Cracks in House Extension on clay soil?

[MAB]

A friend of mine had a two story extension built on the back of his detached 1930's house back in 2006. The ground is South London/Surrey clay. The house rear garden backs onto a golf course with 2 large mature oak trees belonging to the Golf club and dense hedge/shrubs forming the border between his back garden and the golf course. The house extension was built with 2m deep foundations and approved by Building Control but has recently had subsidence cracks appearing which have got worse with this years long hot summer. Oak roots have been found in samples near the foundations and the house owners insurance company are going through a process of monitoring and investigation. My friend is in his 70's and was hoping to downsize house soon but is now worried this could be a long and protracted process making it impossible for him to sell and with much upheaval to come in order to fix the problem. The golf course have offered to reduce the canopy of the trees but want to keep them in place and some say that removing the trees could possibly create a clay heave problem instead. i would be interested to hear any opinions or possible solutions on this problem or any similar experiences and what might be the various possible solutions/outcomes and the best way for my friend to proceed or where he could best get some independent expert advice?

(I will try to get some accurate height and distances of the oak trees from his house and/or photos in the next few days).

[Tyke2]

Hi

It seems like you have explained it pretty accurately already.

If the foundations were deepened due to the trees I would have expected to see something like "Claymaster" :- https://www.jablite.co.uk/application/claymaster/

on the inside of the foundation trench. Otherwise the heave of the clay within the house extension footprint will potentially split the foundation as the clay rehydrates due to the tree roots now been cut off. The claymaster would collapse and prevent damage.

It is possible that if claymaster was used , but the heave was much greater  than predicted, heave damage could still occur. I have seen this where there was a large row of Poplars on a golf course boundary adjacent to a house gable. The clay expanded so much, it crushed the claymaster and lifted the house off its footing, breaking the house in two.

 

[Declan52]

Is it just cracks in the walls or have windows started to crack and doors not closing properly??

Has whoever done the investigation dug down where the cracks are heading to see if the concrete founds have cracked??

Is there any equipment on either side of the cracks to monitor the movement??

It isn't going to be a quick fix. The insurance company will want it monitored for maybe up to a year to see how much it's moving. During this time they will do any cutting down of the trees and will be able to see if this is enough to stop the ground shrinking.

As it will be going through an insurance company anyone who goes to buy the house will have this flagged up to them so might cause a problem.