Any work site planning process must include some form of risk assessment of the trees to be worked upon. Many crews assume that a valid risk assessment has been carried out by whoever bid or evaluated the job, and will either do a superficial evaluation or skip the assessment altogether. An issue of equal severity can be the risk assessment that is done with an eye towards what damage the tree would cause to property or the public if it failed. This is typically what convinced the tree owner or responsible agency to remove the tree. Although this assessment will have some valid and valuable information for the crew, it typically does not evaluate the tree, or trees, for safety and security when placed under the stress and forces of tree care operations. Complete training and education on an integrated risk assessment process of trees for worker safety and tree care operations is not possible in this written format, but the description and discussion of various key topics and issues will assist operational crews in better evaluating not only the safety of work site trees, but also the wisdom of their particular work plans and practices in individual sites or situations.
• Outer perimeter assessment—This is an evaluation of the tree and its structure from outside the drip line, where structural defects or hazards not visible close to the base of the tree may become readily apparent to the evaluator. Care should be taken to evaluate the outer perimeter of the tree by moving completely around it to ensure that nothing is overlooked or missed.
• Inner perimeter assessment—This evaluation is done from a point within the drip line, fairly close to the base of the tree, again attempting to identify structural defects or hazards not visible in the outer perimeter assessment. As with the outer perimeter, the evaluator must move completely around the tree to ensure that no possible problems or hazards are missed.
|Photos by Michael (House) Tain.A|
|An example of using a pull test as part of the operational risk assessment in a class setting.|
• Pull test—This test consists of loading the tree in a horizontal fashion or vector by pulling on a rope or throwline installed in the upper canopy. The tree should be pull tested in all four quadrants (north, south, east and west), and the structure observed for problem areas or defects such as cracks, excessive movement or the failure of smooth movement to travel down a leader or trunk. This failure of movement to travel beyond a particular point could be indicative of a hidden defect dissipating the movement’s force, and require closer evaluation. In addition, another crew member should be observing the tree’s roots during pull testing.
• Roots—The roots of the tree should be evaluated for red flag indicators such as fruiting bodies of fungal organisms, grade changes, trenching, etc., but also observed closely during pull testing. Of particular importance is the static relevant zone, an area around the root flare of the tree equal to 150 percent of its diameter (for example, a 20-inch tree has a static relevant zone of 30 inches). Signs of cracking or heaving within this static relevant zone indicate a risk potential that requires closer scrutiny.
• Pedestal—This is the portion of the tree’s trunk from the root crown up to the first branches, and it should not only be evaluated visually during the inner perimeter assessment, but also observed for hazard indicators during pull testing.
|An example of a catastrophic failure in the pedestal of a tree.|
• Trunk—This is the continuation of the tree’s trunk beyond the first branches, and also merits both visual evaluation during inner and outer analysis and observation during pull testing.
• Scaffolds—The scaffolds are the leaders and branches that make up the structure of the canopy of the tree. As with the other parts of the tree, both a visual examination for red flag indicators and observation during pull testing are key for a comprehensive risk assessment.
|Fruiting bodies of fungal organisms in the rootzone, pedestal, trunk or scaffolds of the tree should be considered red flag indicators and require closer evaluation.|
• Work risk—An evaluation of work risk is what distinguishes an operational risk assessment from a standard one, which is typically only concerned with damage to property or the public. The variety of tree care operations required for individual trees result in a wide variety of forces experienced by the trees themselves. Operations that generate minor forces on the tree and its structure may have little impact if only minimal defects are present, but the results can be catastrophic if both the work forces generated and the tree’s security/stability are not considered and evaluated. A commonsense evaluation of the likely forces generated by the proposed work plan should give crews a reasonable idea of the work risks involved. Pruning or felling operations would present the lowest work risk within the framework of forces generated, while whole tree removal/rigging would generate the most forces and hold the most work risk. Operations such as scaffold limb removal, top removal or cabling/bracing, and top or scaffold rigging would fall somewhere in between. Work risk is increased by rigging activities that will obviously put a greater amount of forces on the tree and its structure than “free falling” limbs or pieces, yet there is still a work risk presented by the bending moments, reactive forces, compression and tension present in some seemingly straightforward felling and pruning, so all activities must be evaluated in conjunction with the tree’s safety and stability
Although at first glance an operational tree risk assessment may seem a complex and overwhelming process, one requiring far too much time and energy to be worthwhile, in reality, with proper training and experience, the assessment can be accomplished quite quickly and efficiently. As for the worth of a comprehensive risk assessment, the alternative of possible injuries or even death due to limb, scaffold or whole tree failure would seem to readily illustrate an assessment’s value.
Tain is a contract climber, splicer, educator and writer currently located in Lancaster, Ky.