Mango tree near our house causing subsidence...

orangegerbera2003

Hi everyone,

We live near Brisbane and have a house which was built in 2005.

In around October 2009 we were rudely awoken by the sound of large cracks....and a very large crack had developed in our house.

We also noticed a few large cracks outside on one corner of our house.

We called the builder as the house is in warranty. They came out and then brought back the engineer (in December 2009) who did the original slab design.

It turns out there is some subsidence in one corner of our house.

The engineer wrote a report (December 2009) where he said that it was likely that the cause is a large mango tree 7.5m from our house on council land.

We requested the original soil report/slab plan/foundation plan (from 2005) for the house which was actually put together by the same engineer (who was brought in by the builder in December 2009).

The 2005 report clearly says in two places that the builder should remove or put a root barrier down near any trees which are in the zone of influence of the house. According to the BSA zones of influence, the tree is clearly within this zone.

When we brought this point to the builder and engineer's attention, they said these were standard points which were put in all reports. They are now saying that they put a thicker waffle slab down (385mm) to deal with the tree. There is nothing in the original report which says that a thicker slab was put down becuase of the tree (it just says that an "H" style waffle raft was put down due to the P/H soil.

The soil is "P" (problem due to the clay reactivity) and "H" (due to controlled fill).

We had an individual engineer come in who, as soon as he saw the tree, pointed it out and said that is most likely the problem as trees suck moisture from under the foundation causing it to hinge.

The builder/their engineer is saying that they want to monitor it for 6 months to see if there anything else contributing.

There may be other things contributing, but we are amazed, at a meeting today, that the builder and engineer suddenly seem to be taking all emphasis away from the mango tree (after the engineer clearly said this was a likely cause just before Christmas and then said we should talk to a root barrier company to get it sorted out). They seem to suddenly have changed their tune.

They took offence when we could suggest that, whatever they did to protect the house from the mango tree, did not seem to have worked.

We have spoken to the council and need to put everything in writing to them.

I have read a few threads on here which talks about how tree roots can damage foundations. We also have large palm trees all along the fence in the neighbouring garden (which were also there at the time our house was built).

How likely is it that the cause is the mango tree?

Has anyone had similar experiences?

Thanks so much!

Done it

You'll find just about everything you need in this thread. Soil subsidence and trees

Eric Frei

Also, Lodge quickly a report with BSA as rumour has it BSA dumps foundation issues after 5 years.

Also find where any building spec or standard supports a 385mm thick slab as suitable, often it is root barrier or peirs. To evade the soil moisture fluctuations and subsidence it is pretty much seen in Brisbane that going deeper with foundations to 1.5m or more is suitable, there is little moisture fluctuation at that depth.

On new estates where large gum trees remain not far from houses I see the same slabs being poured as everywhere else, they don't care. Same thing though, they say the slabs are thicker but that doesn't sit right with all the info I read. As long as the slab is sitting on the surface and the surface expands and contracts you risk subsidence.

In other areas where at say 2m deep ground water is hit (Cleveland) and still in heavy clay they use screw peirs which are screwed in to a predetermined torque.

Once again, lodge initial complaint with BSA. Council wont do a lot and they wont accept liability for anything as the tree was there first and known about.

Also dont forget that to rectify means underpinning, what do they do when they underpin? They pour another foundation deeper beneath the raft slab, validating what I am saying.

__________________

orangegerbera2003

Thanks so much Ekka. I knew you would come back with some good information!

How would I find out if the 385 slab is suitable for these conditions?

Good idea to get on with contacting the BSA.

Thanks!

Eric Frei

Ring BSA and talk to them, ask about any Australian Standards which might be applicable and buy them.

You could also search or ring http://infostore.saiglobal.com/store...ntactingUs.htm

I have attached in PDF the BSA policy.

Attached Files

SubsidencePolicy.pdf (1.62 MB, 56 views)

__________________