We were very lucky to have a week of lovely autumnal weather for shinning up and down poles, and wandering around in the crowns of trees at River Dart Country Park. They have to be able to perform rescues in various scenarios such as where the casualties climbing system is damaged, where the casualty is on a pole, or where they can use a third person to lower a casualty down. It is hard work, a lot to learn in five days and it is physically and mentally challenging, but they rose to the challenges and did it all with good humour and gave it their all. We got to Friday a little wiser, but a lot more stiff!
Their test is next Thursday, so Good Luck to them!
FOCUS on………. Shear bombs!!!
Right in time for bonfire night, some potentially explosive tree action…………
Just a regular tree picture?- look a bit closer.
Ordinarily, I think people would agree, that you would expect a tree not to have a gaping crack in both sides, but in this case, clearly something unusual is happening to this stem.
The tree is on a bank and is heavily weighted on one side, because the trees behind it are causing it to grow out at an angle towards the light.
What can happen sometimes, is that continuous heavy loading, especially in this case where we have a heavy one sided crown and a lean, cause cracks to form deep inside the tree. Essentially the forces pulling the tree into the ground, and the forces bending the tree above ground, can cause two parts of the wood structure to move in opposite directions to each other, and this is called ‘shear’. Here it is happening where a large root on the rear of the tree is anchoring the tree into the ground, and the upper portion of the tree is being forced outwards by the crown weight and the wind. I have mentioned in previous blogs how strongly roots can anchor trees into the ground, they can withstand enormous forces that are strong enough to cause the stem to crack before the roots give up. The tree has formed a crack between this anchored root, and the flexing stem.
These cracks can be present inside the tree for some time, and may not be obvious, but as loading continues, these cracks can get bigger, until they reach the outside of the tree.
Claus Mattheck calls these cracks ‘Shear bombs’, and once the root and tree come apart, they are called ‘Separation cracks’.
They are called ‘bombs’ because they can be a focus for sudden catastrophic failure of the tree structure. Sometimes they can seal, but often they remain as a weak point, and in this case, the crack became visible after Storm Brian, and is now getting longer and wider. I will be monitoring this tree to see what happens over the coming weeks, but my hunch is that this crack will continue to propagate along the length of the trunk, and eventually the stem will fail.