OK, I brought relevant posts over from this thread
Soil subsidence and trees
to here. Lets keep the discussion here otherwise that thread will end up huge losing it's resource value. It's good to refer to it though.
Daz,
From experience with geotechs and engineers here I can say they'll condemn your trees no worries.
They'll apply regs that we dont have.
Some councils face the same problem you do when their street trees cause (allegedly) houses to crack. Now I bumped into something somewhere once but this is how it goes.
The councils criteria was ...
1. Roots must be beneath or adjacent to the building, proven by sample.
2. Soil core with moisture content required and same from side opposite without trees as a test hole (called control), comparison made.
If the moisture content is significantly lower where trees are and roots found >>> BINGO, council could be looking at a claim especially if homeowner requested tree works.
In some places the councils are now installing root barrier along fence lines to prevent these issues.
Also, from the soil core the geotech will give the clay rating. Depending on that rating it will give you the old height and distance rule for the trees. It could be as bad as the closest a tree can be is 1.5x it's ht.
So 10' tree must be 15' away.
Natives are very efficient water scourers, gums, wattles, bottlebrush etc are very efficient water miners.
This is a tough task, and it's going to be fought on engineering codes more than arborists knowledge unfortuneately.
These are the cases that go to court and cost lots of time and money. If the body corporate want to persue for costs of rectifying and underpinning a proportion of that cost can be appropriated to you. In my experience the cost and inconvenience of complying with there requests and moving forward with a new insight could be wise.
To install root barrier to existing trees at that proximity to the trunk might destabilize them or kill them. You did say the trees are within 3m of the flats.
With council trees they're not that close to houses, there's usually 3m wide footpath and then 6m to the house minimum unless there's a relaxation on the property.
Subsidence happens when soil moisture is depleted and the clay shrinks. The major cause of this is drought. Did this just happen in the last couple of years or is it a long term thing?
If you decide to replace and replant I would suggest trying to plant a little further away, install root barrier prior to planting, choose a columnar species with a height cap of 3m ... not something that needs hedging or trimming either. Also, the latest rage for the tighter urban yards is special bamboos which dont run. There's varieties that wont grow too tall now a days.
Hope this helps, I've had my share of these and can say from the legal aspect the tree owner will likely lose if up against geotechs and engineers. We have (pardon the pun) nothing concrete as evidence to the contrary for each specific case.
Remember the 2 point rule above if things go bad.