Are oak tree roots likely to circumvent a 3.3m deep root barrier?

[The Professor]

I am in the final stages of purchasing a 1930's semi-detached house in south-east London. During the purchasing process, I have found out that the house had its rear wall underpinned in 2008 to stop subsidence which was identified as being caused by an circa 80-year old oak tree which is sucking all the moisture from the clay soil. The oak is located on an unadopted side road 16 metres from the house's back wall. The insurance company drilled 3 bore holes around the property and did DNA tests that revealed 80% of roots present were oak (it's the only oak in the area). An ornamental acer and cherry (the other 20% of roots found) were removed from the garden.

Unfortunately, in 2014, the property began to subside again, so the insurance company underpinned the house's rear wall once more with 3.3m of concrete, which they call a 'root barrier'. They tried to carry out works to the oak tree in 2014, but neighbours formed a campaign group which has resulted in the oak being given a TPO by the council.

The vendor, the the vendor's insurance company and the underpinning specialist that did the work all say that the oak roots can no longer get under the house to cause further subsidence as the oak's roots will not go that deep (3.3m), but my building surveyor says that its only a matter of a few years until the roots go under/through/around the barrier in their search for water. In your opinion, will the oak be able to circumvent the 3.3 metre deep root barrier? Any advice welcome! 

[Arctostaphylos uva-ursi]

If the soils are heavy London clay then the likelihood of roots extending to that depth is less likely.  Yes tree roots have been found at greater depths but this is dependent on the conditions.  There is not enough information here to give meaningful advice. If you can get hold of the full subsidence report that would give you a better steer on the what and why.  As a minimum I would be looking for the Arboricultural Report, Soils Investigations, Structural Report and details of any monitoring undertaken.  Monitoring is often continued until the property has stabilised or engineers believe that sufficient measures have been put in place to stop movement.

[The Professor]

I have requested and received a lot of the insurance company and contractors' documents related to the underpinning work. The geology report shows that the soil is 'firm/mid-firm orange, brown mottled grey clay' from a depth of 40cm down to 3m. The deepest roots found in the bore hole samples were at 190cm. Do you think the oak roots could go deeper than 3.3m as the tree continues to grow in such soil? As suggested by others in earlier posts, I'd check that there are no leaking waste-water drains to attract them deeper than they'd grow naturally.

[The Professor]

Here's a satellite photo of the oak in relation to the pair of houses. It is located at the edge of the dirt track used by the garages, owner unknown. It's not in anyone's boundary. The house nearest to the tree has apparently required more underpinning than the one on the right which I'm interested in buying. The trunk of the tree is approx 16m from the rear wall of the house on the right.

[Khriss]

Well , that's opened it up a bit , substantial trees and shrubs , vegetation and managed grass areas plus an active industrial / agricultural ( services and drains ) location near house . On shrinkable clay . Notable Oak in the landscape. A root barrier wont help with this problem . K

[Jake Andrews]

Was the underpinning piled or trench filled foundation at 3.3m?

[The Professor]

Underpinning report says 'C30 grade concrete poured to form pins in sequence' - they excavated underneath the previous underpinning. 

 

[Jon Heuch]

On ‎07‎/‎02‎/‎2018 at 08:32, Khriss said:

Well , that's opened it up a bit , substantial trees and shrubs , vegetation and managed grass areas plus an active industrial / agricultural ( services and drains ) location near house . On shrinkable clay . Notable Oak in the landscape. A root barrier wont help with this problem . K

Why not?

[Khriss]

Cos I would want a lot more detail on the soil Jon ! And a good exploration of the drains and services - it looks a busy site . Certainly to determine where to place - or even IF  a root barrier would help ,  K

[Khriss]

( and looking at the plan view - would enquire if the extension next door has had any problems ! )