Have you ever gone down to do the splits and not be able to get yourself back up without help? Well, trees with structural deficiencies can actually split in a similar manner. Two arms of a trunk may stretch wide during a wind event, but they can't pull themselves back together afterward. The typical split we see is like this one below, with the drill and drill bit through the trunk.
To fix a splitting tree, we apply structural support in the form of cabling or bracing. This hackberry split about 7' up from the ground and we are drilling two holes through the trunk so we can install threaded rods to hold the split together. It's not an easy task. In the above photo, we are using a 4' long drill bit and a drill that weighs around 15 pounds. The hardest part is when the drill bit gets completely stuck in the tree! Its happens too often and can be a scary moment.
Here is a good profile shot of the two rods with the washers and nuts on the end. One of the braces goes through near the top of the split and the other is installed around 20" above the lower brace. The braces are around 30-35" long from end to end. The rods are cut as close to the trunk as possible and then the ends are pounded with a hammer to prevent the nut from backing off.
In most normal tree repairs to a split trunk, the work does not stop at the brace. The top of the tree is where all the leverage happens, and it also requires support in the form of a static steel cable. This cable helps to hold the tree crown together, reducing the forces being placed on the brace, and helps to hold the tree together.
The last picture shows a detail of a brace in a different tree, viewed from above. This brace is about 6 feet long and required lots of effort from many people to get installed. It was well worth it, for this tree a heritage burr oak on Lakeside in Madison. A truly incredible specimen, the back side of this tree was split and had been split for some time. We had to install two cables to hold the top together and then put two super long braces through the trunks.
Each time we do a brace there is a challenge that pops up. These challenges probably explain why this is an underutilized application. Cutting the branch off is usually easier, but often is not what is best for the tree.
In both the above trees, there was no decay present so the bolts and cables replaced the wood that is supposed to hold the trunks together. The combination of bracing and cabling can stabilize a split tree and be an alternative to tree removal. Unlike the braces on a teenager, tree braces and cables are permanent!