniel · Answer

Since the tree has been dying for some time, it any way no longer absorbs huge amounts of water, so no drastic changes in the ground level are likely. If the tree is robust and is drinking huge amounts of water and then suddenly felled the sudden drastic change will cause the water level to  rise, which can effect the structure. Best to have it sorted than to have it come down by itself in the next storm, which could do more damage.

Mac · Answer

You're facing a tricky situation due to the potential structural risks associated with both leaving and removing the tree. Here's an overview of the risks in your scenario and some considerations to help guide your decision:

Roots Continuing to Draw Water:
Even though the tree is decaying, its roots may still be drawing moisture from the clay soil. This might help maintain the current equilibrium of soil moisture content under and around your conservatory.

Risk of Felling the Tree:
Clay Soil Rehydration (Heave): In clay soil, trees extract significant moisture, leading to shrinkage. Once the tree is removed, the soil will start to rehydrate, potentially expanding and pushing up on the foundations of your conservatory. This can cause significant structural damage to shallow or raft foundations like the one under your conservatory.

Considerations for Heave:
Age and Size of the Tree: A mature oak tree would have established a large root system and removed a lot of water from the soil over many years. Its removal will likely lead to significant rehydration.

Foundations Depth:
Raft foundations are often more flexible, which might help them handle some degree of movement, but they can still be vulnerable to heave if the clay expands significantly.

Heave Mitigation: 
Arborists sometimes recommend staged removal of large trees to reduce the shock of sudden rehydration. This involves gradually cutting back parts of the tree over time to allow the soil to rehydrate more slowly, minimizing the risk of heave.

Monitor Soil Moisture: 
Soil specialists can assess the current moisture content and predict the amount of rehydration and movement expected. This information can help determine how much of a risk heave poses in your specific case.

Root Barrier or Mitigation Strategies: 
In some cases, installing root barriers or even under-pinning the conservatory foundations could mitigate the effects of future movement, though these solutions can be costly.

Summary:
Both options come with risk, but the key is balancing these risks with expert advice. If the tree poses an immediate safety risk, removal is likely the best course of action. However, the removal should ideally be accompanied by careful monitoring and potential mitigation steps to reduce the risk of heave. Consulting with a structural engineer and potentially opting for staged removal are strategies to consider.

Best,
Mac
eHealthyHomes

David · Answer

take the advise you have been told they will no whats best

Simon · Answer

Hi if it's was me
 I would remove the tree because it will only get bigger and cost more to remove.
Find a good bricklayer should be able to tell you about possible cost if damaged property or foundations. 
Kind regards

Jasper · Answer

Hi,

In my experience having the stump ground out can reduce heave,

all the best,

Jasper

Felling mature oak. Ground heave?

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