Originally Posted by Ekka

Len, The article mentioned 2/3's of failures are due to poor soil, soil fracturing.

Most wind blown trees tend to heave soil within a 2.5m radius of the trunk regardless of tree size.

Looking over some of Mattheck's work he talked about soil shear too.

Seems when the soil turns to a bog and the tree blows over that it's called a soil failure not root failure. However all blow overs have to break roots at the soil fracture line unless of course it's a small tree.

I mainly see root failures, not too often do I get a 4m high wall of soil like I see in many USA and Euro pics of tree failures. Then again, the soils are much shallower here perhaps putting more emphasis onto roots to hold.

I liked the article and it had some good general rule of thumbs here.