Any comments or suggestions would be greatly appreciated.
I would recommend you take a soil core sample and see what soil layering and profiles you have there. Be careful and avoid tree roots.
For example, do you have a sandy/loam over shale rock? Layering like that means soil profiles can tear away easier especially when wet and loaded by wind force acting against tree roots.
Once you know the layers and profile then there's a few things you can do.
We know that most uprootings of healthy trees occur when ....
* Soil is soaked
* Extreme wind
* Large canopy in leaf (as a bare tree has less sail)
And we also know that most uprootings occur within 2.5m of the trunk, that's the danger zone.
So, knowing what the drainage is like is another aspect you can work on to reduce the soils likelihood of getting bogged.
Where anchorage is required and the subsoil is shale etc boring holes which then get filled with sand or even polystyrene creates fissures the trees roots can exploit, try not to glaze the sides of these holes or roughen them up afterwards with a steel rod etc.
A study of uprooted trees in SE Queensland revealed some facts that throw away contemporary thinking.
Source (not ISA
):
http://eco.confex.com/eco/2007/techprogram/P5725.HTM
Upright trees in our study site did not tend to have their longer and taller buttresses oriented towards the direction of annual prevailing winds, a finding contrary to some other studies. Uprooted trees lacked buttress length on the side opposite direction of fall, supporting the tension member hypothesis, which predicts that buttresses prevent trees from uprooting when subjected to asymmetric loads due to external forces, such as wind. Of the indices of buttress strength, the longest buttress of each tree was the best indicator for direction and magnitude of overall buttress support.
Work with anchorage and soil, of course Woodweasel's recommendations for inoculating the soil with mycorrhiza should also be done, however do work the side of the tree opposite to the prevailing winds.
I've heard of work done where large stainless steel rods were anchored into bedrock that were like a U-Bolt over tree roots. Roots like branches will "swallow" the rods eventually.
Engineers are also placing large trees on roof top gardens, there's a bollard system on top of the building and soil above that, the tree roots intertwine in those bollards to be anchored.
In the attached picture what you can see is a 25cm thick sandy topsoil upon a hard clay base. The tree roots will spread and not want to penetrate that clay base. Now conventional thinking would say, great, spreading root system. However, this is where layering can be a problem and soil separation occurs.
So without knowing what your soil profile and layers are you could be wasting your energy expanding the lateral root surface instead of giving the profiles some clamping capacity. Remember, the vast majority of up rootings occur within 2.5m of the trunk.